Add more detail to type inference error

When encountering code where type inference fails, add more actionable
information:

```
fn main() {
    let foo = Vec::new();
}
```

```
error[E0282]: type annotations needed for `std::vec::Vec<_>`
  --> $DIR/vector-no-ann.rs:2:16
   |
LL |     let foo = Vec::new();
   |         ---   ^^^^^^^^ cannot infer type for `T`
   |         |
   |         consider giving `foo` the type `std::vec::Vec<_>` with the type parameter `T` specified
```

We still need to modify type printing to optionally accept a
`TypeVariableTable` in order to properly print `std::vec::Vec<T>`.

CC #25633.

19 files changed, 82 insertions, 42 deletions

diff --git a/src/librustc/infer/error_reporting/need_type_info.rs b/src/librustc/infer/error_reporting/need_type_info.rs
index 972ffbe1820..173f916c282 100644
--- a/src/librustc/infer/error_reporting/need_type_info.rs
+++ b/src/librustc/infer/error_reporting/need_type_info.rs
@@ -15,17 +15,18 @@ struct FindLocalByTypeVisitor<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
     hir_map: &'a hir::map::Map<'gcx>,
     found_local_pattern: Option<&'gcx Pat>,
     found_arg_pattern: Option<&'gcx Pat>,
+    found_ty: Option<Ty<'tcx>>,
 }
 
 impl<'a, 'gcx, 'tcx> FindLocalByTypeVisitor<'a, 'gcx, 'tcx> {
-    fn node_matches_type(&mut self, hir_id: HirId) -> bool {
+    fn node_matches_type(&mut self, hir_id: HirId) -> Option<Ty<'tcx>> {
         let ty_opt = self.infcx.in_progress_tables.and_then(|tables| {
             tables.borrow().node_type_opt(hir_id)
         });
         match ty_opt {
             Some(ty) => {
                 let ty = self.infcx.resolve_vars_if_possible(&ty);
-                ty.walk().any(|inner_ty| {
+                if ty.walk().any(|inner_ty| {
                     inner_ty == self.target_ty || match (&inner_ty.sty, &self.target_ty.sty) {
                         (&Infer(TyVar(a_vid)), &Infer(TyVar(b_vid))) => {
                             self.infcx
@@ -35,9 +36,13 @@ impl<'a, 'gcx, 'tcx> FindLocalByTypeVisitor<'a, 'gcx, 'tcx> {
                         }
                         _ => false,
                     }
-                })
+                }) {
+                    Some(ty)
+                } else {
+                    None
+                }
             }
-            None => false,
+            None => None,
         }
     }
 }
@@ -48,16 +53,21 @@ impl<'a, 'gcx, 'tcx> Visitor<'gcx> for FindLocalByTypeVisitor<'a, 'gcx, 'tcx> {
     }
 
     fn visit_local(&mut self, local: &'gcx Local) {
-        if self.found_local_pattern.is_none() && self.node_matches_type(local.hir_id) {
+        if let (None, Some(ty)) = (self.found_local_pattern, self.node_matches_type(local.hir_id)) {
             self.found_local_pattern = Some(&*local.pat);
+            self.found_ty = Some(ty);
         }
         intravisit::walk_local(self, local);
     }
 
     fn visit_body(&mut self, body: &'gcx Body) {
         for argument in &body.arguments {
-            if self.found_arg_pattern.is_none() && self.node_matches_type(argument.hir_id) {
+            if let (None, Some(ty)) = (
+                self.found_arg_pattern,
+                self.node_matches_type(argument.hir_id),
+            ) {
                 self.found_arg_pattern = Some(&*argument.pat);
+                self.found_ty = Some(ty);
             }
         }
         intravisit::walk_body(self, body);
@@ -106,6 +116,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             hir_map: &self.tcx.hir(),
             found_local_pattern: None,
             found_arg_pattern: None,
+            found_ty: None,
         };
 
         if let Some(body_id) = body_id {
@@ -113,6 +124,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             local_visitor.visit_expr(expr);
         }
 
