Auto merge of #36338 - estebank:primitive-shadow, r=jseyfried

Be more specific when type parameter shadows primitive type

When a type parameter shadows a primitive type, the error message
was non obvious. For example, given the file `file.rs`:

```rust
trait Parser<T> {
    fn parse(text: &str) -> Option<T>;
}

impl<bool> Parser<bool> for bool {
    fn parse(text: &str) -> Option<bool> {
        Some(true)
    }
}

fn main() {
    println!("{}", bool::parse("ok").unwrap_or(false));
}
```

The output was:

```bash
% rustc file.rs
error[E0308]: mismatched types
 --> file.rs:7:14
  |
7 |         Some(true)
  |              ^^^^ expected type parameter, found bool a
  |
  = note: expected type `bool`
  = note:    found type `bool`

error: aborting due to previous error
```

We now show extra information about the type:

```bash
% rustc file.rs
error[E0308]: mismatched types
 --> file.rs:7:14
  |
7 |         Some(true)
  |              ^^^^ expected type parameter, found bool a
  |
  = note: expected type `bool` (type parameter)
  = note:    found type `bool` (bool)

error: aborting due to previous error
```

Fixes #35030

Author: bors

src/librustc/infer/error_reporting.rs

@@ -547,7 +547,18 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             };
 
             if !is_simple_error {
-                diag.note_expected_found(&"type", &expected, &found);
+                if expected == found {
+                    if let &TypeError::Sorts(ref values) = terr {
+                        diag.note_expected_found_extra(
+                            &"type", &expected, &found,
+                            &format!(" ({})", values.expected.sort_string(self.tcx)),
+                            &format!(" ({})", values.found.sort_string(self.tcx)));
+                    } else {
+                        diag.note_expected_found(&"type", &expected, &found);
+                    }
+                } else {
+                    diag.note_expected_found(&"type", &expected, &found);
+                }
             }
         }
 

src/librustc/ty/error.rs

@@ -210,7 +210,7 @@ impl<'tcx> fmt::Display for TypeError<'tcx> {
 }
 
 impl<'a, 'gcx, 'lcx, 'tcx> ty::TyS<'tcx> {
-    fn sort_string(&self, tcx: TyCtxt<'a, 'gcx, 'lcx>) -> String {
+    pub fn sort_string(&self, tcx: TyCtxt<'a, 'gcx, 'lcx>) -> String {
         match self.sty {
             ty::TyBool | ty::TyChar | ty::TyInt(_) |
             ty::TyUint(_) | ty::TyFloat(_) | ty::TyStr | ty::TyNever => self.to_string(),

src/librustc_errors/lib.rs

@@ -273,10 +273,21 @@ impl<'a> DiagnosticBuilder<'a> {
                                expected: &fmt::Display,
                                found: &fmt::Display)
                                -> &mut DiagnosticBuilder<'a>
+    {
+        self.note_expected_found_extra(label, expected, found, &"", &"")
+    }
+
+    pub fn note_expected_found_extra(&mut self,
+                                     label: &fmt::Display,
+                                     expected: &fmt::Display,
+                                     found: &fmt::Display,
+                                     expected_extra: &fmt::Display,
+                                     found_extra: &fmt::Display)
+                                     -> &mut DiagnosticBuilder<'a>
     {
         // For now, just attach these as notes
-        self.note(&format!("expected {} `{}`", label, expected));
-        self.note(&format!("   found {} `{}`", label, found));
+        self.note(&format!("expected {} `{}`{}", label, expected, expected_extra));
+        self.note(&format!("   found {} `{}`{}", label, found, found_extra));
         self
     }
 
@@ -764,4 +775,4 @@ pub fn expect<T, M>(diag: &Handler, opt: Option<T>, msg: M) -> T where
         Some(t) => t,
         None => diag.bug(&msg()),
     }
-}
+}

src/test/ui/mismatched_types/issue-35030.rs

@@ -0,0 +1,25 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// rustc-env:RUST_NEW_ERROR_FORMAT
+
+trait Parser<T> {
+    fn parse(text: &str) -> Option<T>;
+}
+
+impl<bool> Parser<bool> for bool {
+    fn parse(text: &str) -> Option<bool> {
+        Some(true)
+    }
+}
+
+fn main() {
+    println!("{}", bool::parse("ok").unwrap_or(false));
+}

src/test/ui/mismatched_types/issue-35030.stderr

@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-35030.rs:19:14
+   |
+19 |         Some(true)
+   |              ^^^^ expected type parameter, found bool
+   |
+   = note: expected type `bool` (type parameter)
+   = note:    found type `bool` (bool)
+
+error: aborting due to previous error
+