Provide more context when an expected type was a type parameter

compiler/rustc_hir_analysis/src/check/compare_eii.rs

@@ -511,7 +511,7 @@ fn report_eii_mismatch<'tcx>(
     infcx.err_ctxt().note_type_err(
         &mut diag,
         &cause,
-        trait_err_span.map(|sp| (sp, Cow::from("type in declaration"), false)),
+        trait_err_span.map(|sp| (sp.into(), Cow::from("type in declaration"), false)),
         Some(param_env.and(infer::ValuePairs::PolySigs(ExpectedFound {
             expected: ty::Binder::dummy(declaration_sig),
             found: ty::Binder::dummy(external_impl_sig),

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -18,7 +18,7 @@ use rustc_middle::ty::{
     TypeFolder, TypeSuperFoldable, TypeVisitable, TypeVisitableExt, TypeVisitor, TypingMode,
     Unnormalized, Upcast,
 };
-use rustc_span::{BytePos, DUMMY_SP, Span, bug, span_bug};
+use rustc_span::{BytePos, DUMMY_SP, Span, bug, kw, span_bug};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::regions::InferCtxtRegionExt;
@@ -624,12 +624,16 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
                 "method `{}` has an incompatible return type for trait",
                 trait_m.name()
             );
+            diag.span_context(tcx.def_span(impl_m.container_id(tcx)).shrink_to_lo());
             infcx.err_ctxt().note_type_err(
                 &mut diag,
                 &cause,
-                tcx.hir_get_if_local(impl_m.def_id)
-                    .and_then(|node| node.fn_decl())
-                    .map(|decl| (decl.output.span(), Cow::from("return type in trait"), false)),
+                tcx.hir_get_if_local(trait_m.def_id).and_then(|node| node.fn_decl()).map(|decl| {
+                    let mut span: MultiSpan = decl.output.span().into();
+                    span.push_span_context(tcx.def_span(trait_m.container_id(tcx)).shrink_to_lo());
+                    span.push_span_context(tcx.def_span(trait_m.def_id).shrink_to_lo());
+                    (span, Cow::from("return type in trait"), false)
+                }),
                 Some(param_env.and(infer::ValuePairs::Terms(ExpectedFound {
                     expected: trait_return_ty.into(),
                     found: impl_return_ty.into(),
@@ -1013,6 +1017,7 @@ fn report_trait_method_mismatch<'tcx>(
         "method `{}` has an incompatible type for trait",
         trait_m.name()
     );
+    diag.span_context(tcx.def_span(impl_m.container_id(tcx)).shrink_to_lo());
     match &terr {
         TypeError::ArgumentMutability(0) | TypeError::ArgumentSorts(_, 0)
             if trait_m.is_method() =>
@@ -1078,10 +1083,53 @@ fn report_trait_method_mismatch<'tcx>(
     }
 
     cause.span = impl_err_span;
+
+    let mut labels = vec![];
+    let mut context = vec![];
+    if let TypeError::ArgumentSorts(_, i) = &terr
+        && let Some(ty) =
+            tcx.fn_sig(trait_m.def_id).skip_binder().inputs_and_output().skip_binder().get(*i)
+        && let ty::Param(param) = ty.kind()
+        // The expected type corresponds to a type parameter, which migh thave a default type or
+        // have been explicitly set in the `impl` with a different type.
+        && let generics = tcx.generics_of(trait_m.def_id)
+        && let param = generics.type_param(*param, tcx)
+        && let GenericParamDefKind::Type { has_default: true, .. } = param.kind
+    {
+        context.push(tcx.def_span(trait_m.container_id(tcx)).shrink_to_lo());
+
+        let param_name = param.name;
+        let msg = if let Some(ty) = trait_sig.inputs_and_output.get(*i)
+            && infcx.can_eq(param_env, *ty, tcx.type_of(param.def_id).skip_binder())
+        {
+            format!("expected to match the default of type parameter `{param_name}`")
+        } else if let Some(ty) = trait_sig.inputs_and_output.get(*i)
+            && let ty::Param(p) = tcx.type_of(param.def_id).skip_binder().kind()
+            && p.name == kw::SelfUpper
+            && infcx.can_eq(param_env, *ty, impl_trait_ref.self_ty())
+        {
+            format!("expected to match the `Self` default of type parameter `{param_name}`")
+        } else {
+            format!("expected to match type parameter `{param_name}`")
+        };
+        labels.push((tcx.def_span(param.def_id), msg));
+    }
+
     infcx.err_ctxt().note_type_err(
         &mut diag,
         &cause,
-        trait_err_span.map(|sp| (sp, Cow::from("type in trait"), false)),
+        trait_err_span.map(|sp| {
+            let mut span: MultiSpan = sp.into();
+            span.push_span_context(tcx.def_span(trait_m.container_id(tcx)).shrink_to_lo());
+            span.push_span_context(tcx.def_span(trait_m.def_id).shrink_to_lo());
+            for (sp, label) in labels {
+                span.push_span_label(sp, label);
+            }
+            for sp in context {
+                span.push_span_context(sp);
+            }
+            (span, Cow::from("type in trait"), false)
+        }),
         Some(param_env.and(infer::ValuePairs::PolySigs(ExpectedFound {
             expected: ty::Binder::dummy(trait_sig),
             found: ty::Binder::dummy(impl_sig),
@@ -2284,6 +2332,8 @@ fn compare_const_clause_entailment<'tcx>(
             "implemented const `{}` has an incompatible type for trait",
             trait_ct.name()
         );
+        diag.span_context(tcx.def_span(impl_ct.container_id(tcx)).shrink_to_lo());
+        diag.span_context(tcx.def_span(impl_ct.def_id).shrink_to_lo());
 
