Add suggestion to diagnostic when user has array but trait wants slice.

For #90528.

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

@@ -1035,7 +1035,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             // Can't show anything else useful, try to find similar impls.
                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
                             if !self.report_similar_impl_candidates(
-                                impl_candidates,
+                                &impl_candidates,
                                 trait_ref,
                                 body_hir_id,
                                 &mut err,
@@ -1071,14 +1071,21 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                     let impl_candidates =
                                         self.find_similar_impl_candidates(trait_pred);
                                     self.report_similar_impl_candidates(
-                                        impl_candidates,
+                                        &impl_candidates,
                                         trait_ref,
                                         body_hir_id,
                                         &mut err,
                                         true,
                                     );
                                 }
                             }
+
+                            self.maybe_suggest_convert_to_slice(
+                                &mut err,
+                                trait_ref,
+                                impl_candidates.as_slice(),
+                                span,
+                            );
                         }
 
                         // Changing mutability doesn't make a difference to whether we have
@@ -1529,7 +1536,7 @@ trait InferCtxtPrivExt<'tcx> {
 
     fn report_similar_impl_candidates(
         &self,
-        impl_candidates: Vec<ImplCandidate<'tcx>>,
+        impl_candidates: &[ImplCandidate<'tcx>],
         trait_ref: ty::PolyTraitRef<'tcx>,
         body_id: hir::HirId,
         err: &mut Diagnostic,
@@ -2027,7 +2034,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 
     fn report_similar_impl_candidates(
         &self,
-        impl_candidates: Vec<ImplCandidate<'tcx>>,
+        impl_candidates: &[ImplCandidate<'tcx>],
         trait_ref: ty::PolyTraitRef<'tcx>,
         body_id: hir::HirId,
         err: &mut Diagnostic,
@@ -2138,7 +2145,8 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         // Prefer more similar candidates first, then sort lexicographically
         // by their normalized string representation.
         let mut normalized_impl_candidates_and_similarities = impl_candidates
-            .into_iter()
+            .iter()
+            .copied()
             .map(|ImplCandidate { trait_ref, similarity }| {
                 // FIXME(compiler-errors): This should be using `NormalizeExt::normalize`
                 let normalized = self
@@ -2351,7 +2359,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 let hir =
                                     self.tcx.hir().local_def_id_to_hir_id(obligation.cause.body_id);
                                 self.report_similar_impl_candidates(
-                                    impl_candidates,
+                                    impl_candidates.as_slice(),
                                     trait_ref,
                                     body_id.map(|id| id.hir_id).unwrap_or(hir),
                                     &mut err,

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1,7 +1,7 @@
 // ignore-tidy-filelength
 
 use super::{
-    DefIdOrName, FindExprBySpan, Obligation, ObligationCause, ObligationCauseCode,
+    DefIdOrName, FindExprBySpan, ImplCandidate, Obligation, ObligationCause, ObligationCauseCode,
     PredicateObligation,
 };
 
@@ -382,6 +382,14 @@ pub trait TypeErrCtxtExt<'tcx> {
         body_id: hir::HirId,
         param_env: ty::ParamEnv<'tcx>,
     ) -> Vec<Option<(Span, (DefId, Ty<'tcx>))>>;
+
+    fn maybe_suggest_convert_to_slice(
+        &self,
+        err: &mut Diagnostic,
+        trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
+        candidate_impls: &[ImplCandidate<'tcx>],
+        span: Span,
+    );
 }
 
