try to add flag

config.example.toml

@@ -414,6 +414,9 @@
 # Specify the location of the Android NDK. Used when targeting Android.
 #android-ndk = "/path/to/android-ndk-r26d"
 
+# What custom diff tool to use for displaying compiletest tests.
+#display-diff-tool = "difft --color=always --background=light --display=side-by-side"
+
 # =============================================================================
 # General install configuration options
 # =============================================================================

src/bootstrap/src/core/build_steps/test.rs

@@ -1839,6 +1839,9 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
 
         let mut flags = if is_rustdoc { Vec::new() } else { vec!["-Crpath".to_string()] };
         flags.push(format!("-Cdebuginfo={}", builder.config.rust_debuginfo_level_tests));
+        if let Some(display_diff_tool) = &builder.config.display_diff_tool {
+            flags.push(format!(r#"--display-diff-tool="{display_diff_tool}""#));
+        }
         flags.extend(builder.config.cmd.compiletest_rustc_args().iter().map(|s| s.to_string()));
 
         if suite != "mir-opt" {

src/bootstrap/src/core/config/config.rs

@@ -365,6 +365,9 @@ pub struct Config {
     /// The paths to work with. For example: with `./x check foo bar` we get
     /// `paths=["foo", "bar"]`.
     pub paths: Vec<PathBuf>,
+
+    /// What custom diff tool to use for displaying compiletest tests.
+    pub display_diff_tool: Option<String>,
 }
 
 #[derive(Clone, Debug, Default)]
@@ -885,6 +888,7 @@ define_config! {
         metrics: Option<bool> = "metrics",
         android_ndk: Option<PathBuf> = "android-ndk",
         optimized_compiler_builtins: Option<bool> = "optimized-compiler-builtins",
+        display_diff_tool: Option<String> = "display-diff-tool",
     }
 }
 
@@ -1504,6 +1508,7 @@ impl Config {
             metrics: _,
             android_ndk,
             optimized_compiler_builtins,
+            display_diff_tool,
         } = toml.build.unwrap_or_default();
 
         if let Some(file_build) = build {
@@ -2141,6 +2146,7 @@ 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.display_diff_tool = display_diff_tool;
 
         let download_rustc = config.download_rustc_commit.is_some();
         // See https://github.com/rust-lang/compiler-team/issues/326

src/tools/compiletest/src/lib.rs

@@ -171,7 +171,9 @@ pub fn parse_config(args: Vec<String>) -> Config {
             "git-merge-commit-email",
             "email address used for finding merge commits",
             "EMAIL",
-        );
+        )
+        .optopt("", "display-diff-tool", "What custom diff tool to use for displaying compiletest tests.", "COMMAND")
+    ;
 
     let (argv0, args_) = args.split_first().unwrap();
     if args.len() == 1 || args[1] == "-h" || args[1] == "--help" {
@@ -360,7 +362,7 @@ pub fn parse_config(args: Vec<String>) -> Config {
         git_merge_commit_email: matches.opt_str("git-merge-commit-email").unwrap(),
 
         profiler_support: matches.opt_present("profiler-support"),
-        diff_command: env::var("COMPILETEST_DIFF_TOOL").ok(),
+        diff_command: matches.opt_str("display-diff-tool"),
     }
 }
 

src/tools/compiletest/src/runtest.rs

@@ -2555,7 +2555,8 @@ impl<'test> TestCx<'test> {
                             ));
                         }
                         Ok(output) => {
-                            io::stdout().lock().write_all(&output.stdout).unwrap();
+                            let output = String::from_utf8_lossy_owned(output.stdout).unwrap();
+                            print!("{output}");
                         }
                     }
                 } else {