         let trait_c_span = trait_ct.def_id.as_local().map(|trait_ct_def_id| {
             // Add a label to the Span containing just the type of the const
@@ -2294,7 +2344,12 @@ fn compare_const_clause_entailment<'tcx>(
         infcx.err_ctxt().note_type_err(
             &mut diag,
             &cause,
-            trait_c_span.map(|span| (span, Cow::from("type in trait"), false)),
+            trait_c_span.map(|span| {
+                let mut span: MultiSpan = span.into();
+                span.push_span_context(tcx.def_span(trait_ct.container_id(tcx)).shrink_to_lo());
+                span.push_span_context(tcx.def_span(trait_ct.def_id).shrink_to_lo());
+                (span, Cow::from("type in trait"), false)
+            }),
             Some(param_env.and(infer::ValuePairs::Terms(ExpectedFound {
                 expected: trait_ty.into(),
                 found: impl_ty.into(),

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

@@ -54,7 +54,9 @@ use rustc_abi::ExternAbi;
 use rustc_attr_ir::diagnostic::{CustomDiagnostic, Directive, FormatArgs};
 use rustc_attr_ir::find_attr;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
-use rustc_errors::{Applicability, Diag, DiagStyledString, IntoDiagArg, StringPart, pluralize};
+use rustc_errors::{
+    Applicability, Diag, DiagStyledString, IntoDiagArg, MultiSpan, StringPart, pluralize,
+};
 use rustc_hir as hir;
 use rustc_hir::def_id::{CRATE_DEF_ID, DefId};
 use rustc_hir::intravisit::Visitor;
@@ -1463,7 +1465,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         &self,
         diag: &mut Diag<'_>,
         cause: &ObligationCause<'tcx>,
-        secondary_span: Option<(Span, Cow<'static, str>, bool)>,
+        secondary_span: Option<(MultiSpan, Cow<'static, str>, bool)>,
         mut values: Option<ty::ParamEnvAnd<'tcx, ValuePairs<'tcx>>>,
         terr: TypeError<'tcx>,
         prefer_label: bool,
@@ -1639,9 +1641,15 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             }
         };
 
-        let mut label_or_note = |span: Span, msg: Cow<'static, str>| {
-            if (prefer_label && is_simple_error) || &[span] == diag.span.primary_spans() {
-                diag.span_label(span, msg);
+        let mut label_or_note = |span: MultiSpan, msg: Cow<'static, str>| {
+            if !span.has_span_labels()
+                && span.span_context().is_empty()
+                && ((prefer_label && is_simple_error)
+                    || span.primary_spans() == diag.span.primary_spans())
+            {
+                for span in span.primary_spans() {
+                    diag.span_label(*span, msg.clone());
+                }
             } else {
                 diag.span_note(span, msg);
             }
@@ -1657,9 +1665,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     terr.to_string(self.tcx)
                 };
                 label_or_note(secondary_span, terr);
-                label_or_note(span, secondary_msg);
+                label_or_note(span.into(), secondary_msg);
             } else {
-                label_or_note(span, terr.to_string(self.tcx));
+                label_or_note(span.into(), terr.to_string(self.tcx));
                 label_or_note(secondary_span, secondary_msg);
             }
         } else if let Some(values) = values
@@ -1671,12 +1679,15 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             let expected = with_forced_trimmed_paths!(e.sort_string(self.tcx));
             let found = with_forced_trimmed_paths!(f.sort_string(self.tcx));
             if expected == found {
-                label_or_note(span, terr.to_string(self.tcx));
+                label_or_note(span.into(), terr.to_string(self.tcx));
             } else {
-                label_or_note(span, Cow::from(format!("expected {expected}, found {found}")));
+                label_or_note(
+                    span.into(),
+                    Cow::from(format!("expected {expected}, found {found}")),
+                );
             }
         } else {
-            label_or_note(span, terr.to_string(self.tcx));
+            label_or_note(span.into(), terr.to_string(self.tcx));
         }
 
