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.

9 files changed, 86 insertions, 4 deletions

diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index 18846a6dbe1..9277bae94ec 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/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 {
diff --git a/compiler/rustc_errors/src/styled_buffer.rs b/compiler/rustc_errors/src/styled_buffer.rs
index 5ca9e9b18f3..b0f4ec84a89 100644
--- a/compiler/rustc_errors/src/styled_buffer.rs
+++ b/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) {
diff --git a/tests/ui/diagnostic-width/long-span.long.stderr b/tests/ui/diagnostic-width/long-span.long.stderr
new file mode 100644
index 00000000000..81edde85b33
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/diagnostic-width/long-span.longest.stderr b/tests/ui/diagnostic-width/long-span.longest.stderr
new file mode 100644
index 00000000000..77aafc5f426
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/diagnostic-width/long-span.rs b/tests/ui/diagnostic-width/long-span.rs
new file mode 100644
index 00000000000..2feacdd9604
--- /dev/null
+++ b/tests/ui/diagnostic-width/long-span.rs
@@ -0,0 +1,9 @@
+//@ revisions: shortest short long longest
+//@[shortest] compile-flags: --diagnostic-width=4
+//@[short] compile-flags: --diagnostic-width=12 -Zunstable-options --json=diagnostic-unicode
+//@[long] compile-flags: --diagnostic-width=80 -Zunstable-options --json=diagnostic-unicode
+//@[longest] compile-flags: --diagnostic-width=120
+// ignore-tidy-linelength
+const C: 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 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, 0, 0, 0, 0, 0, 0, 0, 0, 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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
+//~^ ERROR E0308
+fn main() {}
diff --git a/tests/ui/diagnostic-width/long-span.short.stderr b/tests/ui/diagnostic-width/long-span.short.stderr
new file mode 100644
index 00000000000..8e62acc936c
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/diagnostic-width/long-span.shortest.stderr b/tests/ui/diagnostic-width/long-span.shortest.stderr
new file mode 100644
index 00000000000..d9cec96ad8f
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/parser/raw/too-many-hash.stderr b/tests/ui/parser/raw/too-many-hash.stderr
index 1c46b5385cd..61fcbcee159 100644
--- a/tests/ui/parser/raw/too-many-hash.stderr
+++ b/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
 
diff --git a/tests/ui/rust-2024/reserved-guarded-strings.stderr b/tests/ui/rust-2024/reserved-guarded-strings.stderr
index 0f3b06147c4..5c0f5064546 100644
--- a/tests/ui/rust-2024/reserved-guarded-strings.stderr
+++ b/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