fix(fingerprint): Don't throwaway the cache on RUSTFLAGS changes (rust-lang#14830)

### What does this PR try to resolve?

Fixes rust-lang#8716

This splits `-C metdata` and `-C extra-filename` and adds `RUSTFLAGS` to
`-C extra-filename` in the hope that we can still make PGO and
reproducible builds work while caching artifacts per RUSTFLAGS used.

### How should we test and review this PR?

### Additional information

Author: weihanglo

src/cargo/core/compiler/build_runner/compilation_files.rs

@@ -41,24 +41,25 @@ impl fmt::Debug for UnitHash {
     }
 }
 
-/// The `Metadata` is a hash used to make unique file names for each unit in a
-/// build. It is also used for symbol mangling.
+/// [`Metadata`] tracks several [`UnitHash`]s, including
+/// [`Metadata::unit_id`], [`Metadata::c_metadata`], and [`Metadata::c_extra_filename`].
 ///
-/// For example:
+/// We use a hash because it is an easy way to guarantee
+/// that all the inputs can be converted to a valid path.
+///
+/// [`Metadata::unit_id`] is used to uniquely identify a unit in the build graph.
+/// This serves as a similar role as [`Metadata::c_extra_filename`] in that it uniquely identifies output
+/// on the filesystem except that its always present.
+///
+/// [`Metadata::c_extra_filename`] is needed for cases like:
 /// - A project may depend on crate `A` and crate `B`, so the package name must be in the file name.
 /// - Similarly a project may depend on two versions of `A`, so the version must be in the file name.
 ///
-/// In general this must include all things that need to be distinguished in different parts of
+/// This also acts as the main layer of caching provided by Cargo
+/// so this must include all things that need to be distinguished in different parts of
 /// the same build. This is absolutely required or we override things before
 /// we get chance to use them.
 ///
-/// It is also used for symbol mangling, because if you have two versions of
-/// the same crate linked together, their symbols need to be differentiated.
-///
-/// We use a hash because it is an easy way to guarantee
-/// that all the inputs can be converted to a valid path.
-///
-/// This also acts as the main layer of caching provided by Cargo.
 /// For example, we want to cache `cargo build` and `cargo doc` separately, so that running one
 /// does not invalidate the artifacts for the other. We do this by including [`CompileMode`] in the
 /// hash, thus the artifacts go in different folders and do not override each other.
@@ -73,37 +74,41 @@ impl fmt::Debug for UnitHash {
 /// more space than needed. This makes not including something in `Metadata`
 /// a form of cache invalidation.
 ///
-/// You should also avoid anything that would interfere with reproducible
+/// Note that the `Fingerprint` is in charge of tracking everything needed to determine if a
+/// rebuild is needed.
+///
+/// [`Metadata::c_metadata`] is used for symbol mangling, because if you have two versions of
+/// the same crate linked together, their symbols need to be differentiated.
+///
+/// You should avoid anything that would interfere with reproducible
 /// builds. For example, *any* absolute path should be avoided. This is one
-/// reason that `RUSTFLAGS` is not in `Metadata`, because it often has
+/// reason that `RUSTFLAGS` is not in [`Metadata::c_metadata`], because it often has
 /// absolute paths (like `--remap-path-prefix` which is fundamentally used for
 /// reproducible builds and has absolute paths in it). Also, in some cases the
 /// mangled symbols need to be stable between different builds with different
 /// settings. For example, profile-guided optimizations need to swap
 /// `RUSTFLAGS` between runs, but needs to keep the same symbol names.
-///
-/// Note that the `Fingerprint` is in charge of tracking everything needed to determine if a
-/// rebuild is needed.
 #[derive(Copy, Clone, Debug)]
 pub struct Metadata {
-    meta_hash: UnitHash,
-    use_extra_filename: bool,
+    unit_id: UnitHash,
+    c_metadata: UnitHash,
+    c_extra_filename: Option<UnitHash>,
 }
 
 impl Metadata {
     /// A hash to identify a given [`Unit`] in the build graph
     pub fn unit_id(&self) -> UnitHash {
-        self.meta_hash
+        self.unit_id
     }
 
     /// A hash to add to symbol naming through `-C metadata`
     pub fn c_metadata(&self) -> UnitHash {
-        self.meta_hash
+        self.c_metadata
     }
 
     /// A hash to add to file names through `-C extra-filename`
     pub fn c_extra_filename(&self) -> Option<UnitHash> {
-        self.use_extra_filename.then_some(self.meta_hash)
+        self.c_extra_filename
     }
 }
 
