Auto merge of rust-lang#14664 - HKalbasi:mir, r=HKalbasi

MIR episode 4

In lowering, it now supports overloaded and arith assignment binary operators, statics. and constants in patterns. There is now 252 functions that we fail to emit mir for them, and the majority of them are due type mismatches or other deep and unrelated issues (but it isn't done yet, for example slice patterns and destructing assignment is not implemented yet).

In evaluating, it now can evaluate associated constants in traits (so now typenum's `U5::ToConst` should work), allocator functions, atomic intrinsics, and some more things. It also provides a (hacky) basis for making progress in rust-lang#14275. I also added a `Interpret` code lens to `Run` and `Debug` when the experimental `interpret tests` is enabled (previously it showed result in hover, which was unusable even for debugging)

Changes in unrelated files are:
* Changes substitutions of closures, now it includes parent substs ~~before~~ after `sig_ty`.
* ~~A salsa input for retrieving the path of a file id, used in emitting stack trace for interpret result.~~
* Normalizing associated types in layout computing

Author: bors

crates/base-db/src/change.rs

@@ -34,7 +34,7 @@ impl fmt::Debug for Change {
 }
 
 impl Change {
-    pub fn new() -> Change {
+    pub fn new() -> Self {
         Change::default()
     }
 

crates/hir-def/src/body/lower.rs

@@ -17,7 +17,7 @@ use rustc_hash::FxHashMap;
 use smallvec::SmallVec;
 use syntax::{
     ast::{
-        self, ArrayExprKind, AstChildren, BlockExpr, HasArgList, HasLoopBody, HasName,
+        self, ArrayExprKind, AstChildren, BlockExpr, HasArgList, HasAttrs, HasLoopBody, HasName,
         SlicePatComponents,
     },
     AstNode, AstPtr, SyntaxNodePtr,
@@ -302,16 +302,29 @@ impl ExprCollector<'_> {
                 self.alloc_expr(Expr::For { iterable, pat, body, label }, syntax_ptr)
             }
             ast::Expr::CallExpr(e) => {
-                let callee = self.collect_expr_opt(e.expr());
-                let args = if let Some(arg_list) = e.arg_list() {
-                    arg_list.args().filter_map(|e| self.maybe_collect_expr(e)).collect()
-                } else {
-                    Box::default()
+                let is_rustc_box = {
+                    let attrs = e.attrs();
+                    attrs.filter_map(|x| x.as_simple_atom()).any(|x| x == "rustc_box")
                 };
-                self.alloc_expr(
-                    Expr::Call { callee, args, is_assignee_expr: self.is_lowering_assignee_expr },
-                    syntax_ptr,
-                )
+                if is_rustc_box {
+                    let expr = self.collect_expr_opt(e.arg_list().and_then(|x| x.args().next()));
+                    self.alloc_expr(Expr::Box { expr }, syntax_ptr)
+                } else {
+                    let callee = self.collect_expr_opt(e.expr());
+                    let args = if let Some(arg_list) = e.arg_list() {
+                        arg_list.args().filter_map(|e| self.maybe_collect_expr(e)).collect()
+                    } else {
+                        Box::default()
+                    };
+                    self.alloc_expr(
+                        Expr::Call {
+                            callee,
+                            args,
+                            is_assignee_expr: self.is_lowering_assignee_expr,
+                        },
+                        syntax_ptr,
+                    )
+                }
             }
             ast::Expr::MethodCallExpr(e) => {
                 let receiver = self.collect_expr_opt(e.receiver());