+        let ty_msg = match local_visitor.found_ty {
+            Some(ty) if &ty.to_string() != "_" => format!(" for `{}`", ty),
+            _ => String::new(),
+        };
         if let Some(pattern) = local_visitor.found_arg_pattern {
             err_span = pattern.span;
             // We don't want to show the default label for closures.
@@ -131,13 +146,35 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             //          ^ consider giving this closure parameter a type
             // ```
             labels.clear();
-            labels.push(
-                (pattern.span, "consider giving this closure parameter a type".to_owned()));
+            labels.push((pattern.span, format!(
+                "consider giving this closure parameter {}",
+                match local_visitor.found_ty {
+                    Some(ty) if &ty.to_string() != "_" => format!(
+                        "the type `{}` with the type parameter `{}` specified",
+                        ty,
+                        name,
+                    ),
+                    _ => "a type".to_owned(),
+                },
+            )));
         } else if let Some(pattern) = local_visitor.found_local_pattern {
             if let Some(simple_ident) = pattern.simple_ident() {
                 match pattern.span.compiler_desugaring_kind() {
-                    None => labels.push((pattern.span,
-                                         format!("consider giving `{}` a type", simple_ident))),
+                    None => labels.push((
+                        pattern.span,
+                        format!(
+                            "consider giving `{}` {}",
+                            simple_ident,
+                            match local_visitor.found_ty {
+                                Some(ty) if &ty.to_string() != "_" => format!(
+                                    "the type `{}` with the type parameter `{}` specified",
+                                    ty,
+                                    name,
+                                ),
+                                _ => "a type".to_owned(),
+                            },
+                        ),
+                    )),
                     Some(CompilerDesugaringKind::ForLoop) => labels.push((
                         pattern.span,
                         "the element type for this iterator is not specified".to_owned(),
@@ -147,12 +184,15 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             } else {
                 labels.push((pattern.span, "consider giving the pattern a type".to_owned()));
             }
-        }
+        };
 
-        let mut err = struct_span_err!(self.tcx.sess,
-                                       err_span,
-                                       E0282,
-                                       "type annotations needed");
+        let mut err = struct_span_err!(
+            self.tcx.sess,
+            err_span,
+            E0282,
+            "type annotations needed{}",
+            ty_msg,
+        );
 
         for (target_span, label_message) in labels {
             err.span_label(target_span, label_message);
diff --git a/src/test/ui/issues/issue-12187-1.rs b/src/test/ui/issues/issue-12187-1.rs
index 37ff468e032..86128ed94bd 100644
--- a/src/test/ui/issues/issue-12187-1.rs
+++ b/src/test/ui/issues/issue-12187-1.rs
@@ -4,5 +4,5 @@ fn new<T>() -> &'static T {
 
 fn main() {
     let &v = new();
-    //~^ ERROR type annotations needed [E0282]
+    //~^ ERROR type annotations needed
 }
diff --git a/src/test/ui/issues/issue-12187-1.stderr b/src/test/ui/issues/issue-12187-1.stderr
index f8df4f82e7e..77aa736873b 100644
--- a/src/test/ui/issues/issue-12187-1.stderr
+++ b/src/test/ui/issues/issue-12187-1.stderr
@@ -1,4 +1,4 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `&_`
   --> $DIR/issue-12187-1.rs:6:10
    |
 LL |     let &v = new();
diff --git a/src/test/ui/issues/issue-12187-2.rs b/src/test/ui/issues/issue-12187-2.rs
index a1cdb849779..080a6206be7 100644
--- a/src/test/ui/issues/issue-12187-2.rs
+++ b/src/test/ui/issues/issue-12187-2.rs
@@ -4,5 +4,5 @@ fn new<'r, T>() -> &'r T {
 
