Rollup merge of rust-lang#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
```
```

Author: Manishearth

src/librustc/util/ppaux.rs

@@ -974,7 +974,9 @@ impl fmt::Display for ty::InferTy {
             ty::TyVar(ref vid) if print_var_ids => write!(f, "{:?}", vid),
             ty::IntVar(ref vid) if print_var_ids => write!(f, "{:?}", vid),
             ty::FloatVar(ref vid) if print_var_ids => write!(f, "{:?}", vid),
-            ty::TyVar(_) | ty::IntVar(_) | ty::FloatVar(_) => write!(f, "_"),
+            ty::TyVar(_) => write!(f, "_"),
+            ty::IntVar(_) => write!(f, "{}", "{integer}"),
+            ty::FloatVar(_) => write!(f, "{}", "{float}"),
             ty::FreshTy(v) => write!(f, "FreshTy({})", v),
             ty::FreshIntTy(v) => write!(f, "FreshIntTy({})", v),
             ty::FreshFloatTy(v) => write!(f, "FreshFloatTy({})", v)

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];

src/test/compile-fail/bad-const-type.rs

@@ -11,6 +11,6 @@
 static i: String = 10;
 //~^ ERROR mismatched types
 //~| expected type `std::string::String`
-//~| found type `_`
+//~| found type `{integer}`
 //~| expected struct `std::string::String`, found integral variable
 fn main() { println!("{}", i); }

src/test/compile-fail/coerce-mut.rs

@@ -15,6 +15,6 @@ fn main() {
     f(&x);
     //~^ ERROR mismatched types
     //~| expected type `&mut i32`
-    //~| found type `&_`
+    //~| found type `&{integer}`
     //~| values differ in mutability
 }

src/test/compile-fail/coercion-slice.rs

@@ -14,6 +14,6 @@ fn main() {
     let _: &[i32] = [0];
     //~^ ERROR mismatched types
     //~| expected type `&[i32]`
-    //~| found type `[_; 1]`
+    //~| found type `[{integer}; 1]`
     //~| expected &-ptr, found array of 1 elements
 }

src/test/compile-fail/fully-qualified-type-name1.rs

@@ -15,6 +15,6 @@ fn main() {
     x = 5;
     //~^ ERROR mismatched types
     //~| expected type `std::option::Option<usize>`
-    //~| found type `_`
+    //~| found type `{integer}`
     //~| expected enum `std::option::Option`, found integral variable
 }

src/test/compile-fail/if-let-arm-types.rs

@@ -12,7 +12,7 @@ fn main() {
     if let Some(b) = None { //~ ERROR: `if let` arms have incompatible types
         //~^ expected (), found integral variable
         //~| expected type `()`
-        //~| found type `_`
+        //~| found type `{integer}`
         ()
     } else {                //~ NOTE: `if let` arm with an incompatible type
         1

src/test/compile-fail/indexing-requires-a-uint.rs

@@ -13,7 +13,7 @@
 
 fn main() {
     fn bar<T>(_: T) {}
-    [0][0u8]; //~ ERROR: `[_]: std::ops::Index<u8>` is not satisfied
+    [0][0u8]; //~ ERROR: `[{integer}]: std::ops::Index<u8>` is not satisfied
 
     [0][0]; // should infer to be a usize
 

src/test/compile-fail/integral-variable-unification-error.rs

@@ -12,7 +12,7 @@ fn main() {
     let mut x = 2;
     x = 5.0;
     //~^ ERROR mismatched types
-    //~| expected type `_`
-    //~| found type `_`
+    //~| expected type `{integer}`
+    //~| found type `{float}`
     //~| expected integral variable, found floating-point variable
 }

src/test/compile-fail/issue-13466.rs

@@ -17,13 +17,13 @@ pub fn main() {
     let _x: usize = match Some(1) {
         Ok(u) => u,
         //~^ ERROR mismatched types
-        //~| expected type `std::option::Option<_>`
+        //~| expected type `std::option::Option<{integer}>`
         //~| found type `std::result::Result<_, _>`
         //~| expected enum `std::option::Option`, found enum `std::result::Result`
 
         Err(e) => panic!(e)
         //~^ ERROR mismatched types
-        //~| expected type `std::option::Option<_>`
+        //~| expected type `std::option::Option<{integer}>`
         //~| found type `std::result::Result<_, _>`
         //~| expected enum `std::option::Option`, found enum `std::result::Result`
     };