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

src/test/ui/lifetime-errors/ex2a-push-one-existing-name.stderr

@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 16 |     x.push(y);
    |            ^ lifetime mismatch
    |
-   = note: expected type `Ref<'a, i32>`
-              found type `Ref<'_, i32>`
+   = note: expected type `Ref<'a, _>`
+              found type `Ref<'_, _>`
 note: the anonymous lifetime #2 defined on the body at 15:51...
   --> $DIR/ex2a-push-one-existing-name.rs:15:52
    |

src/test/ui/lifetime-errors/ex2b-push-no-existing-names.stderr

@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 16 |     x.push(y);
    |            ^ lifetime mismatch
    |
-   = note: expected type `Ref<'_, i32>`
-              found type `Ref<'_, i32>`
+   = note: expected type `Ref<'_, _>`
+              found type `Ref<'_, _>`
 note: the anonymous lifetime #3 defined on the body at 15:43...
   --> $DIR/ex2b-push-no-existing-names.rs:15:44
    |

src/test/ui/lifetime-errors/ex2c-push-inference-variable.stderr

@@ -27,7 +27,7 @@ note: but, the lifetime must be valid for the lifetime 'b as defined on the body
 17 | |     x.push(z);
 18 | | }
    | |_^ ...ending here
-note: ...so that expression is assignable (expected Ref<'b, i32>, found Ref<'_, i32>)
+note: ...so that expression is assignable (expected Ref<'b, _>, found Ref<'_, _>)
   --> $DIR/ex2c-push-inference-variable.rs:17:12
    |
 17 |     x.push(z);

src/test/ui/mismatched_types/abridged.rs

@@ -0,0 +1,61 @@
+// Copyright 2017 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.
+
+enum Bar {
+    Qux,
+    Zar,
+}
+
+struct Foo {
+    bar: usize,
+}
+
+struct X<T1, T2> {
+    x: T1,
+    y: T2,
+}
+
+fn a() -> Foo {
+    Some(Foo { bar: 1 })
+}
+
+fn a2() -> Foo {
+    Ok(Foo { bar: 1})
+}
+
+fn b() -> Option<Foo> {
+    Foo { bar: 1 }
+}
+
+fn c() -> Result<Foo, Bar> {
+    Foo { bar: 1 }
+}
+
+fn d() -> X<X<String, String>, String> {
+    X {
+        x: X {
+            x: "".to_string(),
+            y: 2,
+        },
+        y: 3,
+    }
+}
+
+fn e() -> X<X<String, String>, String> {
+    X {
+        x: X {
+            x: "".to_string(),
+            y: 2,
+        },
+        y: "".to_string(),
+    }
+}
+
+fn main() {}

src/test/ui/mismatched_types/abridged.stderr

@@ -0,0 +1,70 @@
+error[E0308]: mismatched types
+  --> $DIR/abridged.rs:26:5
+   |
+26 |     Some(Foo { bar: 1 })
+   |     ^^^^^^^^^^^^^^^^^^^^ expected struct `Foo`, found enum `std::option::Option`
+   |
+   = note: expected type `Foo`
+              found type `std::option::Option<Foo>`
+
+error[E0308]: mismatched types
+  --> $DIR/abridged.rs:30:5
+   |
+30 |     Ok(Foo { bar: 1})
+   |     ^^^^^^^^^^^^^^^^^ expected struct `Foo`, found enum `std::result::Result`
+   |
+   = note: expected type `Foo`
+              found type `std::result::Result<Foo, _>`
+
+error[E0308]: mismatched types
+  --> $DIR/abridged.rs:34:5
+   |
+34 |     Foo { bar: 1 }
+   |     ^^^^^^^^^^^^^^ expected enum `std::option::Option`, found struct `Foo`
+   |
+   = note: expected type `std::option::Option<Foo>`
+              found type `Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/abridged.rs:38:5
+   |
+38 |     Foo { bar: 1 }
+   |     ^^^^^^^^^^^^^^ expected enum `std::result::Result`, found struct `Foo`
+   |
+   = note: expected type `std::result::Result<Foo, Bar>`
+              found type `Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/abridged.rs:42:5
+   |
+42 |       X {
+   |  _____^ starting here...
+43 | |         x: X {
+44 | |             x: "".to_string(),
+45 | |             y: 2,
+46 | |         },
+47 | |         y: 3,
+48 | |     }
+   | |_____^ ...ending here: expected struct `std::string::String`, found integral variable
+   |
+   = note: expected type `X<X<_, std::string::String>, std::string::String>`
+              found type `X<X<_, {integer}>, {integer}>`
+
+error[E0308]: mismatched types
+  --> $DIR/abridged.rs:52:5
+   |
+52 |       X {
+   |  _____^ starting here...
+53 | |         x: X {
+54 | |             x: "".to_string(),
+55 | |             y: 2,
+56 | |         },
+57 | |         y: "".to_string(),
+58 | |     }
+   | |_____^ ...ending here: expected struct `std::string::String`, found integral variable
+   |
+   = note: expected type `X<X<_, std::string::String>, _>`
+              found type `X<X<_, {integer}>, _>`
+
+error: aborting due to 6 previous errors
+