Add cfg to always treat target as host

build.rs

@@ -5,39 +5,12 @@ use std::path::Path;
 
 fn main() -> io::Result<()> {
     let out_dir = env::var_os("OUT_DIR").unwrap();
-    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 target = env::var("TARGET").ok();
     let path = Path::new(&out_dir).join("target.rs");
     let value = match target {
         Some(target) => format!(r#"Some("{}")"#, target.escape_debug()),
         None => "None".to_owned(),
     };
-    let content = format!("const TARGET: Option<&'static str> = {};", value);
+    let content = format!("const TARGET: Option<&str> = {};", value);
     fs::write(path, content)
 }

src/lib.rs

@@ -245,6 +245,22 @@ mod normalize;
 mod run;
 mod rustflags;
 
+// 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.
+//
+// Currently, cargo does not have a way to tell the build script whether
+// --target has been passed or not, and there is no heuristic that can handle
+// this well.
+//
+// Therefore, expose a cfg to always treat the target as host.
+#[cfg(trybuild_no_target)]
+const TARGET: Option<&str> = None;
+#[cfg(not(trybuild_no_target))]
 include!(concat!(env!("OUT_DIR"), "/target.rs"));
 
 use std::cell::RefCell;