Pass certain flags when building all compiler/ crates.

Currently they are passed to the ones satisfying `Mode::Rustc` via
RUSTFLAGS. But RUSTFLAGS is ignored for proc macro crates. This commit
makes those flags get added directly to the command for proc macro
crates.

Author: nnethercote

src/bootstrap/src/bin/rustc.rs

@@ -21,8 +21,8 @@ use std::process::{Child, Command};
 use std::time::Instant;
 
 use shared_helpers::{
-    dylib_path, dylib_path_var, exe, maybe_dump, parse_rustc_stage, parse_rustc_verbose,
-    parse_value_from_args,
+    FLAGS_FOR_RUSTC, dylib_path, dylib_path_var, exe, maybe_dump, parse_rustc_stage,
+    parse_rustc_verbose, parse_value_from_args,
 };
 
 #[path = "../utils/shared_helpers.rs"]
@@ -145,6 +145,35 @@ fn main() {
         cmd.arg("-Z").arg("on-broken-pipe=kill");
     }
 
+    // In `cargo.rs` we add the `FLAGS_FOR_RUSTC` flags for all `compiler/`
+    // crates to RUSTFLAGS. However, RUSTFLAGS is ignored for `compiler/` proc
+    // macro crates, because RUSTFLAGS is ignored when `--target` is given. So,
+    // we add the `FLAGS_FOR_RUSTC` flags directly to the command here if they
+    // are also in RUSTFLAGS.
+    let crate_type = parse_value_from_args(&orig_args, "--crate-type");
+    if crate_type == Some("proc-macro") {
+        if let Ok(rustflags) = env::var("RUSTFLAGS") {
+            // Count how many of flags from `FLAGS_FOR_RUSTC` are in RUSTFLAGS.
+            let n = FLAGS_FOR_RUSTC.iter().filter(|&flag| rustflags.contains(flag)).count();
+            if n == FLAGS_FOR_RUSTC.len() {
+                // All the FLAGS_FOR_RUSTC flags are in RUSTFLAGS, which means
+                // this is a `compiler/` proc macro crate. Insert the flags
+                // directly into the command.
+                for flag in shared_helpers::FLAGS_FOR_RUSTC {
+                    cmd.arg(flag);
+                }
+            } else if n == 0 {
+                // None of the FLAGS_FOR_RUSTC flags are in RUSTFLAGS.
+                // Therefore it's a non-`compiler/` proc macro crate:
+                // third-party, clippy, etc. Do nothing.
+            } else {
+                // Some but not all of the FLAGS_FOR_RUSTC flags are in
+                // RUSTFLAGS. Huh?
+                panic!("RUSTFLAGS is missing some expected flags");
+            }
+        }
+    }
+
     if target.is_some() {
         // The stage0 compiler has a special sysroot distinct from what we
         // actually downloaded, so we just always pass the `--sysroot` option,
@@ -165,7 +194,6 @@ fn main() {
             cmd.arg("-C").arg("panic=abort");
         }
 
-        let crate_type = parse_value_from_args(&orig_args, "--crate-type");
         // `-Ztls-model=initial-exec` must not be applied to proc-macros, see
         // issue https://github.com/rust-lang/rust/issues/100530
         if env::var("RUSTC_TLS_MODEL_INITIAL_EXEC").is_ok()

src/bootstrap/src/core/builder/cargo.rs

@@ -1071,13 +1071,13 @@ impl Builder<'_> {
             rustdocflags.arg("-Wrustdoc::invalid_codeblock_attributes");
         }
 
+        // Add some flags to RUSTFLAGS for `compiler/` crates. Note: proc macro
+        // crates need additional special handling. See the `FLAGS_FOR_RUSTC`
+        // use in `rustc.rs` for details.
         if mode == Mode::Rustc {
-            rustflags.arg("-Wrustc::internal");
-            rustflags.arg("-Drustc::symbol_intern_string_literal");
-            // FIXME(edition_2024): Change this to `-Wrust_2024_idioms` when all
-            // of the individual lints are satisfied.
-            rustflags.arg("-Wkeyword_idents_2024");
-            rustflags.arg("-Wunsafe_op_in_unsafe_fn");
+            for flag in crate::utils::shared_helpers::FLAGS_FOR_RUSTC {
+                rustflags.arg(flag);
+            }
         }
 
         if self.config.rust_frame_pointers {

src/bootstrap/src/utils/shared_helpers.rs

@@ -113,3 +113,13 @@ pub fn parse_value_from_args<'a>(args: &'a [OsString], key: &str) -> Option<&'a
 
     None
 }
+
+// Flags used to compile all crates in `compiler/`.
+pub const FLAGS_FOR_RUSTC: &[&'static str] = &[
+    "-Wrustc::internal",
+    "-Drustc::symbol_intern_string_literal",
+    // FIXME(edition_2024): Change this to `-Wrust_2024_idioms` when all of the
+    // individual lints are satisfied.
+    "-Wkeyword_idents_2024",
+    "-Wunsafe_op_in_unsafe_fn",
+];