Improve some inference diagnostics

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -313,11 +313,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn emit_inference_failure_err(
         &self,
         body_id: Option<hir::BodyId>,
-        span: Span,
+        failure_span: Span,
         arg: GenericArg<'tcx>,
         // FIXME(#94483): Either use this or remove it.
         _impl_candidates: Vec<ty::TraitRef<'tcx>>,
         error_code: TypeAnnotationNeeded,
+        should_label_span: bool,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let arg = self.resolve_vars_if_possible(arg);
         let arg_data = self.extract_inference_diagnostics_data(arg, None);
@@ -326,7 +327,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             // If we don't have any typeck results we're outside
             // of a body, so we won't be able to get better info
             // here.
-            return self.bad_inference_failure_err(span, arg_data, error_code);
+            return self.bad_inference_failure_err(failure_span, arg_data, error_code);
         };
         let typeck_results = typeck_results.borrow();
         let typeck_results = &typeck_results;
@@ -338,7 +339,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
 
         let Some(InferSource { span, kind }) = local_visitor.infer_source else {
-            return self.bad_inference_failure_err(span, arg_data, error_code)
+            return self.bad_inference_failure_err(failure_span, arg_data, error_code)
         };
 
         let error_code = error_code.into();
@@ -347,6 +348,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             &format!("type annotations needed{}", kind.ty_msg(self)),
             error_code,
         );
+
+        if should_label_span && !failure_span.overlaps(span) {
+            err.span_label(failure_span, "type must be known at this point");
+        }
+
         match kind {
             InferSourceKind::LetBinding { insert_span, pattern_name, ty } => {
                 let suggestion_msg = if let Some(name) = pattern_name {

compiler/rustc_middle/src/ty/mod.rs

@@ -914,9 +914,17 @@ impl<'tcx> Term<'tcx> {
     pub fn ty(&self) -> Option<Ty<'tcx>> {
         if let Term::Ty(ty) = self { Some(*ty) } else { None }
     }
+
     pub fn ct(&self) -> Option<Const<'tcx>> {
         if let Term::Const(c) = self { Some(*c) } else { None }
     }
+
+    pub fn into_arg(self) -> GenericArg<'tcx> {
+        match self {
+            Term::Ty(ty) => ty.into(),
+            Term::Const(c) => c.into(),
+        }
+    }
 }
 
 /// This kind of predicate has no *direct* correspondent in the

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

@@ -1958,26 +1958,6 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 if predicate.references_error() {
                     return;
                 }
-                // Typically, this ambiguity should only happen if
-                // there are unresolved type inference variables
-                // (otherwise it would suggest a coherence
-                // failure). But given #21974 that is not necessarily
-                // the case -- we can have multiple where clauses that
-                // are only distinguished by a region, which results
-                // in an ambiguity even when all types are fully
-                // known, since we don't dispatch based on region
-                // relationships.
-
-                // Pick the first substitution that still contains inference variables as the one
-                // we're going to emit an error for. If there are none (see above), fall back to
-                // the substitution for `Self`.
-                let subst = {
-                    let substs = data.trait_ref.substs;
-                    substs
-                        .iter()
-                        .find(|s| s.has_infer_types_or_consts())
-                        .unwrap_or_else(|| substs[0])
-                };
 
                 // This is kind of a hack: it frequently happens that some earlier
                 // error prevents types from being fully inferred, and then we get
@@ -1999,27 +1979,54 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                         self.emit_inference_failure_err(
                             body_id,
                             span,
-                            subst,
+                            trait_ref.self_ty().skip_binder().into(),
                             vec![],
                             ErrorCode::E0282,
+                            false,
                         )
                         .emit();
                     }
                     return;
                 }
 
-                let impl_candidates = self
-                    .find_similar_impl_candidates(trait_ref)
-                    .into_iter()
-                    .map(|candidate| candidate.trait_ref)
-                    .collect();
-                let mut err = self.emit_inference_failure_err(
-                    body_id,
-                    span,
-                    subst,
-                    impl_candidates,
-                    ErrorCode::E0283,
-                );
+                // Typically, this ambiguity should only happen if
+                // there are unresolved type inference variables
+                // (otherwise it would suggest a coherence
+                // failure). But given #21974 that is not necessarily
+                // the case -- we can have multiple where clauses that
+                // are only distinguished by a region, which results
+                // in an ambiguity even when all types are fully
+                // known, since we don't dispatch based on region
+                // relationships.
+
+                // Pick the first substitution that still contains inference variables as the one
+                // we're going to emit an error for. If there are none (see above), fall back to
+                // a more general error.
+                let subst = data.trait_ref.substs.iter().find(|s| s.has_infer_types_or_consts());
+
+                let mut err = if let Some(subst) = subst {
+                    let impl_candidates = self
+                        .find_similar_impl_candidates(trait_ref)
+                        .into_iter()
+                        .map(|candidate| candidate.trait_ref)
+                        .collect();
+                    self.emit_inference_failure_err(
+                        body_id,
+                        span,
+                        subst,
+                        impl_candidates,
+                        ErrorCode::E0283,
+                        true,
+                    )
+                } else {
+                    struct_span_err!(
+                        self.tcx.sess,
+                        span,
+                        E0283,
+                        "type annotations needed: cannot satisfy `{}`",
+                        predicate,
+                    )
+                };
 
                 let obligation = Obligation::new(
                     obligation.cause.clone(),
@@ -2110,7 +2117,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                     return;
                 }
 
