Rollup merge of #35080 - jonathandturner:fix_numeric_expected_found, r=nikomatsakis

Rename _ to {integer} and {float} for unknown numeric types

This PR renames _ to {integer} or {float} for unknown numeric types, to help people parse error messages that have numeric types that haven't been nailed down.

Example:
```rust
fn main() {
    let x: String = 4;
}
```

Before:
```
error[E0308]: mismatched types
 --> quicktest.rs:2:21
  |
2 |     let x: String = 4;
  |                     ^ expected struct `std::string::String`, found integral variable
  |
  = note: expected type `std::string::String`
  = note:    found type `_`

error: aborting due to previous error
```

after:
```
error[E0308]: mismatched types
 --> quicktest.rs:2:21
  |
2 |     let x: String = 4;
  |                     ^ expected struct `std::string::String`, found integral variable
  |
  = note: expected type `std::string::String`
  = note:    found type `{integer}`

error: aborting due to previous error
```
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/test/compile-fail/array-not-vector.rs b/src/test/compile-fail/array-not-vector.rs
index 1bbccae53a4..47e1c09f380 100644
--- a/src/test/compile-fail/array-not-vector.rs
+++ b/src/test/compile-fail/array-not-vector.rs
@@ -12,7 +12,7 @@ fn main() {
     let _x: i32 = [1, 2, 3];
     //~^ ERROR mismatched types
     //~| expected type `i32`
-    //~| found type `[_; 3]`
+    //~| found type `[{integer}; 3]`
     //~| expected i32, found array of 3 elements
 
     let x: &[i32] = &[1, 2, 3];