Suggest using deref in patterns

Fixes #132784

Author: uellenberg

compiler/rustc_hir_typeck/src/pat.rs

@@ -10,8 +10,12 @@ use rustc_errors::{
 };
 use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::pat_util::EnumerateAndAdjustIterator;
-use rustc_hir::{self as hir, BindingMode, ByRef, HirId, LangItem, Mutability, Pat, PatKind};
+use rustc_hir::{
+    self as hir, BindingMode, ByRef, ExprKind, HirId, LangItem, Mutability, Pat, PatKind,
+    is_range_literal,
+};
 use rustc_infer::infer;
+use rustc_middle::traits::PatternOriginExpr;
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
@@ -94,10 +98,46 @@ struct PatInfo<'a, 'tcx> {
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn pattern_cause(&self, ti: &TopInfo<'tcx>, cause_span: Span) -> ObligationCause<'tcx> {
+        let needs_parens = ti
+            .origin_expr
+            .map(|expr| match expr.kind {
+                // parenthesize if needed (Issue #46756)
+                hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
+                // parenthesize borrows of range literals (Issue #54505)
+                _ if is_range_literal(expr) => true,
+                _ => false,
+            })
+            .unwrap_or(false);
+
+        // If origin_expr exists, then expected represents the type of origin_expr.
+        // If span also exists, then span == origin_expr.span (although it doesn't need to exist).
+        // In that case, we can peel away references from both and treat them
+        // as the same.
+        let origin_expr_info = if let Some(origin_expr) = ti.origin_expr {
+            let mut cur_expr = origin_expr;
+            let mut count = 0;
+
+            // cur_ty may have more layers of references than cur_expr.
+            // We can only make suggestions about cur_expr, however, so we'll
+            // use that as our condition for stopping.
+            while let ExprKind::AddrOf(.., inner) = &cur_expr.kind {
+                cur_expr = inner;
+                count += 1;
+            }
+
+            Some(PatternOriginExpr {
+                peeled_span: cur_expr.span,
+                peeled_count: count,
+                peeled_prefix_suggestion_parentheses: needs_parens,
+            })
+        } else {
+            None
+        };
+
         let code = ObligationCauseCode::Pattern {
             span: ti.span,
             root_ty: ti.expected,
-            origin_expr: ti.origin_expr.is_some(),
+            origin_expr: origin_expr_info,
         };
         self.cause(cause_span, code)
     }

compiler/rustc_middle/src/traits/mod.rs

@@ -312,8 +312,8 @@ pub enum ObligationCauseCode<'tcx> {
         span: Option<Span>,
         /// The root expected type induced by a scrutinee or type expression.
         root_ty: Ty<'tcx>,
-        /// Whether the `Span` came from an expression or a type expression.
-        origin_expr: bool,
+        /// Information about the `Span`, if it came from an expression, otherwise `None`.
+        origin_expr: Option<PatternOriginExpr>,
     },
 
     /// Computing common supertype in an if expression
@@ -526,6 +526,25 @@ pub struct MatchExpressionArmCause<'tcx> {
     pub tail_defines_return_position_impl_trait: Option<LocalDefId>,
 }
 
+/// Information about the origin expression of a pattern, relevant to diagnostics.
+/// Fields here refer to the scrutinee of a pattern.
+/// If the scrutinee isn't given in the diagnostic, then this won't exist.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[derive(TypeFoldable, TypeVisitable, HashStable, TyEncodable, TyDecodable)]
+pub struct PatternOriginExpr {
+    /// A span representing the scrutinee expression, with all leading references
+    /// peeled from the expression.
+    /// Only references in the expression are peeled - if the expression refers to a variable
+    /// whose type is a reference, then that reference is kept because it wasn't created
+    /// in the expression.
+    pub peeled_span: Span,
+    /// The number of references that were peeled to produce `peeled_span`.
+    pub peeled_count: usize,
+    /// Does the peeled expression need to be wrapped in parentheses for
+    /// a prefix suggestion (i.e., dereference) to be valid.
+    pub peeled_prefix_suggestion_parentheses: bool,
+}
+
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 #[derive(TypeFoldable, TypeVisitable, HashStable, TyEncodable, TyDecodable)]
 pub struct IfExpressionCause<'tcx> {