         if let Some(param_env) = param_env {

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

@@ -1875,7 +1875,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             ..
                         }),
                     ) => Some((
-                        ty.span,
+                        ty.span.into(),
                         with_forced_trimmed_paths!(Cow::from(format!(
                             "type mismatch resolving `{}`",
                             self.tcx.short_string(

tests/ui/associated-consts/associated-const-generic-obligations.stderr

@@ -1,12 +1,14 @@
 error[E0326]: implemented const `FROM` has an incompatible type for trait
   --> $DIR/associated-const-generic-obligations.rs:14:17
    |
+LL | impl<T: Foo> Bar for T {
 LL |     const FROM: &'static str = "foo";
    |                 ^^^^^^^^^^^^ expected associated type, found `&'static str`
    |
 note: type in trait
   --> $DIR/associated-const-generic-obligations.rs:10:17
    |
+LL | trait Bar: Foo {
 LL |     const FROM: Self::Out;
    |                 ^^^^^^^^^
    = note: expected associated type `<T as Foo>::Out`

tests/ui/associated-consts/associated-const-impl-wrong-type.stderr

@@ -1,12 +1,14 @@
 error[E0326]: implemented const `BAR` has an incompatible type for trait
   --> $DIR/associated-const-impl-wrong-type.rs:8:16
    |
+LL | impl Foo for SignedBar {
 LL |     const BAR: i32 = -1;
    |                ^^^ expected `u32`, found `i32`
    |
 note: type in trait
   --> $DIR/associated-const-impl-wrong-type.rs:2:16
    |
+LL | trait Foo {
 LL |     const BAR: u32;
    |                ^^^
 

tests/ui/associated-types/defaults-specialization.stderr

@@ -1,12 +1,15 @@
 error[E0053]: method `make` has an incompatible type for trait
   --> $DIR/defaults-specialization.rs:18:18
    |
+LL | default impl<T> Tr for A<T> {
 LL |     fn make() -> u8 { 0 }
    |                  ^^ expected associated type, found `u8`
    |
 note: type in trait
   --> $DIR/defaults-specialization.rs:8:18
    |
+LL | trait Tr {
+...
 LL |     fn make() -> Self::Ty {
    |                  ^^^^^^^^
    = note: expected signature `fn() -> <A<T> as Tr>::Ty`
@@ -20,6 +23,7 @@ LL +     fn make() -> <A<T> as Tr>::Ty { 0 }
 error[E0053]: method `make` has an incompatible type for trait
   --> $DIR/defaults-specialization.rs:34:18
    |
+LL | impl<T> Tr for B<T> {
 LL |     default type Ty = bool;
    |     --------------- associated type is `default` and may be overridden
 LL |
@@ -29,6 +33,8 @@ LL |     fn make() -> bool { true }
 note: type in trait
   --> $DIR/defaults-specialization.rs:8:18
    |
+LL | trait Tr {
+...
 LL |     fn make() -> Self::Ty {
    |                  ^^^^^^^^
    = note: expected signature `fn() -> <B<T> as Tr>::Ty`

tests/ui/async-await/in-trait/async-example-desugared-boxed-in-trait.stderr

@@ -1,12 +1,14 @@
 error[E0053]: method `foo` has an incompatible type for trait
   --> $DIR/async-example-desugared-boxed-in-trait.rs:11:28
    |
+LL | impl MyTrait for i32 {
 LL |     async fn foo(&self) -> i32 {
    |                            ^^^ expected `Pin<Box<dyn Future<Output = i32>>>`, found future
    |
 note: type in trait
   --> $DIR/async-example-desugared-boxed-in-trait.rs:7:22
    |
+LL | trait MyTrait {
 LL |     fn foo(&self) -> Pin<Box<dyn Future<Output = i32> + '_>>;
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: expected signature `fn(&_) -> Pin<Box<dyn Future<Output = i32>>>`

