Pass more host flags using RUSTC_HOST_FLAGS

src/bootstrap/bin/_helper.rs

@@ -14,6 +14,7 @@ fn parse_rustc_verbose() -> usize {
 /// Parses the value of the "RUSTC_STAGE" environment variable and returns it as a `String`.
 ///
 /// If "RUSTC_STAGE" was not set, the program will be terminated with 101.
+#[allow(unused)]
 fn parse_rustc_stage() -> String {
     std::env::var("RUSTC_STAGE").unwrap_or_else(|_| {
         // Don't panic here; it's reasonable to try and run these shims directly. Give a helpful error instead.

src/bootstrap/bin/rustc.rs

@@ -27,7 +27,6 @@ fn main() {
     let args = env::args_os().skip(1).collect::<Vec<_>>();
     let arg = |name| args.windows(2).find(|args| args[0] == name).and_then(|args| args[1].to_str());
 
-    let stage = parse_rustc_stage();
     let verbose = parse_rustc_verbose();
 
     // Detect whether or not we're a build script depending on whether --target
@@ -108,27 +107,13 @@ fn main() {
             cmd.arg("-Ztls-model=initial-exec");
         }
     } else {
-        // FIXME(rust-lang/cargo#5754) we shouldn't be using special env vars
-        // here, but rather Cargo should know what flags to pass rustc itself.
-
         // Find any host flags that were passed by bootstrap.
         // The flags are stored in a RUSTC_HOST_FLAGS variable, separated by spaces.
         if let Ok(flags) = std::env::var("RUSTC_HOST_FLAGS") {
             for flag in flags.split(' ') {
                 cmd.arg(flag);
             }
         }
-
-        // Cargo doesn't pass RUSTFLAGS to proc_macros:
-        // https://github.com/rust-lang/cargo/issues/4423
-        // Thus, if we are on stage 0, we explicitly set `--cfg=bootstrap`.
-        // We also declare that the flag is expected, which we need to do to not
-        // get warnings about it being unexpected.
-        if stage == "0" {
-            cmd.arg("--cfg=bootstrap");
-        }
-        cmd.arg("-Zunstable-options");
-        cmd.arg("--check-cfg=values(bootstrap)");
     }
 
     if let Ok(map) = env::var("RUSTC_DEBUGINFO_MAP") {

src/bootstrap/builder.rs

@@ -1438,6 +1438,20 @@ impl<'a> Builder<'a> {
             }
         }
 
+        // FIXME(rust-lang/cargo#5754) we shouldn't be using special command arguments
+        // to the host invocation here, but rather Cargo should know what flags to pass rustc
+        // itself.
+        if stage == 0 {
+            hostflags.arg("--cfg=bootstrap");
+        }
+        // Cargo doesn't pass RUSTFLAGS to proc_macros:
+        // https://github.com/rust-lang/cargo/issues/4423
+        // Thus, if we are on stage 0, we explicitly set `--cfg=bootstrap`.
+        // We also declare that the flag is expected, which we need to do to not
+        // get warnings about it being unexpected.
+        hostflags.arg("-Zunstable-options");
+        hostflags.arg("--check-cfg=values(bootstrap)");
+
         // FIXME: It might be better to use the same value for both `RUSTFLAGS` and `RUSTDOCFLAGS`,
         // but this breaks CI. At the very least, stage0 `rustdoc` needs `--cfg bootstrap`. See
         // #71458.