expr.rs

use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};

use rustc_ast::attr;
use rustc_ast::ptr::P as AstP;
use rustc_ast::*;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_data_structures::thin_vec::ThinVec;
use rustc_errors::struct_span_err;
use rustc_hir as hir;
use rustc_hir::def::Res;
use rustc_hir::definitions::DefPathData;
use rustc_span::hygiene::ExpnId;
use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
use rustc_span::symbol::{sym, Ident, Symbol};
use rustc_span::{hygiene::ForLoopLoc, BytePos, DUMMY_SP};

impl<'hir> LoweringContext<'_, 'hir> {
    fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
        self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
    }

    pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
        self.arena.alloc(self.lower_expr_mut(e))
    }

    pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
        ensure_sufficient_stack(|| {
            let kind = match e.kind {
                ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
                ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
                ExprKind::ConstBlock(ref anon_const) => {
                    let anon_const = self.lower_anon_const(anon_const);
                    hir::ExprKind::ConstBlock(anon_const)
                }
                ExprKind::Repeat(ref expr, ref count) => {
                    let expr = self.lower_expr(expr);
                    let count = self.lower_anon_const(count);
                    hir::ExprKind::Repeat(expr, count)
                }
                ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
                ExprKind::Call(ref f, ref args) => {
                    if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
                        self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
                    } else {
                        let f = self.lower_expr(f);
                        hir::ExprKind::Call(f, self.lower_exprs(args))
                    }
                }
                ExprKind::MethodCall(ref seg, ref args, span) => {
                    let hir_seg = self.arena.alloc(self.lower_path_segment(
                        e.span,
                        seg,
                        ParamMode::Optional,
                        0,
                        ParenthesizedGenericArgs::Err,
                        ImplTraitContext::disallowed(),
                    ));
                    let args = self.lower_exprs(args);
                    hir::ExprKind::MethodCall(
                        hir_seg,
                        self.lower_span(seg.ident.span),
                        args,
                        self.lower_span(span),
                    )
                }
                ExprKind::Binary(binop, ref lhs, ref rhs) => {
                    let binop = self.lower_binop(binop);
                    let lhs = self.lower_expr(lhs);
                    let rhs = self.lower_expr(rhs);
                    hir::ExprKind::Binary(binop, lhs, rhs)
                }
                ExprKind::Unary(op, ref ohs) => {
                    let op = self.lower_unop(op);
                    let ohs = self.lower_expr(ohs);
                    hir::ExprKind::Unary(op, ohs)
                }
                ExprKind::Lit(ref l) => {
                    hir::ExprKind::Lit(respan(self.lower_span(l.span), l.kind.clone()))
                }
                ExprKind::Cast(ref expr, ref ty) => {
                    let expr = self.lower_expr(expr);
                    let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
                    hir::ExprKind::Cast(expr, ty)
                }
                ExprKind::Type(ref expr, ref ty) => {
                    let expr = self.lower_expr(expr);
                    let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
                    hir::ExprKind::Type(expr, ty)
                }
                ExprKind::AddrOf(k, m, ref ohs) => {
                    let ohs = self.lower_expr(ohs);
                    hir::ExprKind::AddrOf(k, m, ohs)
                }
                ExprKind::Let(ref pat, ref scrutinee, span) => {
                    hir::ExprKind::Let(self.arena.alloc(hir::Let {
                        hir_id: self.next_id(),
                        span: self.lower_span(span),
                        pat: self.lower_pat(pat),
                        ty: None,
                        init: self.lower_expr(scrutinee),
                    }))
                }
                ExprKind::If(ref cond, ref then, ref else_opt) => {
                    self.lower_expr_if(cond, then, else_opt.as_deref())
                }
                ExprKind::While(ref cond, ref body, opt_label) => {
                    self.with_loop_scope(e.id, |this| {
                        let span =
                            this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
                        this.lower_expr_while_in_loop_scope(span, cond, body, opt_label)
                    })
                }
                ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
                    hir::ExprKind::Loop(
                        this.lower_block(body, false),
                        this.lower_label(opt_label),
                        hir::LoopSource::Loop,
                        DUMMY_SP,
                    )
                }),
                ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
                ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
                    self.lower_expr(expr),
                    self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
                    hir::MatchSource::Normal,
                ),
                ExprKind::Async(capture_clause, closure_node_id, ref block) => self
                    .make_async_expr(
                        capture_clause,
                        closure_node_id,
                        None,
                        block.span,
                        hir::AsyncGeneratorKind::Block,
                        |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
                    ),
                ExprKind::Await(ref expr) => {
                    let span = if expr.span.hi() < e.span.hi() {
                        expr.span.shrink_to_hi().with_hi(e.span.hi())
                    } else {
                        // this is a recovered `await expr`
                        e.span
                    };
                    self.lower_expr_await(span, expr)
                }
                ExprKind::Closure(
                    capture_clause,
                    asyncness,
                    movability,
                    ref decl,
                    ref body,
                    fn_decl_span,
                ) => {
                    if let Async::Yes { closure_id, .. } = asyncness {
                        self.lower_expr_async_closure(
                            capture_clause,
                            closure_id,
                            decl,
                            body,
                            fn_decl_span,
                        )
                    } else {
                        self.lower_expr_closure(
                            capture_clause,
                            movability,
                            decl,
                            body,
                            fn_decl_span,
                        )
                    }
                }
                ExprKind::Block(ref blk, opt_label) => {
                    let opt_label = self.lower_label(opt_label);
                    hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
                }
                ExprKind::Assign(ref el, ref er, span) => {
                    self.lower_expr_assign(el, er, span, e.span)
                }
                ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
                    self.lower_binop(op),
                    self.lower_expr(el),
                    self.lower_expr(er),
                ),
                ExprKind::Field(ref el, ident) => {
                    hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
                }
                ExprKind::Index(ref el, ref er) => {
                    hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
                }
                ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
                    self.lower_expr_range_closed(e.span, e1, e2)
                }
                ExprKind::Range(ref e1, ref e2, lims) => {
                    self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
                }
                ExprKind::Underscore => {
                    self.sess
                        .struct_span_err(
                            e.span,
                            "in expressions, `_` can only be used on the left-hand side of an assignment",
                        )
                        .span_label(e.span, "`_` not allowed here")
                        .emit();
                    hir::ExprKind::Err
                }
                ExprKind::Path(ref qself, ref path) => {
                    let qpath = self.lower_qpath(
                        e.id,
                        qself,
                        path,
                        ParamMode::Optional,
                        ImplTraitContext::disallowed(),
                    );
                    hir::ExprKind::Path(qpath)
                }
                ExprKind::Break(opt_label, ref opt_expr) => {
                    let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
                    hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
                }
                ExprKind::Continue(opt_label) => {
                    hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
                }
                ExprKind::Ret(ref e) => {
                    let e = e.as_ref().map(|x| self.lower_expr(x));
                    hir::ExprKind::Ret(e)
                }
                ExprKind::InlineAsm(ref asm) => {
                    hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
                }
                ExprKind::LlvmInlineAsm(ref asm) => self.lower_expr_llvm_asm(asm),
                ExprKind::Struct(ref se) => {
                    let rest = match &se.rest {
                        StructRest::Base(e) => Some(self.lower_expr(e)),
                        StructRest::Rest(sp) => {
                            self.sess
                                .struct_span_err(*sp, "base expression required after `..`")
                                .span_label(*sp, "add a base expression here")
                                .emit();
                            Some(&*self.arena.alloc(self.expr_err(*sp)))
                        }
                        StructRest::None => None,
                    };
                    hir::ExprKind::Struct(
                        self.arena.alloc(self.lower_qpath(
                            e.id,
                            &se.qself,
                            &se.path,
                            ParamMode::Optional,
                            ImplTraitContext::disallowed(),
                        )),
                        self.arena
                            .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
                        rest,
                    )
                }
                ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
                ExprKind::Err => hir::ExprKind::Err,
                ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
                ExprKind::Paren(ref ex) => {
                    let mut ex = self.lower_expr_mut(ex);
                    // Include parens in span, but only if it is a super-span.
                    if e.span.contains(ex.span) {
                        ex.span = self.lower_span(e.span);
                    }
                    // Merge attributes into the inner expression.
                    if !e.attrs.is_empty() {
                        let old_attrs =
                            self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
                        self.attrs.insert(
                            ex.hir_id.local_id,
                            &*self.arena.alloc_from_iter(
                                e.attrs
                                    .iter()
                                    .map(|a| self.lower_attr(a))
                                    .chain(old_attrs.iter().cloned()),
                            ),
                        );
                    }
                    return ex;
                }

