add an option for a custom differ

src/tools/compiletest/src/common.rs

@@ -387,6 +387,8 @@ pub struct Config {
     /// True if the profiler runtime is enabled for this target.
     /// Used by the "needs-profiler-runtime" directive in test files.
     pub profiler_runtime: bool,
+
+    pub diff_command: Option<String>,
 }
 
 impl Config {

src/tools/compiletest/src/lib.rs

@@ -364,6 +364,7 @@ fn make_absolute(path: PathBuf) -> PathBuf {
         git_merge_commit_email: matches.opt_str("git-merge-commit-email").unwrap(),
 
         profiler_runtime: matches.opt_present("profiler-runtime"),
+        diff_command: env::var("COMPILETEST_DIFF_TOOL").ok(),
     }
 }