 fn main() {
     let &v = new();
-    //~^ ERROR type annotations needed [E0282]
+    //~^ ERROR type annotations needed
 }
diff --git a/src/test/ui/issues/issue-12187-2.stderr b/src/test/ui/issues/issue-12187-2.stderr
index c40ae0461ec..4953c2b11b5 100644
--- a/src/test/ui/issues/issue-12187-2.stderr
+++ b/src/test/ui/issues/issue-12187-2.stderr
@@ -1,4 +1,4 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `&_`
   --> $DIR/issue-12187-2.rs:6:10
    |
 LL |     let &v = new();
diff --git a/src/test/ui/issues/issue-17551.stderr b/src/test/ui/issues/issue-17551.stderr
index 40e7727752b..3e99d307b3b 100644
--- a/src/test/ui/issues/issue-17551.stderr
+++ b/src/test/ui/issues/issue-17551.stderr
@@ -1,10 +1,10 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `B<_>`
   --> $DIR/issue-17551.rs:6:15
    |
 LL |     let foo = B(marker::PhantomData);
    |         ---   ^ cannot infer type for `T`
    |         |
-   |         consider giving `foo` a type
+   |         consider giving `foo` the type `B<_>` with the type parameter `T` specified
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-20261.stderr b/src/test/ui/issues/issue-20261.stderr
index 5665f5893f4..c6c3f32dfe7 100644
--- a/src/test/ui/issues/issue-20261.stderr
+++ b/src/test/ui/issues/issue-20261.stderr
@@ -1,4 +1,4 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `&(_,)`
   --> $DIR/issue-20261.rs:4:11
    |
 LL |     for (ref i,) in [].iter() {
diff --git a/src/test/ui/issues/issue-23046.stderr b/src/test/ui/issues/issue-23046.stderr
index aab90a9d440..2ce77576404 100644
--- a/src/test/ui/issues/issue-23046.stderr
+++ b/src/test/ui/issues/issue-23046.stderr
@@ -1,8 +1,8 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `Expr<'_, _>`
   --> $DIR/issue-23046.rs:17:15
    |
 LL |     let ex = |x| {
-   |               ^ consider giving this closure parameter a type
+   |               ^ consider giving this closure parameter the type `Expr<'_, _>` with the type parameter `VAR` specified
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-25368.stderr b/src/test/ui/issues/issue-25368.stderr
index 3ad6a2569be..3b2c3bfe4ff 100644
--- a/src/test/ui/issues/issue-25368.stderr
+++ b/src/test/ui/issues/issue-25368.stderr
@@ -1,4 +1,4 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `(std::sync::mpsc::Sender<Foo<_>>, std::sync::mpsc::Receiver<Foo<_>>)`
   --> $DIR/issue-25368.rs:11:17
    |
 LL |     let (tx, rx) = channel();
diff --git a/src/test/ui/issues/issue-7813.stderr b/src/test/ui/issues/issue-7813.stderr
index 45b9c915885..8e2a69dd9e3 100644
--- a/src/test/ui/issues/issue-7813.stderr
+++ b/src/test/ui/issues/issue-7813.stderr
@@ -1,10 +1,10 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `&[_; 0]`
   --> $DIR/issue-7813.rs:2:13
    |
 LL |     let v = &[];
    |         -   ^^^ cannot infer type
    |         |
-   |         consider giving `v` a type
+   |         consider giving `v` the type `&[_; 0]` with the type parameter `_` specified
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/methods/method-ambig-one-trait-unknown-int-type.rs b/src/test/ui/methods/method-ambig-one-trait-unknown-int-type.rs
index 590e98dc353..e33f23c64db 100644
--- a/src/test/ui/methods/method-ambig-one-trait-unknown-int-type.rs
+++ b/src/test/ui/methods/method-ambig-one-trait-unknown-int-type.rs
@@ -22,7 +22,7 @@ impl Foo for Vec<isize> {
 fn m1() {
     // we couldn't infer the type of the vector just based on calling foo()...
     let mut x = Vec::new();
-    //~^ ERROR type annotations needed [E0282]
+    //~^ ERROR type annotations needed
     x.foo();
 }
 
diff --git a/src/test/ui/methods/method-ambig-one-trait-unknown-int-type.stderr b/src/test/ui/methods/method-ambig-one-trait-unknown-int-type.stderr
index 063a4865b19..bae675526bd 100644
--- a/src/test/ui/methods/method-ambig-one-trait-unknown-int-type.stderr
+++ b/src/test/ui/methods/method-ambig-one-trait-unknown-int-type.stderr
@@ -1,10 +1,10 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `std::vec::Vec<_>`
   --> $DIR/method-ambig-one-trait-unknown-int-type.rs:24:17
    |
 LL |     let mut x = Vec::new();
    |         -----   ^^^^^^^^ cannot infer type for `T`
    |         |
-   |         consider giving `x` a type
+   |         consider giving `x` the type `std::vec::Vec<_>` with the type parameter `T` specified
 
