feat: Right align line numbers

[Muscraft]

As part of my work on getting annotate-snipptes to be used as rustc's renderer, I realized that rustc left-aligned line numbers, while annotate-snippets right-aligned them. This PR switches rustc to right-align the line numbers, matching annotate-snippets. In practice, this change isn't very noticeable in day-to-day output, as it only shows up when a diagnostic span contains line numbers with different lengths (9->10, 99->100, 999->1000, etc.).

rustc

error[E0412]: cannot find type `F` in this scope
  --> $DIR/ui-testing-optout.rs:92:10
   |
4  | type A = B;
   | ----------- similarly named type alias `A` defined here
...
92 | type E = F;
   |          ^ help: a type alias with a similar name exists: `A`

annotate-snippets

error[E0412]: cannot find type `F` in this scope
  --> $DIR/ui-testing-optout.rs:92:10
   |
 4 | type A = B;
   | ----------- similarly named type alias `A` defined here
...
92 | type E = F;
   |          ^ help: a type alias with a similar name exists: `A`

r? @compiler-errors

[rustbot]

compiler-errors is not on the review rotation at the moment.
They may take a while to respond.

[rustbot]

This PR modifies run-make tests.

cc @jieyouxu

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

##[endgroup]
[TIMING] core::build_steps::tool::ToolBuild { build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "tidy", path: "src/tools/tidy", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 12.752
[TIMING] core::build_steps::tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_errors/src/emitter.rs:2127:
                 // Account for a suggestion to completely remove a line(s) with whitespace (#94192).
                 let line_end = sm.lookup_char_pos(parts[0].span.hi()).line;
                 for line in line_start..=line_end {
-                    self.draw_line_num(&mut buffer, line, row_num -1 + line - line_start, max_line_num_len);
+                    self.draw_line_num(
+                        &mut buffer,
+                        line,
+                        row_num - 1 + line - line_start,
+                        max_line_num_len,
+                    );
                     buffer.puts(
                         row_num - 1 + line - line_start,
                         max_line_num_len + 1,
Diff in /checkout/compiler/rustc_errors/src/emitter.rs:2623:
             let last_line_index = file_lines.lines[file_lines.lines.len() - 1].line_index;
             let last_line = &file_lines.file.get_line(last_line_index).unwrap();
             if last_line != line_to_add {
-                self.draw_line_num(buffer, line_num + file_lines.lines.len() - 1, *row_num - 1, max_line_num_len);
+                self.draw_line_num(
+                    buffer,
+                    line_num + file_lines.lines.len() - 1,
+                    *row_num - 1,
+                    max_line_num_len,
+                );
                 buffer.puts(*row_num - 1, max_line_num_len + 1, "- ", Style::Removal);
                 buffer.puts(
                     *row_num - 1,
fmt: checked 6266 files
Build completed unsuccessfully in 0:00:48
  local time: Mon Jul 28 22:30:25 UTC 2025
  network time: Mon, 28 Jul 2025 22:30:25 GMT
##[error]Process completed with exit code 1.

[compiler-errors]

@bors r+ rollup

[bors]

📌 Commit aa97672 has been approved by compiler-errors

It is now in the queue for this repository.