                // Desugar `ExprForLoop`
                // from: `[opt_ident]: for <pat> in <head> <body>`
                ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
                    return self.lower_expr_for(e, pat, head, body, opt_label);
                }
                ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
            };

            let hir_id = self.lower_node_id(e.id);
            self.lower_attrs(hir_id, &e.attrs);
            hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
        })
    }

    fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
        match u {
            UnOp::Deref => hir::UnOp::Deref,
            UnOp::Not => hir::UnOp::Not,
            UnOp::Neg => hir::UnOp::Neg,
        }
    }

    fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
        Spanned {
            node: match b.node {
                BinOpKind::Add => hir::BinOpKind::Add,
                BinOpKind::Sub => hir::BinOpKind::Sub,
                BinOpKind::Mul => hir::BinOpKind::Mul,
                BinOpKind::Div => hir::BinOpKind::Div,
                BinOpKind::Rem => hir::BinOpKind::Rem,
                BinOpKind::And => hir::BinOpKind::And,
                BinOpKind::Or => hir::BinOpKind::Or,
                BinOpKind::BitXor => hir::BinOpKind::BitXor,
                BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
                BinOpKind::BitOr => hir::BinOpKind::BitOr,
                BinOpKind::Shl => hir::BinOpKind::Shl,
                BinOpKind::Shr => hir::BinOpKind::Shr,
                BinOpKind::Eq => hir::BinOpKind::Eq,
                BinOpKind::Lt => hir::BinOpKind::Lt,
                BinOpKind::Le => hir::BinOpKind::Le,
                BinOpKind::Ne => hir::BinOpKind::Ne,
                BinOpKind::Ge => hir::BinOpKind::Ge,
                BinOpKind::Gt => hir::BinOpKind::Gt,
            },
            span: self.lower_span(b.span),
        }
    }

    fn lower_legacy_const_generics(
        &mut self,
        mut f: Expr,
        args: Vec<AstP<Expr>>,
        legacy_args_idx: &[usize],
    ) -> hir::ExprKind<'hir> {
        let path = match f.kind {
            ExprKind::Path(None, ref mut path) => path,
            _ => unreachable!(),
        };

        // Split the arguments into const generics and normal arguments
        let mut real_args = vec![];
        let mut generic_args = vec![];
        for (idx, arg) in args.into_iter().enumerate() {
            if legacy_args_idx.contains(&idx) {
                let parent_def_id = self.current_hir_id_owner;
                let node_id = self.resolver.next_node_id();

                // Add a definition for the in-band const def.
                self.resolver.create_def(
                    parent_def_id,
                    node_id,
                    DefPathData::AnonConst,
                    ExpnId::root(),
                    arg.span,
                );

                let anon_const = AnonConst { id: node_id, value: arg };
                generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
            } else {
                real_args.push(arg);
            }
        }

        // Add generic args to the last element of the path.
        let last_segment = path.segments.last_mut().unwrap();
        assert!(last_segment.args.is_none());
        last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
            span: DUMMY_SP,
            args: generic_args,
        })));

        // Now lower everything as normal.
        let f = self.lower_expr(&f);
        hir::ExprKind::Call(f, self.lower_exprs(&real_args))
    }

    fn lower_expr_if(
        &mut self,
        cond: &Expr,
        then: &Block,
        else_opt: Option<&Expr>,
    ) -> hir::ExprKind<'hir> {
        let lowered_cond = self.lower_expr(cond);
        let new_cond = self.manage_let_cond(lowered_cond);
        let then_expr = self.lower_block_expr(then);
        if let Some(rslt) = else_opt {
            hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
        } else {
            hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
        }
    }

    // If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
    // in a temporary block.
    fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
        match cond.kind {
            hir::ExprKind::Let(..) => cond,
            _ => {
                let span_block =
                    self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
                self.expr_drop_temps(span_block, cond, AttrVec::new())
            }
        }
    }