@@ -588,56 +593,54 @@ fn compute_metadata(
     metas: &mut HashMap<Unit, Metadata>,
 ) -> Metadata {
     let bcx = &build_runner.bcx;
-    let mut hasher = StableHasher::new();
+    let deps_metadata = build_runner
+        .unit_deps(unit)
+        .iter()
+        .map(|dep| *metadata_of(&dep.unit, build_runner, metas))
+        .collect::<Vec<_>>();
+    let use_extra_filename = use_extra_filename(bcx, unit);
 
-    METADATA_VERSION.hash(&mut hasher);
+    let mut shared_hasher = StableHasher::new();
+
+    METADATA_VERSION.hash(&mut shared_hasher);
 
     // Unique metadata per (name, source, version) triple. This'll allow us
     // to pull crates from anywhere without worrying about conflicts.
     unit.pkg
         .package_id()
         .stable_hash(bcx.ws.root())
-        .hash(&mut hasher);
+        .hash(&mut shared_hasher);
 
     // Also mix in enabled features to our metadata. This'll ensure that
     // when changing feature sets each lib is separately cached.
-    unit.features.hash(&mut hasher);
-
-    // Mix in the target-metadata of all the dependencies of this target.
-    let mut deps_metadata = build_runner
-        .unit_deps(unit)
-        .iter()
-        .map(|dep| metadata_of(&dep.unit, build_runner, metas).meta_hash)
-        .collect::<Vec<_>>();
-    deps_metadata.sort();
-    deps_metadata.hash(&mut hasher);
+    unit.features.hash(&mut shared_hasher);
 
     // Throw in the profile we're compiling with. This helps caching
     // `panic=abort` and `panic=unwind` artifacts, additionally with various
     // settings like debuginfo and whatnot.
-    unit.profile.hash(&mut hasher);
-    unit.mode.hash(&mut hasher);
-    build_runner.lto[unit].hash(&mut hasher);
+    unit.profile.hash(&mut shared_hasher);
+    unit.mode.hash(&mut shared_hasher);
+    build_runner.lto[unit].hash(&mut shared_hasher);
 
     // Artifacts compiled for the host should have a different
     // metadata piece than those compiled for the target, so make sure
     // we throw in the unit's `kind` as well.  Use `fingerprint_hash`
     // so that the StableHash doesn't change based on the pathnames
     // of the custom target JSON spec files.
-    unit.kind.fingerprint_hash().hash(&mut hasher);
+    unit.kind.fingerprint_hash().hash(&mut shared_hasher);
 
     // Finally throw in the target name/kind. This ensures that concurrent
     // compiles of targets in the same crate don't collide.
-    unit.target.name().hash(&mut hasher);
-    unit.target.kind().hash(&mut hasher);
+    unit.target.name().hash(&mut shared_hasher);
+    unit.target.kind().hash(&mut shared_hasher);
 
-    hash_rustc_version(bcx, &mut hasher, unit);
+    hash_rustc_version(bcx, &mut shared_hasher, unit);
 
     if build_runner.bcx.ws.is_member(&unit.pkg) {
         // This is primarily here for clippy. This ensures that the clippy
         // artifacts are separate from the `check` ones.
         if let Some(path) = &build_runner.bcx.rustc().workspace_wrapper {
-            path.hash(&mut hasher);
+            path.hash(&mut shared_hasher);
         }
     }
 
@@ -648,7 +651,7 @@ fn compute_metadata(
         .gctx
         .get_env("__CARGO_DEFAULT_LIB_METADATA")
     {
-        channel.hash(&mut hasher);
+        channel.hash(&mut shared_hasher);
     }
 
     // std units need to be kept separate from user dependencies. std crates
@@ -658,7 +661,7 @@ fn compute_metadata(
     // don't need unstable support. A future experiment might be to set
     // `is_std` to false for build dependencies so that they can be shared
     // with user dependencies.
-    unit.is_std.hash(&mut hasher);
+    unit.is_std.hash(&mut shared_hasher);
 
     // While we don't hash RUSTFLAGS because it may contain absolute paths that
     // hurts reproducibility, we track whether a unit's RUSTFLAGS is from host
@@ -677,15 +680,71 @@ fn compute_metadata(
                 .target_config(CompileKind::Host)
                 .links_overrides
                 != unit.links_overrides;
-        target_configs_are_different.hash(&mut hasher);
+        target_configs_are_different.hash(&mut shared_hasher);
     }
 
