Add more context for type parameters

src/librustc/infer/error_reporting/need_type_info.rs

@@ -9,6 +9,7 @@ use syntax::source_map::DesugaringKind;
 use syntax::symbol::kw;
 use syntax_pos::Span;
 use errors::{Applicability, DiagnosticBuilder};
+use std::borrow::Cow;
 
 use rustc_error_codes::*;
 
@@ -113,6 +114,7 @@ fn closure_return_type_suggestion(
     err: &mut DiagnosticBuilder<'_>,
     output: &FunctionRetTy,
     body: &Body,
+    descr: &str,
     name: &str,
     ret: &str,
 ) {
@@ -136,7 +138,7 @@ fn closure_return_type_suggestion(
         suggestion,
         Applicability::HasPlaceholders,
     );
-    err.span_label(span, InferCtxt::missing_type_msg(&name));
+    err.span_label(span, InferCtxt::missing_type_msg(&name, &descr));
 }
 
 /// Given a closure signature, return a `String` containing a list of all its argument types.
@@ -175,13 +177,17 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         &self,
         ty: Ty<'tcx>,
         highlight: Option<ty::print::RegionHighlightMode>,
-    ) -> (String, Option<Span>) {
+    ) -> (String, Option<Span>, Cow<'static, str>) {
         if let ty::Infer(ty::TyVar(ty_vid)) = ty.kind {
             let ty_vars = self.type_variables.borrow();
             let var_origin = ty_vars.var_origin(ty_vid);
             if let TypeVariableOriginKind::TypeParameterDefinition(name) = var_origin.kind {
                 if name != kw::SelfUpper {
-                    return (name.to_string(), Some(var_origin.span));
+                    return (
+                        name.to_string(),
+                        Some(var_origin.span),
+                        "type parameter".into(),
+                    );
                 }
             }
         }
@@ -192,7 +198,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             printer.region_highlight_mode = highlight;
         }
         let _ = ty.print(printer);
-        (s, None)
+        (s, None, ty.prefix_string())
     }
 
     pub fn need_type_info_err(
@@ -203,7 +209,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         error_code: TypeAnnotationNeeded,
     ) -> DiagnosticBuilder<'tcx> {
         let ty = self.resolve_vars_if_possible(&ty);
-        let (name, name_sp) = self.extract_type_name(&ty, None);
+        let (name, name_sp, descr) = self.extract_type_name(&ty, None);
 
         let mut local_visitor = FindLocalByTypeVisitor::new(&self, ty, &self.tcx.hir());
         let ty_to_string = |ty: Ty<'tcx>| -> String {
@@ -308,6 +314,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             &mut err,
                             &decl.output,
                             &body,
+                            &descr,
                             &name,
                             &ret,
                         );
@@ -427,7 +434,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 span_label.label.is_some() && span_label.span == span
             }) && local_visitor.found_arg_pattern.is_none()
         { // Avoid multiple labels pointing at `span`.
-            err.span_label(span, InferCtxt::missing_type_msg(&name));
+            err.span_label(span, InferCtxt::missing_type_msg(&name, &descr));
         }
 
         err
@@ -468,10 +475,15 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     );
                 } else {
                     let sig = self.tcx.fn_sig(did);
-                    err.span_label(e.span, &format!(
-                        "this method call resolves to `{:?}`",
-                        sig.output().skip_binder(),
-                    ));
+                    let bound_output = sig.output();
+                    let output = bound_output.skip_binder();
+                    err.span_label(e.span, &format!("this method call resolves to `{:?}`", output));
+                    let kind = &output.kind;
+                    if let ty::Projection(proj) | ty::UnnormalizedProjection(proj) = kind {
+                        if let Some(span) = self.tcx.hir().span_if_local(proj.item_def_id) {
+                            err.span_label(span, &format!("`{:?}` defined here", output));
+                        }
+                    }
                 }
             }
         }
@@ -484,19 +496,19 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         ty: Ty<'tcx>,
     ) -> DiagnosticBuilder<'tcx> {
         let ty = self.resolve_vars_if_possible(&ty);
-        let name = self.extract_type_name(&ty, None).0;
+        let (name, _, descr) = self.extract_type_name(&ty, None);
         let mut err = struct_span_err!(
             self.tcx.sess, span, E0698, "type inside {} must be known in this context", kind,
         );
-        err.span_label(span, InferCtxt::missing_type_msg(&name));
+        err.span_label(span, InferCtxt::missing_type_msg(&name, &descr));
         err
     }
 