-                self.emit_inference_failure_err(body_id, span, arg, vec![], ErrorCode::E0282)
+                self.emit_inference_failure_err(body_id, span, arg, vec![], ErrorCode::E0282, false)
             }
 
             ty::PredicateKind::Subtype(data) => {
@@ -2124,26 +2131,38 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 let SubtypePredicate { a_is_expected: _, a, b } = data;
                 // both must be type variables, or the other would've been instantiated
                 assert!(a.is_ty_var() && b.is_ty_var());
-                self.emit_inference_failure_err(body_id, span, a.into(), vec![], ErrorCode::E0282)
+                self.emit_inference_failure_err(
+                    body_id,
+                    span,
+                    a.into(),
+                    vec![],
+                    ErrorCode::E0282,
+                    true,
+                )
             }
             ty::PredicateKind::Projection(data) => {
-                let self_ty = data.projection_ty.self_ty();
-                let term = data.term;
                 if predicate.references_error() || self.is_tainted_by_errors() {
                     return;
                 }
-                if self_ty.needs_infer() && term.needs_infer() {
-                    // We do this for the `foo.collect()?` case to produce a suggestion.
+                let subst = data
+                    .projection_ty
+                    .substs
+                    .iter()
+                    .chain(Some(data.term.into_arg()))
+                    .find(|g| g.has_infer_types_or_consts());
+                if let Some(subst) = subst {
                     let mut err = self.emit_inference_failure_err(
                         body_id,
                         span,
-                        self_ty.into(),
+                        subst,
                         vec![],
                         ErrorCode::E0284,
+                        true,
                     );
                     err.note(&format!("cannot satisfy `{}`", predicate));
                     err
                 } else {
+                    // If we can't find a substitution, just print a generic error
                     let mut err = struct_span_err!(
                         self.tcx.sess,
                         span,

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -1538,9 +1538,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ty
         } else {
             if !self.is_tainted_by_errors() {
-                self.emit_inference_failure_err((**self).body_id, sp, ty.into(), vec![], E0282)
-                    .note("type must be known at this point")
-                    .emit();
+                self.emit_inference_failure_err(
+                    (**self).body_id,
+                    sp,
+                    ty.into(),
+                    vec![],
+                    E0282,
+                    true,
+                )
+                .emit();
             }
             let err = self.tcx.ty_error();
             self.demand_suptype(sp, err, ty);

compiler/rustc_typeck/src/check/writeback.rs

@@ -694,6 +694,7 @@ impl<'cx, 'tcx> Resolver<'cx, 'tcx> {
                     t.into(),
                     vec![],
                     E0282,
+                    false,
                 )
                 .emit();
         }
@@ -708,6 +709,7 @@ impl<'cx, 'tcx> Resolver<'cx, 'tcx> {
                     c.into(),
                     vec![],
                     E0282,
+                    false,
                 )
                 .emit();
         }

src/test/ui/array-slice-vec/infer_array_len.stderr

@@ -4,7 +4,6 @@ error[E0282]: type annotations needed
 LL |     let [_, _] = a.into();
    |         ^^^^^^
    |
-   = note: type must be known at this point
 help: consider giving this pattern a type
    |
 LL |     let [_, _]: _ = a.into();

src/test/ui/array-slice-vec/slice-pat-type-mismatches.stderr

@@ -27,8 +27,6 @@ error[E0282]: type annotations needed
    |
 LL |         [] => {}
    |         ^^ cannot infer type
-   |
-   = note: type must be known at this point
 
 error: aborting due to 5 previous errors
 

src/test/ui/cast/issue-85586.stderr

@@ -3,8 +3,6 @@ error[E0282]: type annotations needed
    |
 LL |     let b = (a + 1) as usize;
    |             ^^^^^^^ cannot infer type
-   |
-   = note: type must be known at this point
 
 error: aborting due to previous error
 

src/test/ui/coherence/coherence-overlap-trait-alias.stderr

@@ -1,8 +1,8 @@
-error[E0283]: type annotations needed
+error[E0283]: type annotations needed: cannot satisfy `u32: C`
   --> $DIR/coherence-overlap-trait-alias.rs:15:6
    |
 LL | impl C for u32 {}
-   |      ^ cannot infer type for type `u32`
+   |      ^
    |
 note: multiple `impl`s satisfying `u32: C` found
   --> $DIR/coherence-overlap-trait-alias.rs:14:1

src/test/ui/const-generics/generic_arg_infer/issue-91614.stderr

@@ -2,7 +2,7 @@ error[E0283]: type annotations needed for `Mask<_, LANES>`
   --> $DIR/issue-91614.rs:6:9
    |
 LL |     let y = Mask::<_, _>::splat(false);
-   |         ^
+   |         ^   ------------------- type must be known at this point
    |
    = note: cannot satisfy `_: MaskElement`
 note: required by a bound in `Mask::<T, LANES>::splat`

src/test/ui/const-generics/generic_const_exprs/issue-72787.min.stderr

@@ -34,19 +34,19 @@ LL |     IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
    = help: const parameters may only be used as standalone arguments, i.e. `J`
    = help: use `#![feature(generic_const_exprs)]` to allow generic const expressions
 