    // We desugar: `'label: while $cond $body` into:
    //
    // ```
    // 'label: loop {
    //   if { let _t = $cond; _t } {
    //     $body
    //   }
    //   else {
    //     break;
    //   }
    // }
    // ```
    //
    // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
    // to preserve drop semantics since `while $cond { ... }` does not
    // let temporaries live outside of `cond`.
    fn lower_expr_while_in_loop_scope(
        &mut self,
        span: Span,
        cond: &Expr,
        body: &Block,
        opt_label: Option<Label>,
    ) -> hir::ExprKind<'hir> {
        let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
        let new_cond = self.manage_let_cond(lowered_cond);
        let then = self.lower_block_expr(body);
        let expr_break = self.expr_break(span, ThinVec::new());
        let stmt_break = self.stmt_expr(span, expr_break);
        let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
        let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
        let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
        let if_expr = self.expr(span, if_kind, ThinVec::new());
        let block = self.block_expr(self.arena.alloc(if_expr));
        let span = self.lower_span(span.with_hi(cond.span.hi()));
        let opt_label = self.lower_label(opt_label);
        hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
    }

    /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
    /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
    /// and save the block id to use it as a break target for desugaring of the `?` operator.
    fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
        self.with_catch_scope(body.id, |this| {
            let mut block = this.lower_block_noalloc(body, true);

            // Final expression of the block (if present) or `()` with span at the end of block
            let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
                (
                    this.mark_span_with_reason(
                        DesugaringKind::TryBlock,
                        expr.span,
                        this.allow_try_trait.clone(),
                    ),
                    expr,
                )
            } else {
                let try_span = this.mark_span_with_reason(
                    DesugaringKind::TryBlock,
                    this.sess.source_map().end_point(body.span),
                    this.allow_try_trait.clone(),
                );

                (try_span, this.expr_unit(try_span))
            };

            let ok_wrapped_span =
                this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);

            // `::std::ops::Try::from_output($tail_expr)`
            block.expr = Some(this.wrap_in_try_constructor(
                hir::LangItem::TryTraitFromOutput,
                try_span,
                tail_expr,
                ok_wrapped_span,
            ));

            hir::ExprKind::Block(this.arena.alloc(block), None)
        })
    }

    fn wrap_in_try_constructor(
        &mut self,
        lang_item: hir::LangItem,
        method_span: Span,
        expr: &'hir hir::Expr<'hir>,
        overall_span: Span,
    ) -> &'hir hir::Expr<'hir> {
        let constructor = self.arena.alloc(self.expr_lang_item_path(
            method_span,
            lang_item,
            ThinVec::new(),
            None,
        ));
        self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
    }

    fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
        let pat = self.lower_pat(&arm.pat);
        let guard = arm.guard.as_ref().map(|cond| {
            if let ExprKind::Let(ref pat, ref scrutinee, _) = cond.kind {
                hir::Guard::IfLet(self.lower_pat(pat), self.lower_expr(scrutinee))
            } else {
                hir::Guard::If(self.lower_expr(cond))
            }
        });
        let hir_id = self.next_id();
        self.lower_attrs(hir_id, &arm.attrs);
        hir::Arm {
            hir_id,
            pat,
            guard,
            body: self.lower_expr(&arm.body),
            span: self.lower_span(arm.span),
        }
    }

    /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
    ///
    /// This results in:
    ///
    /// ```text
    /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
    ///     <body>
    /// })
    /// ```
    pub(super) fn make_async_expr(
        &mut self,
        capture_clause: CaptureBy,
        closure_node_id: NodeId,
        ret_ty: Option<AstP<Ty>>,
        span: Span,
        async_gen_kind: hir::AsyncGeneratorKind,
        body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
    ) -> hir::ExprKind<'hir> {
        let output = match ret_ty {
            Some(ty) => hir::FnRetTy::Return(self.lower_ty(&ty, ImplTraitContext::disallowed())),
            None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
        };

        // Resume argument type. We let the compiler infer this to simplify the lowering. It is
        // fully constrained by `future::from_generator`.
        let input_ty = hir::Ty {
            hir_id: self.next_id(),
            kind: hir::TyKind::Infer,
            span: self.lower_span(span),
        };

        // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
        let decl = self.arena.alloc(hir::FnDecl {
            inputs: arena_vec![self; input_ty],
            output,
            c_variadic: false,
            implicit_self: hir::ImplicitSelfKind::None,
        });

        // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
        let (pat, task_context_hid) = self.pat_ident_binding_mode(
            span,
            Ident::with_dummy_span(sym::_task_context),
            hir::BindingAnnotation::Mutable,
        );
        let param = hir::Param {
            hir_id: self.next_id(),
            pat,
            ty_span: self.lower_span(span),
            span: self.lower_span(span),
        };
        let params = arena_vec![self; param];

        let body_id = self.lower_body(move |this| {
            this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));

            let old_ctx = this.task_context;
            this.task_context = Some(task_context_hid);
            let res = body(this);
            this.task_context = old_ctx;
            (params, res)
        });

        // `static |_task_context| -> <ret_ty> { body }`:
        let generator_kind = hir::ExprKind::Closure(
            capture_clause,
            decl,
            body_id,
            self.lower_span(span),
            Some(hir::Movability::Static),
        );
        let generator = hir::Expr {
            hir_id: self.lower_node_id(closure_node_id),
            kind: generator_kind,
            span: self.lower_span(span),
        };

        // `future::from_generator`:
        let unstable_span =
            self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
        let gen_future = self.expr_lang_item_path(
            unstable_span,
            hir::LangItem::FromGenerator,
            ThinVec::new(),
            None,
        );

        // `future::from_generator(generator)`:
        hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
    }

    /// Desugar `<expr>.await` into:
    /// ```rust
    /// match ::std::future::IntoFuture::into_future(<expr>) {
    ///     mut pinned => loop {
    ///         match unsafe { ::std::future::Future::poll(
    ///             <::std::pin::Pin>::new_unchecked(&mut pinned),
    ///             ::std::future::get_context(task_context),
    ///         ) } {
    ///             ::std::task::Poll::Ready(result) => break result,
    ///             ::std::task::Poll::Pending => {}
    ///         }
    ///         task_context = yield ();
    ///     }
    /// }
    /// ```
    fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
        let dot_await_span = expr.span.shrink_to_hi().to(await_span);
        match self.generator_kind {
            Some(hir::GeneratorKind::Async(_)) => {}
            Some(hir::GeneratorKind::Gen) | None => {
                let mut err = struct_span_err!(
                    self.sess,
                    await_span,
                    E0728,
                    "`await` is only allowed inside `async` functions and blocks"
                );
                err.span_label(await_span, "only allowed inside `async` functions and blocks");
                if let Some(item_sp) = self.current_item {
                    err.span_label(item_sp, "this is not `async`");
                }
                err.emit();
            }
        }
        let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
        let gen_future_span = self.mark_span_with_reason(
            DesugaringKind::Await,
            await_span,
            self.allow_gen_future.clone(),
        );
        let expr = self.lower_expr_mut(expr);
        let expr_hir_id = expr.hir_id;

        let pinned_ident = Ident::with_dummy_span(sym::pinned);
        let (pinned_pat, pinned_pat_hid) =
            self.pat_ident_binding_mode(span, pinned_ident, hir::BindingAnnotation::Mutable);

        let task_context_ident = Ident::with_dummy_span(sym::_task_context);