 fn predicate_constraint(generics: &hir::Generics<'_>, pred: ty::Predicate<'_>) -> (Span, String) {
@@ -3826,6 +3834,73 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         }
         assocs_in_this_method
     }
+
+    /// If the type that failed selection is an array or a reference to an array,
+    /// but the trait is implemented for slices, suggest that the user converts
+    /// the array into a slice.
+    fn maybe_suggest_convert_to_slice(
+        &self,
+        err: &mut Diagnostic,
+        trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
+        candidate_impls: &[ImplCandidate<'tcx>],
+        span: Span,
+    ) {
+        // Three cases where we can make a suggestion:
+        // 1. `[T; _]` (array of T)
+        // 2. `&[T; _]` (reference to array of T)
+        // 3. `&mut [T; _]` (mutable reference to array of T)
+        let (element_ty, mut mutability) = match *trait_ref.skip_binder().self_ty().kind() {
+            ty::Array(element_ty, _) => (element_ty, None),
+
+            ty::Ref(_, pointee_ty, mutability) => match *pointee_ty.kind() {
+                ty::Array(element_ty, _) => (element_ty, Some(mutability)),
+                _ => return,
+            },
+
+            _ => return,
+        };
+
+        // Go through all the candidate impls to see if any of them is for
+        // slices of `element_ty` with `mutability`.
+        let mut is_slice = |candidate: Ty<'tcx>| match *candidate.kind() {
+            ty::RawPtr(ty::TypeAndMut { ty: t, mutbl: m }) | ty::Ref(_, t, m) => {
+                if matches!(*t.kind(), ty::Slice(e) if e == element_ty)
+                    && m == mutability.unwrap_or(m)
+                {
+                    // Use the candidate's mutability going forward.
+                    mutability = Some(m);
+                    true
+                } else {
+                    false
+                }
+            }
+            _ => false,
+        };
+
+        // Grab the first candidate that matches, if any, and make a suggestion.
+        if let Some(slice_ty) = candidate_impls
+            .iter()
+            .map(|trait_ref| trait_ref.trait_ref.self_ty())
+            .filter(|t| is_slice(*t))
+            .next()
+        {
+            let msg = &format!("convert the array to a `{}` slice instead", slice_ty);
+
+            if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+                let mut suggestions = vec![];
+                if snippet.starts_with('&') {
+                } else if let Some(hir::Mutability::Mut) = mutability {
+                    suggestions.push((span.shrink_to_lo(), "&mut ".into()));
+                } else {
+                    suggestions.push((span.shrink_to_lo(), "&".into()));
+                }
+                suggestions.push((span.shrink_to_hi(), "[..]".into()));
+                err.multipart_suggestion_verbose(msg, suggestions, Applicability::MaybeIncorrect);
+            } else {
+                err.span_help(span, msg);
+            }
+        }
+    }
 }
 
 /// Add a hint to add a missing borrow or remove an unnecessary one.

tests/ui/dst/issue-90528-unsizing-suggestion-1.rs

@@ -0,0 +1,20 @@
+// Issue #90528: provide helpful suggestions when a trait bound is unsatisfied
+// due to a missed unsizing coercion.
+//
+// This test exercises array literals and a trait implemented on immutable slices.
+
+trait Read {}
+
+impl Read for &[u8] {}
+
+fn wants_read(_: impl Read) {}
+
+fn main() {
+    wants_read([0u8]);
+    //~^ ERROR the trait bound `[u8; 1]: Read` is not satisfied
+    wants_read(&[0u8]);
+    //~^ ERROR the trait bound `&[u8; 1]: Read` is not satisfied
+    wants_read(&[0u8][..]);
+    wants_read(&mut [0u8]);
+    //~^ ERROR the trait bound `&mut [u8; 1]: Read` is not satisfied
+}

tests/ui/dst/issue-90528-unsizing-suggestion-1.stderr

@@ -0,0 +1,56 @@
+error[E0277]: the trait bound `[u8; 1]: Read` is not satisfied
+  --> $DIR/issue-90528-unsizing-suggestion-1.rs:13:16
+   |
+LL |     wants_read([0u8]);
+   |     ---------- ^^^^^ the trait `Read` is not implemented for `[u8; 1]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Read` is implemented for `&[u8]`
+note: required by a bound in `wants_read`
+  --> $DIR/issue-90528-unsizing-suggestion-1.rs:10:23
+   |
+LL | fn wants_read(_: impl Read) {}
+   |                       ^^^^ required by this bound in `wants_read`
+help: convert the array to a `&[u8]` slice instead
+   |
+LL |     wants_read(&[0u8][..]);
+   |                +     ++++
+
+error[E0277]: the trait bound `&[u8; 1]: Read` is not satisfied
+  --> $DIR/issue-90528-unsizing-suggestion-1.rs:15:16
+   |
+LL |     wants_read(&[0u8]);
+   |     ---------- ^^^^^^ the trait `Read` is not implemented for `&[u8; 1]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Read` is implemented for `&[u8]`
+note: required by a bound in `wants_read`
+  --> $DIR/issue-90528-unsizing-suggestion-1.rs:10:23
+   |
+LL | fn wants_read(_: impl Read) {}
+   |                       ^^^^ required by this bound in `wants_read`
+help: convert the array to a `&[u8]` slice instead
+   |
+LL |     wants_read(&[0u8][..]);
+   |                      ++++
+
+error[E0277]: the trait bound `&mut [u8; 1]: Read` is not satisfied
+  --> $DIR/issue-90528-unsizing-suggestion-1.rs:18:16
+   |
+LL |     wants_read(&mut [0u8]);
+   |     ---------- ^^^^^^^^^^ the trait `Read` is not implemented for `&mut [u8; 1]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Read` is implemented for `&[u8]`
+note: required by a bound in `wants_read`
+  --> $DIR/issue-90528-unsizing-suggestion-1.rs:10:23
+   |
+LL | fn wants_read(_: impl Read) {}
+   |                       ^^^^ required by this bound in `wants_read`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/dst/issue-90528-unsizing-suggestion-2.rs

