On long spans, trim the middle of them to make them fit in the terminal width

When encountering a single line span that is wider than the terminal, we keep context at the start and end of the span but otherwise remove the code from the middle. This is somewhat independent from whether the left and right margins of the output have been trimmed as well.

```
error[E0308]: mismatched types
  --> $DIR/long-span.rs:6:15
   |
LL | ... = [0, 0, 0, 0, ..., 0, 0];
   |       ^^^^^^^^^^^^^...^^^^^^^ expected `u8`, found `[{integer}; 1681]`
```

Address part of #137680 (missing handling of the long suggestion). Fix #125581.

compiler/rustc_errors/src/emitter.rs

@@ -1238,6 +1238,33 @@ impl HumanEmitter {
                 );
             }
         }
+
+        // We look for individual *long* spans, and we trim the *middle*, so that we render
+        // LL | ...= [0, 0, 0, ..., 0, 0];
+        //    |      ^^^^^^^^^^...^^^^^^^ expected `&[u8]`, found `[{integer}; 1680]`
+        for &(pos, annotation) in &annotations_position {
+            let AnnotationType::Singleline = annotation.annotation_type else { continue };
+            let width = annotation.end_col.display - annotation.start_col.display;
+            if pos == 0 && width > margin.column_width && width > 10 {
+                // If the terminal is *too* small, we keep at least a tiny bit of the span for
+                // display.
+                let pad = max(margin.column_width / 2, 5);
+                // Code line
+                buffer.replace(
+                    line_offset,
+                    annotation.start_col.file + pad,
+                    annotation.end_col.file - pad,
+                    self.margin(),
+                );
+                // Underline line
+                buffer.replace(
+                    line_offset + 1,
+                    annotation.start_col.file + pad,
+                    annotation.end_col.file - pad,
+                    self.margin(),
+                );
+            }
+        }
         annotations_position
             .iter()
             .filter_map(|&(_, annotation)| match annotation.annotation_type {

compiler/rustc_errors/src/styled_buffer.rs

@@ -89,6 +89,16 @@ impl StyledBuffer {
         }
     }
 
+    pub(crate) fn replace(&mut self, line: usize, start: usize, end: usize, string: &str) {
+        if start == end {
+            return;
+        }
+        let _ = self.lines[line].drain(start..(end - string.chars().count()));
+        for (i, c) in string.chars().enumerate() {
+            self.lines[line][start + i] = StyledChar::new(c, Style::LineNumber);
+        }
+    }
+
     /// For given `line` inserts `string` with `style` before old content of that line,
     /// adding lines if needed
     pub(crate) fn prepend(&mut self, line: usize, string: &str, style: Style) {

tests/ui/diagnostic-width/long-span.long.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+   ╭▸ $DIR/long-span.rs:7:15
+   │
+LL │ …u8 = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, …, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]…
+   ╰╴      ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━…━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ expected `u8`, found `[{integer}; 1680]`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/diagnostic-width/long-span.longest.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+  --> $DIR/long-span.rs:7:15
+   |
+LL | ... = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,... 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `u8`, found `[{integer}; 1680]`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/diagnostic-width/long-span.short.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+   ╭▸ $DIR/long-span.rs:7:15
+   │
+LL │ …u8 = [0, 0, 0…0]…
+   ╰╴      ━━━━━━━━…━━ expected `u8`, found `[{integer}; 1680]`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/diagnostic-width/long-span.shortest.stderr

@@ -0,0 +1,9 @@
+error[E0308]: mismatched types
+  --> $DIR/long-span.rs:7:15
+   |
+LL | ... = [0, 0, 0......
+   |       ^^^^^^^^...^^ expected `u8`, found `[{integer}; 1680]`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/parser/raw/too-many-hash.stderr

@@ -1,8 +1,8 @@
 error: too many `#` symbols: raw strings may be delimited by up to 255 `#` symbols, but found 256
   --> $DIR/too-many-hash.rs:4:19
    |
-LL | ... = r################################################################################################################################################################################################################################################################"very raw"##############################################################################################################################################################################################################################################################...
-   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | ... = r############################################################################...#############################################################...
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/rust-2024/reserved-guarded-strings.stderr

@@ -241,8 +241,8 @@ LL |     demo2!(#"foo"## #);
 error: invalid string literal
   --> $DIR/reserved-guarded-strings.rs:71:12
    |
-LL | ...n!(####################################################################################################################################################################################################################################################################"foo...
-   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | ...n!(######################################################################...#################################################################"foo...
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: unprefixed guarded string literals are reserved for future use since Rust 2024
 help: consider inserting whitespace here