        // unsafe {
        //     ::std::future::Future::poll(
        //         ::std::pin::Pin::new_unchecked(&mut pinned),
        //         ::std::future::get_context(task_context),
        //     )
        // }
        let poll_expr = {
            let pinned = self.expr_ident(span, pinned_ident, pinned_pat_hid);
            let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
            let task_context = if let Some(task_context_hid) = self.task_context {
                self.expr_ident_mut(span, task_context_ident, task_context_hid)
            } else {
                // Use of `await` outside of an async context, we cannot use `task_context` here.
                self.expr_err(span)
            };
            let new_unchecked = self.expr_call_lang_item_fn_mut(
                span,
                hir::LangItem::PinNewUnchecked,
                arena_vec![self; ref_mut_pinned],
                Some(expr_hir_id),
            );
            let get_context = self.expr_call_lang_item_fn_mut(
                gen_future_span,
                hir::LangItem::GetContext,
                arena_vec![self; task_context],
                Some(expr_hir_id),
            );
            let call = self.expr_call_lang_item_fn(
                span,
                hir::LangItem::FuturePoll,
                arena_vec![self; new_unchecked, get_context],
                Some(expr_hir_id),
            );
            self.arena.alloc(self.expr_unsafe(call))
        };

        // `::std::task::Poll::Ready(result) => break result`
        let loop_node_id = self.resolver.next_node_id();
        let loop_hir_id = self.lower_node_id(loop_node_id);
        let ready_arm = {
            let x_ident = Ident::with_dummy_span(sym::result);
            let (x_pat, x_pat_hid) = self.pat_ident(span, x_ident);
            let x_expr = self.expr_ident(span, x_ident, x_pat_hid);
            let ready_field = self.single_pat_field(span, x_pat);
            let ready_pat = self.pat_lang_item_variant(
                span,
                hir::LangItem::PollReady,
                ready_field,
                Some(expr_hir_id),
            );
            let break_x = self.with_loop_scope(loop_node_id, move |this| {
                let expr_break =
                    hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
                this.arena.alloc(this.expr(span, expr_break, ThinVec::new()))
            });
            self.arm(ready_pat, break_x)
        };

        // `::std::task::Poll::Pending => {}`
        let pending_arm = {
            let pending_pat = self.pat_lang_item_variant(
                span,
                hir::LangItem::PollPending,
                &[],
                Some(expr_hir_id),
            );
            let empty_block = self.expr_block_empty(span);
            self.arm(pending_pat, empty_block)
        };

        let inner_match_stmt = {
            let match_expr = self.expr_match(
                span,
                poll_expr,
                arena_vec![self; ready_arm, pending_arm],
                hir::MatchSource::AwaitDesugar,
            );
            self.stmt_expr(span, match_expr)
        };

        // task_context = yield ();
        let yield_stmt = {
            let unit = self.expr_unit(span);
            let yield_expr = self.expr(
                span,
                hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
                ThinVec::new(),
            );
            let yield_expr = self.arena.alloc(yield_expr);

            if let Some(task_context_hid) = self.task_context {
                let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
                let assign = self.expr(
                    span,
                    hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
                    AttrVec::new(),
                );
                self.stmt_expr(span, assign)
            } else {
                // Use of `await` outside of an async context. Return `yield_expr` so that we can
                // proceed with type checking.
                self.stmt(span, hir::StmtKind::Semi(yield_expr))
            }
        };

        let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);

        // loop { .. }
        let loop_expr = self.arena.alloc(hir::Expr {
            hir_id: loop_hir_id,
            kind: hir::ExprKind::Loop(
                loop_block,
                None,
                hir::LoopSource::Loop,
                self.lower_span(span),
            ),
            span: self.lower_span(span),
        });

        // mut pinned => loop { ... }
        let pinned_arm = self.arm(pinned_pat, loop_expr);

        // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
        let into_future_span = self.mark_span_with_reason(
            DesugaringKind::Await,
            await_span,
            self.allow_into_future.clone(),
        );
        let into_future_expr = self.expr_call_lang_item_fn(
            into_future_span,
            hir::LangItem::IntoFutureIntoFuture,
            arena_vec![self; expr],
            Some(expr_hir_id),
        );

        // match <into_future_expr> {
        //     mut pinned => loop { .. }
        // }
        hir::ExprKind::Match(
            into_future_expr,
            arena_vec![self; pinned_arm],
            hir::MatchSource::AwaitDesugar,
        )
    }

    fn lower_expr_closure(
        &mut self,
        capture_clause: CaptureBy,
        movability: Movability,
        decl: &FnDecl,
        body: &Expr,
        fn_decl_span: Span,
    ) -> hir::ExprKind<'hir> {
        let (body_id, generator_option) = self.with_new_scopes(move |this| {
            let prev = this.current_item;
            this.current_item = Some(fn_decl_span);
            let mut generator_kind = None;
            let body_id = this.lower_fn_body(decl, |this| {
                let e = this.lower_expr_mut(body);
                generator_kind = this.generator_kind;
                e
            });
            let generator_option =
                this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
            this.current_item = prev;
            (body_id, generator_option)
        });