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

@@ -61,10 +61,11 @@ use rustc_hir::{self as hir};
 use rustc_macros::extension;
 use rustc_middle::bug;
 use rustc_middle::dep_graph::DepContext;
+use rustc_middle::traits::PatternOriginExpr;
 use rustc_middle::ty::error::{ExpectedFound, TypeError, TypeErrorToStringExt};
 use rustc_middle::ty::print::{PrintError, PrintTraitRefExt as _, with_forced_trimmed_paths};
 use rustc_middle::ty::{
-    self, List, Region, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable, TypeVisitable,
+    self, List, ParamEnv, Region, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable, TypeVisitable,
     TypeVisitableExt,
 };
 use rustc_span::{BytePos, DesugaringKind, Pos, Span, sym};
@@ -74,7 +75,7 @@ use crate::error_reporting::TypeErrCtxt;
 use crate::errors::{ObligationCauseFailureCode, TypeErrorAdditionalDiags};
 use crate::infer;
 use crate::infer::relate::{self, RelateResult, TypeRelation};
-use crate::infer::{InferCtxt, TypeTrace, ValuePairs};
+use crate::infer::{InferCtxt, InferCtxtExt as _, TypeTrace, ValuePairs};
 use crate::solve::deeply_normalize_for_diagnostics;
 use crate::traits::{
     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
@@ -339,38 +340,71 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         cause: &ObligationCause<'tcx>,
         exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
         terr: TypeError<'tcx>,
+        param_env: Option<ParamEnv<'tcx>>,
     ) {
         match *cause.code() {
-            ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
-                let ty = self.resolve_vars_if_possible(root_ty);
-                if !matches!(ty.kind(), ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_)))
-                {
+            ObligationCauseCode::Pattern {
+                origin_expr: Some(origin_expr),
+                span: Some(span),
+                root_ty,
+            } => {
+                let expected_ty = self.resolve_vars_if_possible(root_ty);
+                if !matches!(
+                    expected_ty.kind(),
+                    ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_))
+                ) {
                     // don't show type `_`
                     if span.desugaring_kind() == Some(DesugaringKind::ForLoop)
-                        && let ty::Adt(def, args) = ty.kind()
+                        && let ty::Adt(def, args) = expected_ty.kind()
                         && Some(def.did()) == self.tcx.get_diagnostic_item(sym::Option)
                     {
                         err.span_label(
                             span,
                             format!("this is an iterator with items of type `{}`", args.type_at(0)),
                         );
                     } else {
-                        err.span_label(span, format!("this expression has type `{ty}`"));
+                        err.span_label(span, format!("this expression has type `{expected_ty}`"));
                     }
                 }
                 if let Some(ty::error::ExpectedFound { found, .. }) = exp_found
-                    && ty.boxed_ty() == Some(found)
-                    && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
+                    && let Ok(mut peeled_snippet) =
+                        self.tcx.sess.source_map().span_to_snippet(origin_expr.peeled_span)
                 {
-                    err.span_suggestion(
-                        span,
-                        "consider dereferencing the boxed value",
-                        format!("*{snippet}"),
-                        Applicability::MachineApplicable,
-                    );
+                    // Parentheses are needed for cases like as casts.
+                    // We use the peeled_span for deref suggestions.
+                    // It's also safe to use for box, since box only triggers if there
+                    // wasn't a reference to begin with.
+                    if origin_expr.peeled_prefix_suggestion_parentheses {
+                        peeled_snippet = format!("({peeled_snippet})");
+                    }
+
+                    // Try giving a box suggestion first, as it is a special case of the
+                    // deref suggestion.
+                    if expected_ty.boxed_ty() == Some(found) {
+                        err.span_suggestion_verbose(
+                            span,
+                            "consider dereferencing the boxed value",
+                            format!("*{peeled_snippet}"),
+                            Applicability::MachineApplicable,
+                        );
+                    } else if let Some(param_env) = param_env
+                        && let Some(prefix) = self.should_deref_suggestion_on_mismatch(
+                            param_env,
+                            found,
+                            expected_ty,
+                            origin_expr,
+                        )
+                    {
+                        err.span_suggestion_verbose(
+                            span,
+                            "consider dereferencing to access the inner value using the Deref trait",
+                            format!("{prefix}{peeled_snippet}"),
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
                 }
             }
-            ObligationCauseCode::Pattern { origin_expr: false, span: Some(span), .. } => {
+            ObligationCauseCode::Pattern { origin_expr: None, span: Some(span), .. } => {
                 err.span_label(span, "expected due to this");
             }
             ObligationCauseCode::BlockTailExpression(
@@ -524,6 +558,45 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         }
     }
 
+    /// Determines whether deref_to == <deref_from as Deref>::Target, and if so,
+    /// returns a prefix that should be added to deref_from as a suggestion.
+    fn should_deref_suggestion_on_mismatch(
+        &self,
+        param_env: ParamEnv<'tcx>,
+        deref_to: Ty<'tcx>,
+        deref_from: Ty<'tcx>,
+        origin_expr: PatternOriginExpr,
+    ) -> Option<String> {
+        // origin_expr contains stripped away versions of our expression.
+        // We'll want to use that to avoid suggesting things like *&x.
+        // However, the type that we have access to hasn't been stripped away,
+        // so we need to ignore the first n dereferences, where n is the number
+        // that's been stripped away in origin_expr.
+
+        // Find a way to autoderef from deraf_from to deref_to.
+        let Some((num_derefs, (after_deref_ty, _))) = (self.autoderef_steps)(deref_from)
+            .into_iter()
+            .enumerate()
+            .find(|(_, (ty, _))| self.infcx.can_eq(param_env, *ty, deref_to))
+        else {
+            return None;
+        };
+
+        if num_derefs <= origin_expr.peeled_count {
+            return None;
+        }
+
+        let deref_part = "*".repeat(num_derefs - origin_expr.peeled_count);
+
+        // If the user used a reference in the original expression, they probably
+        // want the suggestion to still give a reference.
+        if deref_from.is_ref() && !after_deref_ty.is_ref() {
+            Some(format!("&{deref_part}"))
+        } else {
+            Some(deref_part)
+        }
+    }
+
     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
     /// populate `other_value` with `other_ty`.
@@ -1312,8 +1385,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             Variable(ty::error::ExpectedFound<Ty<'a>>),
             Fixed(&'static str),
         }
-        let (expected_found, exp_found, is_simple_error, values) = match values {
-            None => (None, Mismatch::Fixed("type"), false, None),
+        let (expected_found, exp_found, is_simple_error, values, param_env) = match values {
+            None => (None, Mismatch::Fixed("type"), false, None, None),
             Some(ty::ParamEnvAnd { param_env, value: values }) => {
                 let mut values = self.resolve_vars_if_possible(values);
                 if self.next_trait_solver() {
@@ -1365,7 +1438,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     diag.downgrade_to_delayed_bug();
                     return;
                 };
-                (Some(vals), exp_found, is_simple_error, Some(values))
+                (Some(vals), exp_found, is_simple_error, Some(values), Some(param_env))
             }
         };
 
