allow overwriting the output of rustc --version

compiler/rustc_driver_impl/src/lib.rs

@@ -909,6 +909,15 @@ pub fn version_at_macro_invocation(
 ) {
     let verbose = matches.opt_present("verbose");
 
+    let mut version = version;
+    let mut release = release;
+    let tmp;
+    if let Ok(force_version) = std::env::var("RUSTC_OVERRIDE_VERSION_STRING") {
+        tmp = force_version;
+        version = &tmp;
+        release = &tmp;
+    }
+
     safe_println!("{binary} {version}");
 
     if verbose {

src/tools/compiletest/src/header.rs

@@ -330,6 +330,7 @@ impl TestProps {
     pub fn from_file(testfile: &Path, revision: Option<&str>, config: &Config) -> Self {
         let mut props = TestProps::new();
         props.load_from(testfile, revision, config);
+        props.exec_env.push(("RUSTC".to_string(), config.rustc_path.display().to_string()));
 
         match (props.pass_mode, props.fail_mode) {
             (None, None) if config.mode == Mode::Ui => props.fail_mode = Some(FailMode::Check),

tests/ui/feature-gates/version_check.rs

@@ -0,0 +1,17 @@
+//@ run-pass
+//@ only-linux
+//@ only-x86
+// FIXME: this should be more like //@ needs-subprocesses
+use std::process::Command;
+
+fn main() {
+    let signalled_version = "Ceci n'est pas une rustc";
+    let version = Command::new(std::env::var_os("RUSTC").unwrap())
+        .env("RUSTC_OVERRIDE_VERSION_STRING", signalled_version)
+        .arg("--version")
+        .output()
+        .unwrap()
+        .stdout;
+    let version = std::str::from_utf8(&version).unwrap().strip_prefix("rustc ").unwrap().trim_end();
+    assert_eq!(version, signalled_version);
+}