-    fn missing_type_msg(type_name: &str) -> String {
+    fn missing_type_msg(type_name: &str, descr: &str) -> Cow<'static, str>{
         if type_name == "_" {
-            "cannot infer type".to_owned()
+            "cannot infer type".into()
         } else {
-            format!("cannot infer type for `{}`", type_name)
+            format!("cannot infer type for {} `{}`", descr, type_name).into()
         }
     }
 }

src/test/ui/associated-types/associated-types-overridden-binding.stderr

@@ -4,7 +4,7 @@ error[E0284]: type annotations needed
 LL | trait Foo: Iterator<Item = i32> {}
    | ------------------------------- required by `Foo`
 LL | trait Bar: Foo<Item = u32> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `Self`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `Self`
    |
    = note: cannot resolve `<Self as std::iter::Iterator>::Item == i32`
 
@@ -14,7 +14,7 @@ error[E0284]: type annotations needed
 LL | trait I32Iterator = Iterator<Item = i32>;
    | ----------------------------------------- required by `I32Iterator`
 LL | trait U32Iterator = I32Iterator<Item = u32>;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `Self`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `Self`
    |
    = note: cannot resolve `<Self as std::iter::Iterator>::Item == i32`
 

src/test/ui/async-await/unresolved_type_param.rs

@@ -8,7 +8,7 @@ async fn bar<T>() -> () {}
 async fn foo() {
     bar().await;
     //~^ ERROR type inside `async fn` body must be known in this context
-    //~| NOTE cannot infer type for `T`
+    //~| NOTE cannot infer type for type parameter `T`
     //~| NOTE the type is part of the `async fn` body because of this `await`
     //~| NOTE in this expansion of desugaring of `await`
 }

src/test/ui/async-await/unresolved_type_param.stderr

@@ -2,7 +2,7 @@ error[E0698]: type inside `async fn` body must be known in this context
   --> $DIR/unresolved_type_param.rs:9:5
    |
 LL |     bar().await;
-   |     ^^^ cannot infer type for `T`
+   |     ^^^ cannot infer type for type parameter `T`
    |
 note: the type is part of the `async fn` body because of this `await`
   --> $DIR/unresolved_type_param.rs:9:5

src/test/ui/const-generics/cannot-infer-const-args.stderr

@@ -10,7 +10,7 @@ error[E0282]: type annotations needed
   --> $DIR/cannot-infer-const-args.rs:9:5
    |
 LL |     foo();
-   |     ^^^ cannot infer type for `fn() -> usize {foo::<_: usize>}`
+   |     ^^^ cannot infer type for fn item `fn() -> usize {foo::<_: usize>}`
 
 error: aborting due to previous error
 

src/test/ui/const-generics/fn-const-param-infer.stderr

@@ -30,7 +30,7 @@ error[E0282]: type annotations needed
   --> $DIR/fn-const-param-infer.rs:22:23
    |
 LL |     let _ = Checked::<generic>;
-   |                       ^^^^^^^ cannot infer type for `T`
+   |                       ^^^^^^^ cannot infer type for type parameter `T`
 
 error[E0308]: mismatched types
   --> $DIR/fn-const-param-infer.rs:25:40

src/test/ui/consts/issue-64662.stderr

@@ -2,13 +2,13 @@ error[E0282]: type annotations needed
   --> $DIR/issue-64662.rs:2:9
    |
 LL |     A = foo(),
-   |         ^^^ cannot infer type for `T`
+   |         ^^^ cannot infer type for type parameter `T`
 