+    let mut c_metadata_hasher = shared_hasher.clone();
+    // Mix in the target-metadata of all the dependencies of this target.
+    let mut dep_c_metadata_hashes = deps_metadata
+        .iter()
+        .map(|m| m.c_metadata)
+        .collect::<Vec<_>>();
+    dep_c_metadata_hashes.sort();
+    dep_c_metadata_hashes.hash(&mut c_metadata_hasher);
+
+    let mut c_extra_filename_hasher = shared_hasher.clone();
+    // Mix in the target-metadata of all the dependencies of this target.
+    let mut dep_c_extra_filename_hashes = deps_metadata
+        .iter()
+        .map(|m| m.c_extra_filename)
+        .collect::<Vec<_>>();
+    dep_c_extra_filename_hashes.sort();
+    dep_c_extra_filename_hashes.hash(&mut c_extra_filename_hasher);
+    // Avoid trashing the caches on RUSTFLAGS changing via `c_extra_filename`
+    //
+    // Limited to `c_extra_filename` to help with reproducible build / PGO issues.
+    let default = Vec::new();
+    let extra_args = build_runner.bcx.extra_args_for(unit).unwrap_or(&default);
+    if !has_remap_path_prefix(&extra_args) {
+        extra_args.hash(&mut c_extra_filename_hasher);
+    }
+    if unit.mode.is_doc() || unit.mode.is_doc_scrape() {
+        if !has_remap_path_prefix(&unit.rustdocflags) {
+            unit.rustdocflags.hash(&mut c_extra_filename_hasher);
+        }
+    } else {
+        if !has_remap_path_prefix(&unit.rustflags) {
+            unit.rustflags.hash(&mut c_extra_filename_hasher);
+        }
+    }
+
+    let c_metadata = UnitHash(c_metadata_hasher.finish());
+    let c_extra_filename = UnitHash(c_extra_filename_hasher.finish());
+    let unit_id = c_extra_filename;
+
+    let c_extra_filename = use_extra_filename.then_some(c_extra_filename);
+
     Metadata {
-        meta_hash: UnitHash(hasher.finish()),
-        use_extra_filename: use_extra_filename(bcx, unit),
+        unit_id,
+        c_metadata,
+        c_extra_filename,
     }
 }
 
+/// HACK: Detect the *potential* presence of `--remap-path-prefix`
+///
+/// As CLI parsing is contextual and dependent on the CLI definition to understand the context, we
+/// can't say for sure whether `--remap-path-prefix` is present, so we guess if anything looks like
+/// it.
+/// If we could, we'd strip it out for hashing.
+/// Instead, we use this to avoid hashing rustflags if it might be present to avoid the risk of taking
+/// a flag that is trying to make things reproducible and making things less reproducible by the
+/// `-Cextra-filename` showing up in the rlib, even with `split-debuginfo`.
+fn has_remap_path_prefix(args: &[String]) -> bool {
+    args.iter()
+        .any(|s| s.starts_with("--remap-path-prefix=") || s == "--remap-path-prefix")
+}
+
 /// Hash the version of rustc being used during the build process.
 fn hash_rustc_version(bcx: &BuildContext<'_, '_>, hasher: &mut StableHasher, unit: &Unit) {
     let vers = &bcx.rustc().version;

