Fix caching issue when building tools.

src/bootstrap/bin/rustc.rs

@@ -76,6 +76,10 @@ fn main() {
  cmd.env("RUST_BACKTRACE", "1");
  }
 
+ if let Ok(lint_flags) = env::var("RUSTC_LINT_FLAGS") {
+ cmd.args(lint_flags.split_whitespace());
+ }
+
  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,

src/bootstrap/builder.rs

@@ -1130,22 +1130,32 @@ impl<'a> Builder<'a> {
  cargo.env("RUSTC_VERBOSE", self.verbosity.to_string());
 
  if source_type == SourceType::InTree {
+ let mut lint_flags = Vec::new();
  // When extending this list, add the new lints to the RUSTFLAGS of the
  // build_bootstrap function of src/bootstrap/bootstrap.py as well as
  // some code doesn't go through this `rustc` wrapper.
- rustflags.arg("-Wrust_2018_idioms");
- rustflags.arg("-Wunused_lifetimes");
+ lint_flags.push("-Wrust_2018_idioms");
+ lint_flags.push("-Wunused_lifetimes");
 
  if self.config.deny_warnings {
- rustflags.arg("-Dwarnings");
+ lint_flags.push("-Dwarnings");
  }
 
  // FIXME(#58633) hide "unused attribute" errors in incremental
  // builds of the standard library, as the underlying checks are
  // not yet properly integrated with incremental recompilation.
  if mode == Mode::Std && compiler.stage == 0 && self.config.incremental {
- rustflags.arg("-Aunused-attributes");
+ lint_flags.push("-Aunused-attributes");
  }
+ // This does not use RUSTFLAGS due to caching issues with Cargo.
+ // Clippy is treated as an "in tree" tool, but shares the same
+ // cache as other "submodule" tools. With these options set in
+ // RUSTFLAGS, that causes *every* shared dependency to be rebuilt.
+ // By injecting this into the rustc wrapper, this circumvents
+ // Cargo's fingerprint detection. This is fine because lint flags
+ // are always ignored in dependencies. Eventually this should be
+ // fixed via better support from Cargo.
+ cargo.env("RUSTC_LINT_FLAGS", lint_flags.join(" "));
  }
 
  if let Mode::Rustc | Mode::Codegen = mode {