add an option for a custom differ

Author: Orion Gonzalez

src/tools/compiletest/src/common.rs

@@ -395,6 +395,8 @@ pub struct Config {
     /// True if the profiler runtime is enabled for this target.
     /// Used by the "needs-profiler-support" header in test files.
     pub profiler_support: bool,
+
+    pub diff_command: Option<String>,
 }
 
 impl Config {

src/tools/compiletest/src/lib.rs

@@ -360,6 +360,7 @@ 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(),
     }
 }