 error[E0308]: mismatched types
   --> $DIR/method-ambig-one-trait-unknown-int-type.rs:33:20
diff --git a/src/test/ui/span/issue-42234-unknown-receiver-type.stderr b/src/test/ui/span/issue-42234-unknown-receiver-type.stderr
index 8923de5705b..9b0d56e38b7 100644
--- a/src/test/ui/span/issue-42234-unknown-receiver-type.stderr
+++ b/src/test/ui/span/issue-42234-unknown-receiver-type.stderr
@@ -1,8 +1,8 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `std::option::Option<_>`
   --> $DIR/issue-42234-unknown-receiver-type.rs:7:5
    |
 LL |     let x: Option<_> = None;
-   |         - consider giving `x` a type
+   |         - consider giving `x` the type `std::option::Option<_>` with the type parameter `T` specified
 LL |     x.unwrap().method_that_could_exist_on_some_type();
    |     ^^^^^^^^^^ cannot infer type for `T`
    |
diff --git a/src/test/ui/type/type-check/cannot_infer_local_or_array.stderr b/src/test/ui/type/type-check/cannot_infer_local_or_array.stderr
index 17be67c6c3a..115808b3b3e 100644
--- a/src/test/ui/type/type-check/cannot_infer_local_or_array.stderr
+++ b/src/test/ui/type/type-check/cannot_infer_local_or_array.stderr
@@ -1,10 +1,10 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `[_; 0]`
   --> $DIR/cannot_infer_local_or_array.rs:2:13
    |
 LL |     let x = [];
    |         -   ^^ cannot infer type
    |         |
-   |         consider giving `x` a type
+   |         consider giving `x` the type `[_; 0]` with the type parameter `_` specified
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/type/type-check/cannot_infer_local_or_vec.stderr b/src/test/ui/type/type-check/cannot_infer_local_or_vec.stderr
index 8215947d49c..49b4330b51e 100644
--- a/src/test/ui/type/type-check/cannot_infer_local_or_vec.stderr
+++ b/src/test/ui/type/type-check/cannot_infer_local_or_vec.stderr
@@ -1,10 +1,10 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `std::vec::Vec<_>`
   --> $DIR/cannot_infer_local_or_vec.rs:2:13
    |
 LL |     let x = vec![];
    |         -   ^^^^^^ cannot infer type for `T`
    |         |
-   |         consider giving `x` a type
+   |         consider giving `x` the type `std::vec::Vec<_>` with the type parameter `T` specified
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
diff --git a/src/test/ui/type/type-check/cannot_infer_local_or_vec_in_tuples.stderr b/src/test/ui/type/type-check/cannot_infer_local_or_vec_in_tuples.stderr
index 1065d49c26b..57eb185a36a 100644
--- a/src/test/ui/type/type-check/cannot_infer_local_or_vec_in_tuples.stderr
+++ b/src/test/ui/type/type-check/cannot_infer_local_or_vec_in_tuples.stderr
@@ -1,4 +1,4 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `(std::vec::Vec<_>,)`
   --> $DIR/cannot_infer_local_or_vec_in_tuples.rs:2:18
    |
 LL |     let (x, ) = (vec![], );
diff --git a/src/test/ui/unboxed-closures/unboxed-closures-failed-recursive-fn-2.stderr b/src/test/ui/unboxed-closures/unboxed-closures-failed-recursive-fn-2.stderr
index bd58e241f0c..d0d8aed0cb8 100644
--- a/src/test/ui/unboxed-closures/unboxed-closures-failed-recursive-fn-2.stderr
+++ b/src/test/ui/unboxed-closures/unboxed-closures-failed-recursive-fn-2.stderr
@@ -1,8 +1,8 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `std::option::Option<_>`
   --> $DIR/unboxed-closures-failed-recursive-fn-2.rs:16:32
    |
 LL |     let mut closure0 = None;
-   |         ------------ consider giving `closure0` a type
+   |         ------------ consider giving `closure0` the type `std::option::Option<_>` with the type parameter `_` specified
 ...
 LL |                         return c();
    |                                ^^^ cannot infer type
diff --git a/src/test/ui/vector-no-ann.rs b/src/test/ui/vector-no-ann.rs
index 200364a5d93..1f11d9c8dff 100644
--- a/src/test/ui/vector-no-ann.rs
+++ b/src/test/ui/vector-no-ann.rs
@@ -1,4 +1,4 @@
 fn main() {
     let _foo = Vec::new();
-    //~^ ERROR type annotations needed [E0282]
+    //~^ ERROR type annotations needed
 }
diff --git a/src/test/ui/vector-no-ann.stderr b/src/test/ui/vector-no-ann.stderr
index 01b569f97f9..6aa925e0715 100644
--- a/src/test/ui/vector-no-ann.stderr
+++ b/src/test/ui/vector-no-ann.stderr
@@ -1,10 +1,10 @@
-error[E0282]: type annotations needed
+error[E0282]: type annotations needed for `std::vec::Vec<_>`
   --> $DIR/vector-no-ann.rs:2:16
    |
 LL |     let _foo = Vec::new();
    |         ----   ^^^^^^^^ cannot infer type for `T`
    |         |
-   |         consider giving `_foo` a type
+   |         consider giving `_foo` the type `std::vec::Vec<_>` with the type parameter `T` specified
 
 error: aborting due to previous error