        // Lower outside new scope to preserve `is_in_loop_condition`.
        let fn_decl = self.lower_fn_decl(decl, None, false, None);

        hir::ExprKind::Closure(
            capture_clause,
            fn_decl,
            body_id,
            self.lower_span(fn_decl_span),
            generator_option,
        )
    }

    fn generator_movability_for_fn(
        &mut self,
        decl: &FnDecl,
        fn_decl_span: Span,
        generator_kind: Option<hir::GeneratorKind>,
        movability: Movability,
    ) -> Option<hir::Movability> {
        match generator_kind {
            Some(hir::GeneratorKind::Gen) => {
                if decl.inputs.len() > 1 {
                    struct_span_err!(
                        self.sess,
                        fn_decl_span,
                        E0628,
                        "too many parameters for a generator (expected 0 or 1 parameters)"
                    )
                    .emit();
                }
                Some(movability)
            }
            Some(hir::GeneratorKind::Async(_)) => {
                panic!("non-`async` closure body turned `async` during lowering");
            }
            None => {
                if movability == Movability::Static {
                    struct_span_err!(self.sess, fn_decl_span, E0697, "closures cannot be static")
                        .span_suggestion(
                            fn_decl_span
                                .with_lo(fn_decl_span.lo() + BytePos("static ".len() as u32)),
                            "remove the `static` keyword to define a regular closure",
                            "".into(),
                            rustc_errors::Applicability::MachineApplicable,
                        )
                        .emit();
                }
                None
            }
        }
    }

    fn lower_expr_async_closure(
        &mut self,
        capture_clause: CaptureBy,
        closure_id: NodeId,
        decl: &FnDecl,
        body: &Expr,
        fn_decl_span: Span,
    ) -> hir::ExprKind<'hir> {
        let outer_decl =
            FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };

        let body_id = self.with_new_scopes(|this| {
            // FIXME(cramertj): allow `async` non-`move` closures with arguments.
            if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
                struct_span_err!(
                    this.sess,
                    fn_decl_span,
                    E0708,
                    "`async` non-`move` closures with parameters are not currently supported",
                )
                .help(
                    "consider using `let` statements to manually capture \
                    variables by reference before entering an `async move` closure",
                )
                .emit();
            }

            // Transform `async |x: u8| -> X { ... }` into
            // `|x: u8| future_from_generator(|| -> X { ... })`.
            let body_id = this.lower_fn_body(&outer_decl, |this| {
                let async_ret_ty =
                    if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
                let async_body = this.make_async_expr(
                    capture_clause,
                    closure_id,
                    async_ret_ty,
                    body.span,
                    hir::AsyncGeneratorKind::Closure,
                    |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
                );
                this.expr(fn_decl_span, async_body, ThinVec::new())
            });
            body_id
        });

        // We need to lower the declaration outside the new scope, because we
        // have to conserve the state of being inside a loop condition for the
        // closure argument types.
        let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);

        hir::ExprKind::Closure(
            capture_clause,
            fn_decl,
            body_id,
            self.lower_span(fn_decl_span),
            None,
        )
    }

    /// Destructure the LHS of complex assignments.
    /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
    fn lower_expr_assign(
        &mut self,
        lhs: &Expr,
        rhs: &Expr,
        eq_sign_span: Span,
        whole_span: Span,
    ) -> hir::ExprKind<'hir> {
        // Return early in case of an ordinary assignment.
        fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
            match &lhs.kind {
                ExprKind::Array(..)
                | ExprKind::Struct(..)
                | ExprKind::Tup(..)
                | ExprKind::Underscore => false,
                // Check for tuple struct constructor.
                ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
                ExprKind::Paren(e) => {
                    match e.kind {
                        // We special-case `(..)` for consistency with patterns.
                        ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
                        _ => is_ordinary(lower_ctx, e),
                    }
                }
                _ => true,
            }
        }
        if is_ordinary(self, lhs) {
            return hir::ExprKind::Assign(
                self.lower_expr(lhs),
                self.lower_expr(rhs),
                self.lower_span(eq_sign_span),
            );
        }

        let mut assignments = vec![];

        // The LHS becomes a pattern: `(lhs1, lhs2)`.
        let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
        let rhs = self.lower_expr(rhs);

        // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
        let destructure_let = self.stmt_let_pat(
            None,
            whole_span,
            Some(rhs),
            pat,
            hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
        );

        // `a = lhs1; b = lhs2;`.
        let stmts = self
            .arena
            .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));

        // Wrap everything in a block.
        hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
    }

    /// If the given expression is a path to a tuple struct, returns that path.
    /// It is not a complete check, but just tries to reject most paths early
    /// if they are not tuple structs.
    /// Type checking will take care of the full validation later.
    fn extract_tuple_struct_path<'a>(
        &mut self,
        expr: &'a Expr,
    ) -> Option<(&'a Option<QSelf>, &'a Path)> {
        if let ExprKind::Path(qself, path) = &expr.kind {
            // Does the path resolve to something disallowed in a tuple struct/variant pattern?
            if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
                if partial_res.unresolved_segments() == 0
                    && !partial_res.base_res().expected_in_tuple_struct_pat()
                {
                    return None;
                }
            }
            return Some((qself, path));
        }
        None
    }

    /// Convert the LHS of a destructuring assignment to a pattern.
    /// Each sub-assignment is recorded in `assignments`.
    fn destructure_assign(
        &mut self,
        lhs: &Expr,
        eq_sign_span: Span,
        assignments: &mut Vec<hir::Stmt<'hir>>,
    ) -> &'hir hir::Pat<'hir> {
        self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
    }