tests/ui/async-await/in-trait/dont-project-to-specializable-projection.stderr

@@ -1,12 +1,14 @@
 error[E0053]: method `foo` has an incompatible type for trait
   --> $DIR/dont-project-to-specializable-projection.rs:14:35
    |
+LL | impl<T> MyTrait<T> for MyStruct {
 LL |     default async fn foo(_: T) -> &'static str {
    |                                   ^^^^^^^^^^^^ expected associated type, found future
    |
 note: type in trait
   --> $DIR/dont-project-to-specializable-projection.rs:10:27
    |
+LL | trait MyTrait<T> {
 LL |     async fn foo(_: T) -> &'static str;
    |                           ^^^^^^^^^^^^
    = note: expected signature `fn(_) -> impl Future<Output = &'static str>`

tests/ui/compare-method/bad-self-type.stderr

@@ -1,6 +1,8 @@
 error[E0053]: method `poll` has an incompatible type for trait
   --> $DIR/bad-self-type.rs:10:13
    |
+LL | impl Future for MyFuture {
+LL |     type Output = ();
 LL |     fn poll(self, _: &mut Context<'_>) -> Poll<()> {
    |             ^^^^ expected `Pin<&mut MyFuture>`, found `MyFuture`
    |
@@ -14,12 +16,14 @@ LL |     fn poll(self: Pin<&mut MyFuture>, _: &mut Context<'_>) -> Poll<()> {
 error[E0053]: method `foo` has an incompatible type for trait
   --> $DIR/bad-self-type.rs:22:18
    |
+LL | impl T for MyFuture {
 LL |     fn foo(self: Box<Self>) {}
    |                  ^^^^^^^^^ expected `MyFuture`, found `Box<MyFuture>`
    |
 note: type in trait
   --> $DIR/bad-self-type.rs:17:12
    |
+LL | trait T {
 LL |     fn foo(self);
    |            ^^^^
    = note: expected signature `fn(MyFuture)`
@@ -33,12 +37,16 @@ LL +     fn foo(self) {}
 error[E0053]: method `bar` has an incompatible type for trait
   --> $DIR/bad-self-type.rs:24:17
    |
+LL | impl T for MyFuture {
+...
 LL |     fn bar(self) {}
    |                 ^ expected `Option<()>`, found `()`
    |
 note: type in trait
   --> $DIR/bad-self-type.rs:18:21
    |
+LL | trait T {
+LL |     fn foo(self);
 LL |     fn bar(self) -> Option<()>;
    |                     ^^^^^^^^^^
    = note: expected signature `fn(MyFuture) -> Option<()>`

tests/ui/compare-method/default-type-parameter.rs

@@ -0,0 +1,14 @@
+trait Trait<T = ()> {
+    fn foo() -> T;
+}
+
+struct S;
+
+impl Trait for S {
+    fn foo() -> u32 { 0 } //~ ERROR
+}
+impl Trait<i32> for () {
+    fn foo() -> u32 { 0 } //~ ERROR
+}
+
+fn main() {}

tests/ui/compare-method/default-type-parameter.stderr

@@ -0,0 +1,47 @@
+error[E0053]: method `foo` has an incompatible type for trait
+  --> $DIR/default-type-parameter.rs:8:17
+   |
+LL | impl Trait for S {
+LL |     fn foo() -> u32 { 0 }
+   |                 ^^^ expected `()`, found `u32`
+   |
+note: type in trait
+  --> $DIR/default-type-parameter.rs:2:17
+   |
+LL | trait Trait<T = ()> {
+   |             ------ expected to match the default of type parameter `T`
+LL |     fn foo() -> T;
+   |                 ^
+   = note: expected signature `fn() -> ()`
+              found signature `fn() -> u32`
+help: change the output type to match the trait
+   |
+LL -     fn foo() -> u32 { 0 }
+LL +     fn foo() -> () { 0 }
+   |
+
+error[E0053]: method `foo` has an incompatible type for trait
+  --> $DIR/default-type-parameter.rs:11:17
+   |
+LL | impl Trait<i32> for () {
+LL |     fn foo() -> u32 { 0 }
+   |                 ^^^ expected `i32`, found `u32`
+   |
+note: type in trait
+  --> $DIR/default-type-parameter.rs:2:17
+   |
+LL | trait Trait<T = ()> {
+   |             ------ expected to match type parameter `T`
+LL |     fn foo() -> T;
+   |                 ^
+   = note: expected signature `fn() -> i32`
+              found signature `fn() -> u32`
+help: change the output type to match the trait
+   |
+LL -     fn foo() -> u32 { 0 }
+LL +     fn foo() -> i32 { 0 }
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0053`.