Fix invalid slice coercion suggestion reported in turbofish

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

@@ -3032,8 +3032,9 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 self.report_similar_impl_candidates_for_root_obligation(&obligation, *trait_predicate, body_def_id, err);
             }
 
-            self.maybe_suggest_convert_to_slice(
+            self.suggest_convert_to_slice(
                 err,
+                obligation,
                 trait_ref,
                 impl_candidates.as_slice(),
                 span,

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

@@ -398,9 +398,10 @@ pub trait TypeErrCtxtExt<'tcx> {
         param_env: ty::ParamEnv<'tcx>,
     ) -> Vec<Option<(Span, (DefId, Ty<'tcx>))>>;
 
-    fn maybe_suggest_convert_to_slice(
+    fn suggest_convert_to_slice(
         &self,
         err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
         trait_ref: ty::PolyTraitRef<'tcx>,
         candidate_impls: &[ImplCandidate<'tcx>],
         span: Span,
@@ -3954,13 +3955,20 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     /// 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(
+    fn suggest_convert_to_slice(
         &self,
         err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
         trait_ref: ty::PolyTraitRef<'tcx>,
         candidate_impls: &[ImplCandidate<'tcx>],
         span: Span,
     ) {
+        // We can only suggest the slice coersion for function arguments since the suggestion
+        // would make no sense in turbofish or call
+        let ObligationCauseCode::FunctionArgumentObligation { .. } = obligation.cause.code() else {
+            return;
+        };
+
         // Three cases where we can make a suggestion:
         // 1. `[T; _]` (array of T)
         // 2. `&[T; _]` (reference to array of T)

tests/ui/dst/issue-90528-unsizing-not-suggestion-110063.rs

@@ -0,0 +1,13 @@
+trait Test {}
+impl Test for &[u8] {}
+
+fn needs_test<T: Test>() -> T {
+    panic!()
+}
+
+fn main() {
+    needs_test::<[u8; 1]>();
+    //~^ ERROR the trait bound
+    let x: [u8; 1] = needs_test();
+    //~^ ERROR the trait bound
+}

tests/ui/dst/issue-90528-unsizing-not-suggestion-110063.stderr

@@ -0,0 +1,29 @@
+error[E0277]: the trait bound `[u8; 1]: Test` is not satisfied
+  --> $DIR/issue-90528-unsizing-not-suggestion-110063.rs:9:18
+   |
+LL |     needs_test::<[u8; 1]>();
+   |                  ^^^^^^^ the trait `Test` is not implemented for `[u8; 1]`
+   |
+   = help: the trait `Test` is implemented for `&[u8]`
+note: required by a bound in `needs_test`
+  --> $DIR/issue-90528-unsizing-not-suggestion-110063.rs:4:18
+   |
+LL | fn needs_test<T: Test>() -> T {
+   |                  ^^^^ required by this bound in `needs_test`
+
+error[E0277]: the trait bound `[u8; 1]: Test` is not satisfied
+  --> $DIR/issue-90528-unsizing-not-suggestion-110063.rs:11:22
+   |
+LL |     let x: [u8; 1] = needs_test();
+   |                      ^^^^^^^^^^ the trait `Test` is not implemented for `[u8; 1]`
+   |
+   = help: the trait `Test` is implemented for `&[u8]`
+note: required by a bound in `needs_test`
+  --> $DIR/issue-90528-unsizing-not-suggestion-110063.rs:4:18
+   |
+LL | fn needs_test<T: Test>() -> T {
+   |                  ^^^^ required by this bound in `needs_test`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.