Rollup merge of #131181 - dev-ardi:custom-differ, r=jieyouxu

Compiletest: Custom differ

This adds support for a custom differ for compiletests. It’s purely visual and helps produce cleaner output when UI tests fail.

I’m using an environment variable for now since it’s experimental and I don’t want to drill the cli arguments all the way down. Also did a bit of general cleanup while I was at it.

This is how it looks [with debug info silenced](https://github.com/rust-lang/rust/pull/131182) (#131182)
`COMPILETEST_DIFF_TOOL="/usr/bin/env difft --color always --background light --display side-by-side" ./x test tests/ui/parser`
![image](https://github.com/user-attachments/assets/f740ce50-7564-4469-be0a-86e24bc50eb8)

3 files changed, 14 insertions, 0 deletions

diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index 27acaff8255..2ad1b39a87c 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -1836,6 +1836,9 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         if builder.config.cmd.only_modified() {
             cmd.arg("--only-modified");
         }
+        if let Some(compiletest_diff_tool) = &builder.config.compiletest_diff_tool {
+            cmd.arg("--compiletest-diff-tool").arg(compiletest_diff_tool);
+        }
 
         let mut flags = if is_rustdoc { Vec::new() } else { vec!["-Crpath".to_string()] };
         flags.push(format!("-Cdebuginfo={}", builder.config.rust_debuginfo_level_tests));
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index 9466b046a65..444c97a2048 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -368,6 +368,9 @@ pub struct Config {
     /// The paths to work with. For example: with `./x check foo bar` we get
     /// `paths=["foo", "bar"]`.
     pub paths: Vec<PathBuf>,
+
+    /// Command for visual diff display, e.g. `diff-tool --color=always`.
+    pub compiletest_diff_tool: Option<String>,
 }
 
 #[derive(Clone, Debug, Default)]
@@ -892,6 +895,7 @@ define_config! {
         android_ndk: Option<PathBuf> = "android-ndk",
         optimized_compiler_builtins: Option<bool> = "optimized-compiler-builtins",
         jobs: Option<u32> = "jobs",
+        compiletest_diff_tool: Option<String> = "compiletest-diff-tool",
     }
 }
 
@@ -1512,6 +1516,7 @@ impl Config {
             android_ndk,
             optimized_compiler_builtins,
             jobs,
+            compiletest_diff_tool,
         } = toml.build.unwrap_or_default();
 
         config.jobs = Some(threads_from_config(flags.jobs.unwrap_or(jobs.unwrap_or(0))));
@@ -2158,6 +2163,7 @@ impl Config {
         config.rust_debuginfo_level_tests = debuginfo_level_tests.unwrap_or(DebuginfoLevel::None);
         config.optimized_compiler_builtins =
             optimized_compiler_builtins.unwrap_or(config.channel != "dev");
+        config.compiletest_diff_tool = compiletest_diff_tool;
 
         let download_rustc = config.download_rustc_commit.is_some();
         // See https://github.com/rust-lang/compiler-team/issues/326
diff --git a/src/bootstrap/src/utils/change_tracker.rs b/src/bootstrap/src/utils/change_tracker.rs
index 9169bc90a45..fac24572a87 100644
--- a/src/bootstrap/src/utils/change_tracker.rs
+++ b/src/bootstrap/src/utils/change_tracker.rs
@@ -280,4 +280,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "Allow setting `--jobs` in config.toml with `build.jobs`.",
     },
+    ChangeInfo {
+        change_id: 131181,
+        severity: ChangeSeverity::Info,
+        summary: "New option `build.compiletest-diff-tool` that adds support for a custom differ for compiletest",
+    },
 ];