Do not pass --target if user is not using --target

build.rs

@@ -5,7 +5,34 @@ use std::path::Path;
 
 fn main() -> io::Result<()> {
     let out_dir = env::var_os("OUT_DIR").unwrap();
-    let target = env::var("TARGET").ok();
+    let mut target = env::var("TARGET").ok();
+
+    // When --target flag is passed, cargo does not pass RUSTFLAGS to rustc when
+    // building proc-macro and build script even if the host and target triples
+    // are the same. Therefore, if we always pass --target to cargo, tools such
+    // as coverage that require RUSTFLAGS do not work for tests run by trybuild.
+    //
+    // To avoid that problem, do not pass --target to cargo if we know that it
+    // has not been passed.
+    //
+    // Cargo does not have a way to tell the build script whether --target has
+    // been passed or not, so we use the following heuristic:
+    //
+    // - The host and target triples are the same.
+    // - And RUSTFLAGS is available when *building* the build script.
+    //
+    // Note that the second is when building, not when running. This is due to:
+    //
+    // - After rust-lang/cargo#9601, cargo does not pass RUSTFLAGS to the build
+    //   script when running.
+    // - CARGO_ENCODED_RUSTFLAGS, which was introduced in rust-lang/cargo#9601,
+    //   cannot be used for this purpose because it contains the value of
+    //   RUSTFLAGS even if --target is passed and the host and target triples
+    //   are the same.
+    if target == env::var("HOST").ok() && option_env!("RUSTFLAGS").is_some() {
+        target = None;
+    }
+
     let path = Path::new(&out_dir).join("target.rs");
     let value = match target {
         Some(target) => format!("Some({:?})", target),