prefer build-time env vars over run-time values

Author: RalfJung

cargo-miri/bin.rs

@@ -655,16 +655,21 @@ fn phase_cargo_runner(binary: &Path, binary_args: env::Args) {
     let info: CrateRunInfo = serde_json::from_reader(file)
         .unwrap_or_else(|_| show_error(format!("file {:?} contains outdated or invalid JSON; try `cargo clean`", binary)));
 
-    // Set missing env vars. Looks like `build.rs` vars are still set at run-time, but
-    // `CARGO_BIN_EXE_*` are not. This means we can give the run-time environment precedence,
-    // to rather do too little than too much.
+    let mut cmd = miri();
+
+    // Set missing env vars. We prefer build-time env vars over run-time ones; see
+    // <https://github.com/rust-lang/miri/issues/1661> for the kind of issue that fixes.
     for (name, val) in info.env {
-        if env::var_os(&name).is_none() {
-            env::set_var(name, val);
+        if verbose {
+            if let Some(old_val) = env::var_os(&name) {
+                if old_val != val {
+                    eprintln!("[cargo-miri runner] Overwriting run-time env var {:?}={:?} with build-time value {:?}", name, old_val, val);
+                }
+            }
         }
+        cmd.env(name, val);
     }
 
-    let mut cmd = miri();
     // Forward rustc arguments.
     // We need to patch "--extern" filenames because we forced a check-only
     // build without cargo knowing about that: replace `.rlib` suffix by