 error[E0282]: type annotations needed
   --> $DIR/issue-64662.rs:3:9
    |
 LL |     B = foo(),
-   |         ^^^ cannot infer type for `T`
+   |         ^^^ cannot infer type for type parameter `T`
 
 error: aborting due to 2 previous errors
 

src/test/ui/error-codes/E0401.stderr

@@ -36,7 +36,7 @@ error[E0282]: type annotations needed
   --> $DIR/E0401.rs:11:5
    |
 LL |     bfnr(x);
-   |     ^^^^ cannot infer type for `U`
+   |     ^^^^ cannot infer type for type parameter `U`
 
 error: aborting due to 4 previous errors
 

src/test/ui/issues/issue-12028.stderr

@@ -2,7 +2,7 @@ error[E0284]: type annotations needed
   --> $DIR/issue-12028.rs:27:14
    |
 LL |         self.input_stream(&mut stream);
-   |              ^^^^^^^^^^^^ cannot infer type for `H`
+   |              ^^^^^^^^^^^^ cannot infer type for type parameter `H`
    |
    = note: cannot resolve `<_ as StreamHasher>::S == <H as StreamHasher>::S`
 

src/test/ui/issues/issue-16966.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/issue-16966.rs:2:5
    |
 LL |     panic!(std::default::Default::default());
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `M`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `M`
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 

src/test/ui/issues/issue-17551.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed for `B<T>`
   --> $DIR/issue-17551.rs:6:15
    |
 LL |     let foo = B(marker::PhantomData);
-   |         ---   ^ cannot infer type for `T`
+   |         ---   ^ cannot infer type for type parameter `T`
    |         |
    |         consider giving `foo` the explicit type `B<T>`, where the type parameter `T` is specified
 

src/test/ui/issues/issue-21974.stderr

@@ -11,7 +11,7 @@ LL | | {
 LL | |     x.foo();
 LL | |     y.foo();
 LL | | }
-   | |_^ cannot infer type for `&'a T`
+   | |_^ cannot infer type for reference `&'a T`
    |
    = note: cannot resolve `&'a T: Foo`
 

src/test/ui/issues/issue-24424.stderr

@@ -5,7 +5,7 @@ LL | trait Trait0<'l0>  {}
    | ----------------- required by `Trait0`
 LL | 
 LL | impl <'l0, 'l1, T0> Trait1<'l0, T0> for bool where T0 : Trait0<'l0>, T0 : Trait0<'l1> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `T0`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T0`
    |
    = note: cannot resolve `T0: Trait0<'l0>`
 

src/test/ui/issues/issue-25368.stderr

@@ -5,7 +5,7 @@ LL |     let (tx, rx) = channel();
    |         -------- consider giving this pattern the explicit type `(std::sync::mpsc::Sender<Foo<T>>, std::sync::mpsc::Receiver<Foo<T>>)`, where the type parameter `T` is specified
 ...
 LL |         tx.send(Foo{ foo: PhantomData });
-   |                 ^^^ cannot infer type for `T`
+   |                 ^^^ cannot infer type for type parameter `T`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-29147.stderr

@@ -5,7 +5,7 @@ LL | trait Foo { fn xxx(&self); }
    |             -------------- required by `Foo::xxx`
 ...
 LL |     let _ = <S5<_>>::xxx;
-   |             ^^^^^^^^^^^^ cannot infer type for `S5<_>`
+   |             ^^^^^^^^^^^^ cannot infer type for struct `S5<_>`
    |
    = note: cannot resolve `S5<_>: Foo`
 