@@ -0,0 +1,28 @@
+// Issue #90528: provide helpful suggestions when a trait bound is unsatisfied
+// due to a missed unsizing coercion.
+//
+// This test exercises array variables and a trait implemented on immmutable slices.
+
+trait Read {}
+
+impl Read for &[u8] {}
+
+fn wants_read(_: impl Read) {}
+
+fn main() {
+    let x = [0u8];
+    wants_read(x);
+    //~^ ERROR the trait bound `[u8; 1]: Read` is not satisfied
+    wants_read(&x);
+    //~^ ERROR the trait bound `&[u8; 1]: Read` is not satisfied
+    wants_read(&x[..]);
+
+    let x = &[0u8];
+    wants_read(x);
+    //~^ ERROR the trait bound `&[u8; 1]: Read` is not satisfied
+    wants_read(&x);
+    //~^ ERROR the trait bound `&&[u8; 1]: Read` is not satisfied
+    wants_read(*x);
+    //~^ ERROR the trait bound `[u8; 1]: Read` is not satisfied
+    wants_read(&x[..]);
+}

tests/ui/dst/issue-90528-unsizing-suggestion-2.stderr

@@ -0,0 +1,94 @@
+error[E0277]: the trait bound `[u8; 1]: Read` is not satisfied
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:14:16
+   |
+LL |     wants_read(x);
+   |     ---------- ^ the trait `Read` is not implemented for `[u8; 1]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Read` is implemented for `&[u8]`
+note: required by a bound in `wants_read`
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:10:23
+   |
+LL | fn wants_read(_: impl Read) {}
+   |                       ^^^^ required by this bound in `wants_read`
+help: convert the array to a `&[u8]` slice instead
+   |
+LL |     wants_read(&x[..]);
+   |                + ++++
+
+error[E0277]: the trait bound `&[u8; 1]: Read` is not satisfied
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:16:16
+   |
+LL |     wants_read(&x);
+   |     ---------- ^^ the trait `Read` is not implemented for `&[u8; 1]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Read` is implemented for `&[u8]`
+note: required by a bound in `wants_read`
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:10:23
+   |
+LL | fn wants_read(_: impl Read) {}
+   |                       ^^^^ required by this bound in `wants_read`
+help: convert the array to a `&[u8]` slice instead
+   |
+LL |     wants_read(&x[..]);
+   |                  ++++
+
+error[E0277]: the trait bound `&[u8; 1]: Read` is not satisfied
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:21:16
+   |
+LL |     wants_read(x);
+   |     ---------- ^ the trait `Read` is not implemented for `&[u8; 1]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Read` is implemented for `&[u8]`
+note: required by a bound in `wants_read`
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:10:23
+   |
+LL | fn wants_read(_: impl Read) {}
+   |                       ^^^^ required by this bound in `wants_read`
+help: convert the array to a `&[u8]` slice instead
+   |
+LL |     wants_read(&x[..]);
+   |                + ++++
+
+error[E0277]: the trait bound `&&[u8; 1]: Read` is not satisfied
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:23:16
+   |
+LL |     wants_read(&x);
+   |     ---------- ^^ the trait `Read` is not implemented for `&&[u8; 1]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Read` is implemented for `&[u8]`
+note: required by a bound in `wants_read`
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:10:23
+   |
+LL | fn wants_read(_: impl Read) {}
+   |                       ^^^^ required by this bound in `wants_read`
+
+error[E0277]: the trait bound `[u8; 1]: Read` is not satisfied
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:25:16
+   |
+LL |     wants_read(*x);
+   |     ---------- ^^ the trait `Read` is not implemented for `[u8; 1]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Read` is implemented for `&[u8]`
+note: required by a bound in `wants_read`
+  --> $DIR/issue-90528-unsizing-suggestion-2.rs:10:23
+   |
+LL | fn wants_read(_: impl Read) {}
+   |                       ^^^^ required by this bound in `wants_read`
+help: convert the array to a `&[u8]` slice instead
+   |
+LL |     wants_read(&*x[..]);
+   |                +  ++++
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/dst/issue-90528-unsizing-suggestion-3.rs

@@ -0,0 +1,22 @@
+// Issue #90528: provide helpful suggestions when a trait bound is unsatisfied
+// due to a missed unsizing coercion.
+//
+// This test exercises array literals and a trait implemented on mutable slices.
+
+trait Write {}
+
+impl Write for &mut [u8] {}
+
+fn wants_write(_: impl Write) {}
+
+fn main() {
+    wants_write([0u8]);
+    //~^ ERROR the trait bound `[u8; 1]: Write` is not satisfied
+    wants_write(&mut [0u8]);
+    //~^ ERROR the trait bound `&mut [u8; 1]: Write` is not satisfied
+    wants_write(&mut [0u8][..]);
+    wants_write(&[0u8]);
+    //~^ ERROR the trait bound `&[u8; 1]: Write` is not satisfied
+    wants_write(&[0u8][..]);
+    //~^ ERROR the trait bound `&[u8]: Write` is not satisfied
+}