-error[E0283]: type annotations needed
+error[E0283]: type annotations needed: cannot satisfy `IsLessOrEqual<I, 8_u32>: True`
   --> $DIR/issue-72787.rs:21:26
    |
 LL |     IsLessOrEqual<I, 8>: True,
-   |                          ^^^^ cannot infer type for struct `IsLessOrEqual<I, 8_u32>`
+   |                          ^^^^
    |
    = note: cannot satisfy `IsLessOrEqual<I, 8_u32>: True`
 
-error[E0283]: type annotations needed
+error[E0283]: type annotations needed: cannot satisfy `IsLessOrEqual<I, 8_u32>: True`
   --> $DIR/issue-72787.rs:21:26
    |
 LL |     IsLessOrEqual<I, 8>: True,
-   |                          ^^^^ cannot infer type for struct `IsLessOrEqual<I, 8_u32>`
+   |                          ^^^^
    |
    = note: cannot satisfy `IsLessOrEqual<I, 8_u32>: True`
 

src/test/ui/const-generics/generic_const_exprs/issue-72787.rs

@@ -19,8 +19,8 @@ struct S<const I: u32, const J: u32>;
 impl<const I: u32, const J: u32> S<I, J>
 where
     IsLessOrEqual<I, 8>: True,
-//[min]~^ Error type annotations needed [E0283]
-//[min]~| Error type annotations needed [E0283]
+//[min]~^ Error type annotations needed
+//[min]~| Error type annotations needed
     IsLessOrEqual<J, 8>: True,
     IsLessOrEqual<{ 8 - I }, { 8 - J }>: True,
 //[min]~^ Error generic parameters may not be used in const operations

src/test/ui/impl-trait/hidden-type-is-opaque-2.stderr

@@ -3,16 +3,12 @@ error[E0282]: type annotations needed
    |
 LL |         cont.reify_as();
    |         ^^^^ cannot infer type
-   |
-   = note: type must be known at this point
 
 error[E0282]: type annotations needed
   --> $DIR/hidden-type-is-opaque-2.rs:18:9
    |
 LL |         cont.reify_as();
    |         ^^^^ cannot infer type
-   |
-   = note: type must be known at this point
 
 error: aborting due to 2 previous errors
 

src/test/ui/inference/cannot-infer-partial-try-return.stderr

@@ -3,6 +3,9 @@ error[E0282]: type annotations needed for `Result<(), QualifiedError<_>>`
    |
 LL |     let x = || -> Result<_, QualifiedError<_>> {
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |
+LL |         infallible()?;
+   |         ------------- type must be known at this point
    |
 help: try giving this closure an explicit return type
    |

src/test/ui/inference/erase-type-params-in-label.stderr

@@ -2,7 +2,7 @@ error[E0283]: type annotations needed for `Foo<i32, &str, W, Z>`
   --> $DIR/erase-type-params-in-label.rs:2:9
    |
 LL |     let foo = foo(1, "");
-   |         ^^^
+   |         ^^^   --- type must be known at this point
    |
    = note: cannot satisfy `_: Default`
 note: required by a bound in `foo`
@@ -23,7 +23,7 @@ error[E0283]: type annotations needed for `Bar<i32, &str, Z>`
   --> $DIR/erase-type-params-in-label.rs:5:9
    |
 LL |     let bar = bar(1, "");
-   |         ^^^
+   |         ^^^   --- type must be known at this point
    |
    = note: cannot satisfy `_: Default`
 note: required by a bound in `bar`

src/test/ui/inference/issue-72616.stderr

@@ -2,7 +2,9 @@ error[E0283]: type annotations needed
   --> $DIR/issue-72616.rs:20:37
    |
 LL |         if String::from("a") == "a".try_into().unwrap() {}
-   |                                     ^^^^^^^^
+   |                              --     ^^^^^^^^
+   |                              |
+   |                              type must be known at this point
    |
    = note: multiple `impl`s satisfying `String: PartialEq<_>` found in the `alloc` crate:
            - impl PartialEq for String;

src/test/ui/inference/issue-72690.stderr

@@ -55,7 +55,7 @@ error[E0283]: type annotations needed for `&T`
   --> $DIR/issue-72690.rs:17:9
    |
 LL |     let _ = "x".as_ref();
-   |         ^
+   |         ^       ------ type must be known at this point
    |
    = note: multiple `impl`s satisfying `str: AsRef<_>` found in the following crates: `core`, `std`:
            - impl AsRef<OsStr> for str;