src/test/ui/issues/issue-5062.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/issue-5062.rs:1:29
    |
 LL | fn main() { format!("{:?}", None); }
-   |                             ^^^^ cannot infer type for `T`
+   |                             ^^^^ cannot infer type for type parameter `T`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-6458-2.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/issue-6458-2.rs:3:21
    |
 LL |     format!("{:?}", None);
-   |                     ^^^^ cannot infer type for `T`
+   |                     ^^^^ cannot infer type for type parameter `T`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-6458-3.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/issue-6458-3.rs:4:5
    |
 LL |     mem::transmute(0);
-   |     ^^^^^^^^^^^^^^ cannot infer type for `U`
+   |     ^^^^^^^^^^^^^^ cannot infer type for type parameter `U`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-6458.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/issue-6458.rs:9:4
    |
 LL |    foo(TypeWithState(marker::PhantomData));
-   |    ^^^ cannot infer type for `State`
+   |    ^^^ cannot infer type for type parameter `State`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-65611.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |     let x = buffer.last().unwrap().0.clone();
    |             -------^^^^--
    |             |      |
-   |             |      cannot infer type for `T`
+   |             |      cannot infer type for type parameter `T`
    |             this method call resolves to `std::option::Option<&T>`
    |
    = note: type must be known at this point

src/test/ui/methods/method-ambig-one-trait-unknown-int-type.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed for `std::vec::Vec<T>`
   --> $DIR/method-ambig-one-trait-unknown-int-type.rs:24:17
    |
 LL |     let mut x = Vec::new();
-   |         -----   ^^^^^^^^ cannot infer type for `T`
+   |         -----   ^^^^^^^^ cannot infer type for type parameter `T`
    |         |
    |         consider giving `x` the explicit type `std::vec::Vec<T>`, where the type parameter `T` is specified
 

src/test/ui/missing/missing-items/missing-type-parameter.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/missing-type-parameter.rs:4:5
    |
 LL |     foo();
-   |     ^^^ cannot infer type for `X`
+   |     ^^^ cannot infer type for type parameter `X`
 
 error: aborting due to previous error
 

src/test/ui/span/issue-42234-unknown-receiver-type.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed for `std::option::Option<_>`
 LL |     let x: Option<_> = None;
    |         - consider giving `x` the explicit type `std::option::Option<_>`, where the type parameter `T` is specified
 LL |     x.unwrap().method_that_could_exist_on_some_type();
-   |       ^^^^^^ cannot infer type for `T`
+   |       ^^^^^^ cannot infer type for type parameter `T`
    |
    = note: type must be known at this point
 

src/test/ui/span/type-annotations-needed-expr.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |     let _ = (vec![1,2,3]).into_iter().sum() as f64;
    |                                       ^^^
    |                                       |
-   |                                       cannot infer type for `S`
+   |                                       cannot infer type for type parameter `S`
    |                                       help: consider specifying the type argument in the method call: `sum::<S>`
    |
    = note: type must be known at this point

src/test/ui/traits/traits-multidispatch-convert-ambig-dest.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/traits-multidispatch-convert-ambig-dest.rs:26:5
    |
 LL |     test(22, std::default::Default::default());
-   |     ^^^^ cannot infer type for `U`
+   |     ^^^^ cannot infer type for type parameter `U`
 
 error: aborting due to previous error
 

src/test/ui/type-inference/or_else-multiple-type-params.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |         .or_else(|err| {
    |          ^^^^^^^
    |          |
-   |          cannot infer type for `F`
+   |          cannot infer type for type parameter `F`
    |          help: consider specifying the type arguments in the method call: `or_else::<F, O>`
 
 error: aborting due to previous error

src/test/ui/type-inference/sort_by_key.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |     lst.sort_by_key(|&(v, _)| v.iter().sum());
    |         ^^^^^^^^^^^                    --- help: consider specifying the type argument in the method call: `sum::<S>`
    |         |
-   |         cannot infer type for `K`
+   |         cannot infer type for type parameter `K`
 
 error: aborting due to previous error
 

src/test/ui/type-inference/unbounded-associated-type.rs

@@ -0,0 +1,16 @@
+trait T {
+    type A;
+    fn foo(&self) -> Self::A {
+        panic!()
+    }
+}
+
+struct S<X>(std::marker::PhantomData<X>);
+
+impl<X> T for S<X> {
+   type A = X;
+}
+
+fn main() {
+    S(std::marker::PhantomData).foo(); //~ ERROR type annotations needed
+}