Rollup merge of #41205 - estebank:shorter-mismatched-types-2, r=nikomatsakis

Highlight and simplify mismatched types

Shorten mismatched types errors by replacing subtypes that are not
different with `_`, and highlighting only the subtypes that are
different.

Given a file

```rust
struct X<T1, T2> {
    x: T1,
    y: T2,
}

fn foo() -> X<X<String, String>, String> {
    X { x: X {x: "".to_string(), y: 2}, y: "".to_string()}
}

fn bar() -> Option<String> {
    "".to_string()
}
```

provide the following output

```rust
error[E0308]: mismatched types
  --> file.rs:6:5
   |
 6 |     X { x: X {x: "".to_string(), y: 2}, y: "".to_string()}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::string::String`, found {integer}
   |
   = note: expected type `X<X<_, std::string::String>, _>`
                                 ^^^^^^^^^^^^^^^^^^^   // < highlighted
              found type `X<X<_, {integer}>, _>`
                                 ^^^^^^^^^             // < highlighted

error[E0308]: mismatched types
  --> file.rs:6:5
   |
10 |     "".to_string()
   |     ^^^^^^^^^^^^^^ expected struct `std::option::Option`, found `std::string::String`
   |
   = note: expected type `Option<std::string::String>`
                          ^^^^^^^                   ^  // < highlighted
              found type `std::string::String`
```

Fix #21025. Re: #40186. Follow up to #39906.

I'm looking to change how this output is accomplished so that it doesn't create list of strings to pass around, but rather add an elided `Ty` placeholder, and use the same string formatting for normal types. I'll be doing that soonish.

r? @nikomatsakis

1 files changed, 6 insertions, 4 deletions

diff --git a/src/librustc_errors/diagnostic_builder.rs b/src/librustc_errors/diagnostic_builder.rs
index 7dfea6b8951..7b27f13951b 100644
--- a/src/librustc_errors/diagnostic_builder.rs
+++ b/src/librustc_errors/diagnostic_builder.rs
@@ -9,6 +9,8 @@
 // except according to those terms.
 
 use Diagnostic;
+use DiagnosticStyledString;
+
 use Level;
 use Handler;
 use std::fmt::{self, Debug};
@@ -115,14 +117,14 @@ impl<'a> DiagnosticBuilder<'a> {
 
     forward!(pub fn note_expected_found(&mut self,
                                         label: &fmt::Display,
-                                        expected: &fmt::Display,
-                                        found: &fmt::Display)
+                                        expected: DiagnosticStyledString,
+                                        found: DiagnosticStyledString)
                                         -> &mut Self);
 
     forward!(pub fn note_expected_found_extra(&mut self,
                                               label: &fmt::Display,
-                                              expected: &fmt::Display,
-                                              found: &fmt::Display,
+                                              expected: DiagnosticStyledString,
+                                              found: DiagnosticStyledString,
                                               expected_extra: &fmt::Display,
                                               found_extra: &fmt::Display)
                                               -> &mut Self);