    fn destructure_assign_mut(
        &mut self,
        lhs: &Expr,
        eq_sign_span: Span,
        assignments: &mut Vec<hir::Stmt<'hir>>,
    ) -> hir::Pat<'hir> {
        match &lhs.kind {
            // Underscore pattern.
            ExprKind::Underscore => {
                return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
            }
            // Slice patterns.
            ExprKind::Array(elements) => {
                let (pats, rest) =
                    self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
                let slice_pat = if let Some((i, span)) = rest {
                    let (before, after) = pats.split_at(i);
                    hir::PatKind::Slice(
                        before,
                        Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
                        after,
                    )
                } else {
                    hir::PatKind::Slice(pats, None, &[])
                };
                return self.pat_without_dbm(lhs.span, slice_pat);
            }
            // Tuple structs.
            ExprKind::Call(callee, args) => {
                if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
                    let (pats, rest) = self.destructure_sequence(
                        args,
                        "tuple struct or variant",
                        eq_sign_span,
                        assignments,
                    );
                    let qpath = self.lower_qpath(
                        callee.id,
                        qself,
                        path,
                        ParamMode::Optional,
                        ImplTraitContext::disallowed(),
                    );
                    // Destructure like a tuple struct.
                    let tuple_struct_pat =
                        hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
                    return self.pat_without_dbm(lhs.span, tuple_struct_pat);
                }
            }
            // Structs.
            ExprKind::Struct(se) => {
                let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
                    let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
                    hir::PatField {
                        hir_id: self.next_id(),
                        ident: self.lower_ident(f.ident),
                        pat,
                        is_shorthand: f.is_shorthand,
                        span: self.lower_span(f.span),
                    }
                }));
                let qpath = self.lower_qpath(
                    lhs.id,
                    &se.qself,
                    &se.path,
                    ParamMode::Optional,
                    ImplTraitContext::disallowed(),
                );
                let fields_omitted = match &se.rest {
                    StructRest::Base(e) => {
                        self.sess
                            .struct_span_err(
                                e.span,
                                "functional record updates are not allowed in destructuring \
                                    assignments",
                            )
                            .span_suggestion(
                                e.span,
                                "consider removing the trailing pattern",
                                String::new(),
                                rustc_errors::Applicability::MachineApplicable,
                            )
                            .emit();
                        true
                    }
                    StructRest::Rest(_) => true,
                    StructRest::None => false,
                };
                let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
                return self.pat_without_dbm(lhs.span, struct_pat);
            }
            // Tuples.
            ExprKind::Tup(elements) => {
                let (pats, rest) =
                    self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
                let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
                return self.pat_without_dbm(lhs.span, tuple_pat);
            }
            ExprKind::Paren(e) => {
                // We special-case `(..)` for consistency with patterns.
                if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
                    let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
                    return self.pat_without_dbm(lhs.span, tuple_pat);
                } else {
                    return self.destructure_assign_mut(e, eq_sign_span, assignments);
                }
            }
            _ => {}
        }
        // Treat all other cases as normal lvalue.
        let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
        let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
        let ident = self.expr_ident(lhs.span, ident, binding);
        let assign =
            hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
        let expr = self.expr(lhs.span, assign, ThinVec::new());
        assignments.push(self.stmt_expr(lhs.span, expr));
        pat
    }

    /// Destructure a sequence of expressions occurring on the LHS of an assignment.
    /// Such a sequence occurs in a tuple (struct)/slice.
    /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
    /// exists.
    /// Each sub-assignment is recorded in `assignments`.
    fn destructure_sequence(
        &mut self,
        elements: &[AstP<Expr>],
        ctx: &str,
        eq_sign_span: Span,
        assignments: &mut Vec<hir::Stmt<'hir>>,
    ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
        let mut rest = None;
        let elements =
            self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
                // Check for `..` pattern.
                if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
                    if let Some((_, prev_span)) = rest {
                        self.ban_extra_rest_pat(e.span, prev_span, ctx);
                    } else {
                        rest = Some((i, e.span));
                    }
                    None
                } else {
                    Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
                }
            }));
        (elements, rest)
    }

    /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
    fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
        let e1 = self.lower_expr_mut(e1);
        let e2 = self.lower_expr_mut(e2);
        let fn_path =
            hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
        let fn_expr =
            self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
        hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
    }

    fn lower_expr_range(
        &mut self,
        span: Span,
        e1: Option<&Expr>,
        e2: Option<&Expr>,
        lims: RangeLimits,
    ) -> hir::ExprKind<'hir> {
        use rustc_ast::RangeLimits::*;

        let lang_item = match (e1, e2, lims) {
            (None, None, HalfOpen) => hir::LangItem::RangeFull,
            (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
            (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
            (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
            (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
            (Some(..), Some(..), Closed) => unreachable!(),
            (_, None, Closed) => self.diagnostic().span_fatal(span, "inclusive range with no end"),
        };

        let fields = self.arena.alloc_from_iter(
            e1.iter().map(|e| ("start", e)).chain(e2.iter().map(|e| ("end", e))).map(|(s, e)| {
                let expr = self.lower_expr(&e);
                let ident = Ident::new(Symbol::intern(s), self.lower_span(e.span));
                self.expr_field(ident, expr, e.span)
            }),
        );

        hir::ExprKind::Struct(
            self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
            fields,
            None,
        )
    }

    fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
        let label = opt_label?;
        Some(Label { ident: self.lower_ident(label.ident) })
    }

    fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
        let target_id = match destination {
            Some((id, _)) => {
                if let Some(loop_id) = self.resolver.get_label_res(id) {
                    Ok(self.lower_node_id(loop_id))
                } else {
                    Err(hir::LoopIdError::UnresolvedLabel)
                }
            }
            None => self
                .loop_scope
                .map(|id| Ok(self.lower_node_id(id)))
                .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
        };
        let label = self.lower_label(destination.map(|(_, label)| label));
        hir::Destination { label, target_id }
    }

    fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
        if self.is_in_loop_condition && opt_label.is_none() {
            hir::Destination {
                label: None,
                target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
            }
        } else {
            self.lower_loop_destination(opt_label.map(|label| (id, label)))
        }
    }

    fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
        let old_scope = self.catch_scope.replace(catch_id);
        let result = f(self);
        self.catch_scope = old_scope;
        result
    }

    fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
        // We're no longer in the base loop's condition; we're in another loop.
        let was_in_loop_condition = self.is_in_loop_condition;
        self.is_in_loop_condition = false;

        let old_scope = self.loop_scope.replace(loop_id);
        let result = f(self);
        self.loop_scope = old_scope;

        self.is_in_loop_condition = was_in_loop_condition;

        result
    }

    fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
        let was_in_loop_condition = self.is_in_loop_condition;
        self.is_in_loop_condition = true;

        let result = f(self);

        self.is_in_loop_condition = was_in_loop_condition;

        result
    }

    fn lower_expr_llvm_asm(&mut self, asm: &LlvmInlineAsm) -> hir::ExprKind<'hir> {
        let inner = hir::LlvmInlineAsmInner {
            inputs: asm.inputs.iter().map(|&(c, _)| c).collect(),
            outputs: asm
                .outputs
                .iter()
                .map(|out| hir::LlvmInlineAsmOutput {
                    constraint: out.constraint,
                    is_rw: out.is_rw,
                    is_indirect: out.is_indirect,
                    span: self.lower_span(out.expr.span),
                })
                .collect(),
            asm: asm.asm,
            asm_str_style: asm.asm_str_style,
            clobbers: asm.clobbers.clone(),
            volatile: asm.volatile,
            alignstack: asm.alignstack,
            dialect: asm.dialect,
        };
        let hir_asm = hir::LlvmInlineAsm {
            inner,
            inputs_exprs: self.arena.alloc_from_iter(
                asm.inputs.iter().map(|&(_, ref input)| self.lower_expr_mut(input)),
            ),
            outputs_exprs: self
                .arena
                .alloc_from_iter(asm.outputs.iter().map(|out| self.lower_expr_mut(&out.expr))),
        };
        hir::ExprKind::LlvmInlineAsm(self.arena.alloc(hir_asm))
    }

    fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
        hir::ExprField {
            hir_id: self.next_id(),
            ident: self.lower_ident(f.ident),
            expr: self.lower_expr(&f.expr),
            span: self.lower_span(f.span),
            is_shorthand: f.is_shorthand,
        }
    }

    fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
        match self.generator_kind {
            Some(hir::GeneratorKind::Gen) => {}
            Some(hir::GeneratorKind::Async(_)) => {
                struct_span_err!(
                    self.sess,
                    span,
                    E0727,
                    "`async` generators are not yet supported"
                )
                .emit();
            }
            None => self.generator_kind = Some(hir::GeneratorKind::Gen),
        }

        let expr =
            opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));

        hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
    }

    /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
    /// ```rust
    /// {
    ///     let result = match IntoIterator::into_iter(<head>) {
    ///         mut iter => {
    ///             [opt_ident]: loop {
    ///                 match Iterator::next(&mut iter) {
    ///                     None => break,
    ///                     Some(<pat>) => <body>,
    ///                 };
    ///             }
    ///         }
    ///     };
    ///     result
    /// }
    /// ```
    fn lower_expr_for(
        &mut self,
        e: &Expr,
        pat: &Pat,
        head: &Expr,
        body: &Block,
        opt_label: Option<Label>,
    ) -> hir::Expr<'hir> {
        let head = self.lower_expr_mut(head);
        let pat = self.lower_pat(pat);
        let for_span =
            self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
        let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
        let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);

        // `None => break`
        let none_arm = {
            let break_expr =
                self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, ThinVec::new()));
            let pat = self.pat_none(for_span);
            self.arm(pat, break_expr)
        };

        // Some(<pat>) => <body>,
        let some_arm = {
            let some_pat = self.pat_some(pat_span, pat);
            let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
            let body_expr = self.arena.alloc(self.expr_block(body_block, ThinVec::new()));
            self.arm(some_pat, body_expr)
        };

        // `mut iter`
        let iter = Ident::with_dummy_span(sym::iter);
        let (iter_pat, iter_pat_nid) =
            self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::Mutable);

        // `match Iterator::next(&mut iter) { ... }`
        let match_expr = {
            let iter = self.expr_ident(head_span, iter, iter_pat_nid);
            let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
            let next_expr = self.expr_call_lang_item_fn(
                head_span,
                hir::LangItem::IteratorNext,
                arena_vec![self; ref_mut_iter],
                None,
            );
            let arms = arena_vec![self; none_arm, some_arm];

            self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
        };
        let match_stmt = self.stmt_expr(for_span, match_expr);

        let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);

        // `[opt_ident]: loop { ... }`
        let kind = hir::ExprKind::Loop(
            loop_block,
            self.lower_label(opt_label),
            hir::LoopSource::ForLoop,
            self.lower_span(for_span.with_hi(head.span.hi())),
        );
        let loop_expr =
            self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });

        // `mut iter => { ... }`
        let iter_arm = self.arm(iter_pat, loop_expr);

        // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
        let into_iter_expr = {
            self.expr_call_lang_item_fn(
                head_span,
                hir::LangItem::IntoIterIntoIter,
                arena_vec![self; head],
                None,
            )
        };

        let match_expr = self.arena.alloc(self.expr_match(
            for_span,
            into_iter_expr,
            arena_vec![self; iter_arm],
            hir::MatchSource::ForLoopDesugar,
        ));

        let attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();

        // This is effectively `{ let _result = ...; _result }`.
        // The construct was introduced in #21984 and is necessary to make sure that
        // temporaries in the `head` expression are dropped and do not leak to the
        // surrounding scope of the `match` since the `match` is not a terminating scope.
        //
        // Also, add the attributes to the outer returned expr node.
        self.expr_drop_temps_mut(for_span, match_expr, attrs.into())
    }

    /// Desugar `ExprKind::Try` from: `<expr>?` into:
    /// ```rust
    /// match Try::branch(<expr>) {
    ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
    ///     ControlFlow::Break(residual) =>
    ///         #[allow(unreachable_code)]
    ///         // If there is an enclosing `try {...}`:
    ///         break 'catch_target Try::from_residual(residual),
    ///         // Otherwise:
    ///         return Try::from_residual(residual),
    /// }
    /// ```
    fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
        let unstable_span = self.mark_span_with_reason(
            DesugaringKind::QuestionMark,
            span,
            self.allow_try_trait.clone(),
        );
        let try_span = self.sess.source_map().end_point(span);
        let try_span = self.mark_span_with_reason(
            DesugaringKind::QuestionMark,
            try_span,
            self.allow_try_trait.clone(),
        );

        // `Try::branch(<expr>)`
        let scrutinee = {
            // expand <expr>
            let sub_expr = self.lower_expr_mut(sub_expr);

            self.expr_call_lang_item_fn(
                unstable_span,
                hir::LangItem::TryTraitBranch,
                arena_vec![self; sub_expr],
                None,
            )
        };

        // `#[allow(unreachable_code)]`
        let attr = {
            // `allow(unreachable_code)`
            let allow = {
                let allow_ident = Ident::new(sym::allow, self.lower_span(span));
                let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
                let uc_nested = attr::mk_nested_word_item(uc_ident);
                attr::mk_list_item(allow_ident, vec![uc_nested])
            };
            attr::mk_attr_outer(allow)
        };
        let attrs = vec![attr];

        // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
        let continue_arm = {
            let val_ident = Ident::with_dummy_span(sym::val);
            let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
            let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
                span,
                val_ident,
                val_pat_nid,
                ThinVec::from(attrs.clone()),
            ));
            let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
            self.arm(continue_pat, val_expr)
        };

