Rollup merge of #50610 - estebank:fmt-str, r=Kimundi

Improve format string errors

Point at format string position inside the formatting string:
```
error: invalid format string: unmatched `}` found
  --> $DIR/format-string-error.rs:21:22
   |
LL |     let _ = format!("}");
   |                      ^ unmatched `}` in format string
```

Explain that argument names can't start with an underscore:
```
error: invalid format string: invalid argument name `_foo`
  --> $DIR/format-string-error.rs:15:23
   |
LL |     let _ = format!("{_foo}", _foo = 6usize);
   |                       ^^^^ invalid argument name in format string
   |
   = note: argument names cannot start with an underscore
```

Fix #23476.

The more accurate spans will only be seen when using `format!` directly, when using `println!` the diagnostics machinery makes the span be the entire statement.

src/test/ui/fmt/format-string-error.rs

@@ -12,5 +12,14 @@ fn main() {
     println!("{");
     println!("{{}}");
     println!("}");
+    let _ = format!("{_foo}", _foo = 6usize);
+    //~^ ERROR invalid format string: invalid argument name `_foo`
+    let _ = format!("{_}", _ = 6usize);
+    //~^ ERROR invalid format string: invalid argument name `_`
+    let _ = format!("{");
+    //~^ ERROR invalid format string: expected `'}'` but string was terminated
+    let _ = format!("}");
+    //~^ ERROR invalid format string: unmatched `}` found
+    let _ = format!("{\\}");
+    //~^ ERROR invalid format string: expected `'}'`, found `'\\'`
 }
-

src/test/ui/fmt/format-string-error.stderr

@@ -2,7 +2,7 @@ error: invalid format string: expected `'}'` but string was terminated
   --> $DIR/format-string-error.rs:12:5
    |
 LL |     println!("{");
-   |     ^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^ expected `'}'` in format string
    |
    = note: if you intended to print `{`, you can escape it using `{{`
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
@@ -11,10 +11,48 @@ error: invalid format string: unmatched `}` found
   --> $DIR/format-string-error.rs:14:5
    |
 LL |     println!("}");
-   |     ^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^ unmatched `}` in format string
    |
    = note: if you intended to print `}`, you can escape it using `}}`
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
-error: aborting due to 2 previous errors
+error: invalid format string: invalid argument name `_foo`
+  --> $DIR/format-string-error.rs:15:23
+   |
+LL |     let _ = format!("{_foo}", _foo = 6usize);
+   |                       ^^^^ invalid argument name in format string
+   |
+   = note: argument names cannot start with an underscore
+
+error: invalid format string: invalid argument name `_`
+  --> $DIR/format-string-error.rs:17:23
+   |
+LL |     let _ = format!("{_}", _ = 6usize);
+   |                       ^ invalid argument name in format string
+   |
+   = note: argument names cannot start with an underscore
+
+error: invalid format string: expected `'}'` but string was terminated
+  --> $DIR/format-string-error.rs:19:23
+   |
+LL |     let _ = format!("{");
+   |                       ^ expected `'}'` in format string
+   |
+   = note: if you intended to print `{`, you can escape it using `{{`
+
+error: invalid format string: unmatched `}` found
+  --> $DIR/format-string-error.rs:21:22
+   |
+LL |     let _ = format!("}");
+   |                      ^ unmatched `}` in format string
+   |
+   = note: if you intended to print `}`, you can escape it using `}}`
+
+error: invalid format string: expected `'}'`, found `'/'`
+  --> $DIR/format-string-error.rs:23:23
+   |
+LL |     let _ = format!("{/}");
+   |                       ^ expected `}` in format string
+
+error: aborting due to 7 previous errors