@@ -1693,7 +1766,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
         // It reads better to have the error origin as the final
         // thing.
-        self.note_error_origin(diag, cause, exp_found, terr);
+        self.note_error_origin(diag, cause, exp_found, terr, param_env);
 
         debug!(?diag);
     }

compiler/rustc_trait_selection/src/error_reporting/infer/suggest.rs

@@ -210,7 +210,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             (Some(ty), _) if self.same_type_modulo_infer(ty, exp_found.found) => match cause.code()
             {
                 ObligationCauseCode::Pattern { span: Some(then_span), origin_expr, .. } => {
-                    origin_expr.then_some(ConsiderAddingAwait::FutureSugg {
+                    origin_expr.is_some().then_some(ConsiderAddingAwait::FutureSugg {
                         span: then_span.shrink_to_hi(),
                     })
                 }

tests/ui/closures/2229_closure_analysis/issue-118144.stderr

@@ -5,6 +5,11 @@ LL |         V(x) = func_arg;
    |         ^^^^   -------- this expression has type `&mut V`
    |         |
    |         expected `&mut V`, found `V`
+   |
+help: consider dereferencing to access the inner value using the Deref trait
+   |
+LL |         V(x) = &*func_arg;
+   |                ~~~~~~~~~~
 
 error: aborting due to 1 previous error
 

tests/ui/issues/issue-57741-dereference-boxed-value/issue-57741.stderr

@@ -2,57 +2,61 @@ error[E0308]: mismatched types
   --> $DIR/issue-57741.rs:20:9
    |
 LL |     let y = match x {
-   |                   -
-   |                   |
-   |                   this expression has type `Box<T>`
-   |                   help: consider dereferencing the boxed value: `*x`
+   |                   - this expression has type `Box<T>`
 LL |         T::A(a) | T::B(a) => a,
    |         ^^^^^^^ expected `Box<T>`, found `T`
    |
    = note: expected struct `Box<T>`
                 found enum `T`
+help: consider dereferencing the boxed value
+   |
+LL |     let y = match *x {
+   |                   ~~
 
 error[E0308]: mismatched types
   --> $DIR/issue-57741.rs:20:19
    |
 LL |     let y = match x {
-   |                   -
-   |                   |
-   |                   this expression has type `Box<T>`
-   |                   help: consider dereferencing the boxed value: `*x`
+   |                   - this expression has type `Box<T>`
 LL |         T::A(a) | T::B(a) => a,
    |                   ^^^^^^^ expected `Box<T>`, found `T`
    |
    = note: expected struct `Box<T>`
                 found enum `T`
+help: consider dereferencing the boxed value
+   |
+LL |     let y = match *x {
+   |                   ~~
 
 error[E0308]: mismatched types
   --> $DIR/issue-57741.rs:27:9
    |
 LL |     let y = match x {
-   |                   -
-   |                   |
-   |                   this expression has type `Box<S>`
-   |                   help: consider dereferencing the boxed value: `*x`
+   |                   - this expression has type `Box<S>`
 LL |         S::A { a } | S::B { b: a } => a,
    |         ^^^^^^^^^^ expected `Box<S>`, found `S`
    |
    = note: expected struct `Box<S>`
                 found enum `S`
+help: consider dereferencing the boxed value
+   |
+LL |     let y = match *x {
+   |                   ~~
 
 error[E0308]: mismatched types
   --> $DIR/issue-57741.rs:27:22
    |
 LL |     let y = match x {
-   |                   -
-   |                   |
-   |                   this expression has type `Box<S>`
-   |                   help: consider dereferencing the boxed value: `*x`
+   |                   - this expression has type `Box<S>`
 LL |         S::A { a } | S::B { b: a } => a,
    |                      ^^^^^^^^^^^^^ expected `Box<S>`, found `S`
    |
    = note: expected struct `Box<S>`
                 found enum `S`
+help: consider dereferencing the boxed value
+   |
+LL |     let y = match *x {
+   |                   ~~
 
 error: aborting due to 4 previous errors