        // `ControlFlow::Break(residual) =>
        //     #[allow(unreachable_code)]
        //     return Try::from_residual(residual),`
        let break_arm = {
            let residual_ident = Ident::with_dummy_span(sym::residual);
            let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
            let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
            let from_residual_expr = self.wrap_in_try_constructor(
                hir::LangItem::TryTraitFromResidual,
                try_span,
                self.arena.alloc(residual_expr),
                unstable_span,
            );
            let thin_attrs = ThinVec::from(attrs);
            let ret_expr = if let Some(catch_node) = self.catch_scope {
                let target_id = Ok(self.lower_node_id(catch_node));
                self.arena.alloc(self.expr(
                    try_span,
                    hir::ExprKind::Break(
                        hir::Destination { label: None, target_id },
                        Some(from_residual_expr),
                    ),
                    thin_attrs,
                ))
            } else {
                self.arena.alloc(self.expr(
                    try_span,
                    hir::ExprKind::Ret(Some(from_residual_expr)),
                    thin_attrs,
                ))
            };

            let break_pat = self.pat_cf_break(try_span, residual_local);
            self.arm(break_pat, ret_expr)
        };

        hir::ExprKind::Match(
            scrutinee,
            arena_vec![self; break_arm, continue_arm],
            hir::MatchSource::TryDesugar,
        )
    }

    // =========================================================================
    // Helper methods for building HIR.
    // =========================================================================

    /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
    ///
    /// In terms of drop order, it has the same effect as wrapping `expr` in
    /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
    ///
    /// The drop order can be important in e.g. `if expr { .. }`.
    pub(super) fn expr_drop_temps(
        &mut self,
        span: Span,
        expr: &'hir hir::Expr<'hir>,
        attrs: AttrVec,
    ) -> &'hir hir::Expr<'hir> {
        self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
    }

    pub(super) fn expr_drop_temps_mut(
        &mut self,
        span: Span,
        expr: &'hir hir::Expr<'hir>,
        attrs: AttrVec,
    ) -> hir::Expr<'hir> {
        self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
    }

    fn expr_match(
        &mut self,
        span: Span,
        arg: &'hir hir::Expr<'hir>,
        arms: &'hir [hir::Arm<'hir>],
        source: hir::MatchSource,
    ) -> hir::Expr<'hir> {
        self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
    }

    fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
        let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
        self.expr(span, expr_break, attrs)
    }

    fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
        let expr_break = self.expr_break(span, attrs);
        self.arena.alloc(expr_break)
    }

    fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
        self.expr(
            span,
            hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
            ThinVec::new(),
        )
    }

    fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
        self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
    }

    fn expr_call_mut(
        &mut self,
        span: Span,
        e: &'hir hir::Expr<'hir>,
        args: &'hir [hir::Expr<'hir>],
    ) -> hir::Expr<'hir> {
        self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
    }

    fn expr_call(
        &mut self,
        span: Span,
        e: &'hir hir::Expr<'hir>,
        args: &'hir [hir::Expr<'hir>],
    ) -> &'hir hir::Expr<'hir> {
        self.arena.alloc(self.expr_call_mut(span, e, args))
    }

    fn expr_call_lang_item_fn_mut(
        &mut self,
        span: Span,
        lang_item: hir::LangItem,
        args: &'hir [hir::Expr<'hir>],
        hir_id: Option<hir::HirId>,
    ) -> hir::Expr<'hir> {
        let path =
            self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new(), hir_id));
        self.expr_call_mut(span, path, args)
    }

    fn expr_call_lang_item_fn(
        &mut self,
        span: Span,
        lang_item: hir::LangItem,
        args: &'hir [hir::Expr<'hir>],
        hir_id: Option<hir::HirId>,
    ) -> &'hir hir::Expr<'hir> {
        self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
    }

    fn expr_lang_item_path(
        &mut self,
        span: Span,
        lang_item: hir::LangItem,
        attrs: AttrVec,
        hir_id: Option<hir::HirId>,
    ) -> hir::Expr<'hir> {
        self.expr(
            span,
            hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
            attrs,
        )
    }

    pub(super) fn expr_ident(
        &mut self,
        sp: Span,
        ident: Ident,
        binding: hir::HirId,
    ) -> &'hir hir::Expr<'hir> {
        self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
    }

    pub(super) fn expr_ident_mut(
        &mut self,
        sp: Span,
        ident: Ident,
        binding: hir::HirId,
    ) -> hir::Expr<'hir> {
        self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
    }

    fn expr_ident_with_attrs(
        &mut self,
        span: Span,
        ident: Ident,
        binding: hir::HirId,
        attrs: AttrVec,
    ) -> hir::Expr<'hir> {
        let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
            None,
            self.arena.alloc(hir::Path {
                span: self.lower_span(span),
                res: Res::Local(binding),
                segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
            }),
        ));

        self.expr(span, expr_path, attrs)
    }

    fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
        let hir_id = self.next_id();
        let span = expr.span;
        self.expr(
            span,
            hir::ExprKind::Block(
                self.arena.alloc(hir::Block {
                    stmts: &[],
                    expr: Some(expr),
                    hir_id,
                    rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
                    span: self.lower_span(span),
                    targeted_by_break: false,
                }),
                None,
            ),
            ThinVec::new(),
        )
    }

    fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
        let blk = self.block_all(span, &[], None);
        let expr = self.expr_block(blk, ThinVec::new());
        self.arena.alloc(expr)
    }

    pub(super) fn expr_block(
        &mut self,
        b: &'hir hir::Block<'hir>,
        attrs: AttrVec,
    ) -> hir::Expr<'hir> {
        self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
    }

    pub(super) fn expr(
        &mut self,
        span: Span,
        kind: hir::ExprKind<'hir>,
        attrs: AttrVec,
    ) -> hir::Expr<'hir> {
        let hir_id = self.next_id();
        self.lower_attrs(hir_id, &attrs);
        hir::Expr { hir_id, kind, span: self.lower_span(span) }
    }

    fn expr_field(
        &mut self,
        ident: Ident,
        expr: &'hir hir::Expr<'hir>,
        span: Span,
    ) -> hir::ExprField<'hir> {
        hir::ExprField {
            hir_id: self.next_id(),
            ident,
            span: self.lower_span(span),
            expr,
            is_shorthand: false,
        }
    }

    fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
        hir::Arm {
            hir_id: self.next_id(),
            pat,
            guard: None,
            span: self.lower_span(expr.span),
            body: expr,
        }
    }
}