support passing -C instrument-coverage in RUSTFLAGS

Author: davidhewitt

src/rustflags.rs

@@ -8,5 +8,14 @@ pub(crate) fn toml() -> toml::Value {
         rustflags.push(lint);
     }
 
+    if let Some(flags) = std::env::var("RUSTFLAGS").ok() {
+        // TODO: could parse this properly and allowlist (or blocklist?)
+        // certain flags. This is good enough to at least support
+        // `cargo-llvm-cov`.
+        if flags.contains("-C instrument-coverage") {
+            rustflags.extend(["-C", "instrument-coverage"]);
+        }
+    }
+
     toml::Value::try_from(rustflags).unwrap()
 }