src/cargo/core/compiler/fingerprint/mod.rs

@@ -47,46 +47,48 @@
 //! reliable and reproducible builds at the cost of being complex, slow, and
 //! platform-dependent.
 //!
-//! ## Fingerprints and Metadata
+//! ## Fingerprints and [`UnitHash`]s
+//!
+//! [`Metadata`] tracks several [`UnitHash`]s, including
+//! [`Metadata::unit_id`], [`Metadata::c_metadata`], and [`Metadata::c_extra_filename`].
+//! See its documentation for more details.
 //!
-//! The [`Metadata`] hash is a hash added to the output filenames to isolate
-//! each unit. See its documentationfor more details.
 //! NOTE: Not all output files are isolated via filename hashes (like dylibs).
 //! The fingerprint directory uses a hash, but sometimes units share the same
 //! fingerprint directory (when they don't have Metadata) so care should be
 //! taken to handle this!
 //!
-//! Fingerprints and Metadata are similar, and track some of the same things.
-//! The Metadata contains information that is required to keep Units separate.
+//! Fingerprints and [`UnitHash`]s are similar, and track some of the same things.
+//! [`UnitHash`]s contains information that is required to keep Units separate.
 //! The Fingerprint includes additional information that should cause a
 //! recompile, but it is desired to reuse the same filenames. A comparison
 //! of what is tracked:
 //!
-//! Value                                      | Fingerprint | Metadata
-//! -------------------------------------------|-------------|----------
-//! rustc                                      | ✓           | ✓
-//! [`Profile`]                                | ✓           | ✓
-//! `cargo rustc` extra args                   | ✓           |
-//! [`CompileMode`]                            | ✓           | ✓
-//! Target Name                                | ✓           | ✓
-//! `TargetKind` (bin/lib/etc.)                | ✓           | ✓
-//! Enabled Features                           | ✓           | ✓
-//! Declared Features                          | ✓           |
-//! Immediate dependency’s hashes              | ✓[^1]       | ✓
-//! [`CompileKind`] (host/target)              | ✓           | ✓
-//! `__CARGO_DEFAULT_LIB_METADATA`[^4]         |             | ✓
-//! `package_id`                               |             | ✓
-//! authors, description, homepage, repo       | ✓           |
-//! Target src path relative to ws             | ✓           |
-//! Target flags (test/bench/for_host/edition) | ✓           |
-//! -C incremental=… flag                      | ✓           |
-//! mtime of sources                           | ✓[^3]       |
-//! RUSTFLAGS/RUSTDOCFLAGS                     | ✓           |
-//! [`Lto`] flags                              | ✓           | ✓
-//! config settings[^5]                        | ✓           |
-//! `is_std`                                   |             | ✓
-//! `[lints]` table[^6]                        | ✓           |
-//! `[lints.rust.unexpected_cfgs.check-cfg]`   | ✓           |
+//! Value                                      | Fingerprint | `Metadata::unit_id` | `Metadata::c_metadata` | `Metadata::c_extra_filename`
+//! -------------------------------------------|-------------|---------------------|------------------------|----------
+//! rustc                                      | ✓           | ✓                   | ✓                      | ✓
+//! [`Profile`]                                | ✓           | ✓                   | ✓                      | ✓
+//! `cargo rustc` extra args                   | ✓           | ✓[^7]               |                        | ✓[^7]
+//! [`CompileMode`]                            | ✓           | ✓                   | ✓                      | ✓
+//! Target Name                                | ✓           | ✓                   | ✓                      | ✓
+//! `TargetKind` (bin/lib/etc.)                | ✓           | ✓                   | ✓                      | ✓
+//! Enabled Features                           | ✓           | ✓                   | ✓                      | ✓
+//! Declared Features                          | ✓           |                     |                        |
+//! Immediate dependency’s hashes              | ✓[^1]       | ✓                   | ✓                      | ✓
+//! [`CompileKind`] (host/target)              | ✓           | ✓                   | ✓                      | ✓
+//! `__CARGO_DEFAULT_LIB_METADATA`[^4]         |             | ✓                   | ✓                      | ✓
+//! `package_id`                               |             | ✓                   | ✓                      | ✓
+//! authors, description, homepage, repo       | ✓           |                     |                        |
+//! Target src path relative to ws             | ✓           |                     |                        |
+//! Target flags (test/bench/for_host/edition) | ✓           |                     |                        |
+//! -C incremental=… flag                      | ✓           |                     |                        |
+//! mtime of sources                           | ✓[^3]       |                     |                        |
+//! RUSTFLAGS/RUSTDOCFLAGS                     | ✓           | ✓[^7]               |                        | ✓[^7]
+//! [`Lto`] flags                              | ✓           | ✓                   | ✓                      | ✓
+//! config settings[^5]                        | ✓           |                     |                        |
+//! `is_std`                                   |             | ✓                   | ✓                      | ✓
+//! `[lints]` table[^6]                        | ✓           |                     |                        |
+//! `[lints.rust.unexpected_cfgs.check-cfg]`   | ✓           |                     |                        |
 //!
 //! [^1]: Build script and bin dependencies are not included.
 //!
@@ -100,6 +102,9 @@
 //!
 //! [^6]: Via [`Manifest::lint_rustflags`][crate::core::Manifest::lint_rustflags]
 //!
+//! [^7]: extra-flags and RUSTFLAGS are conditionally excluded when `--remap-path-prefix` is
+//!       present to avoid breaking build reproducibility while we wait for trim-paths
+//!
 //! When deciding what should go in the Metadata vs the Fingerprint, consider
 //! that some files (like dylibs) do not have a hash in their filename. Thus,
 //! if a value changes, only the fingerprint will detect the change (consider,
@@ -348,6 +353,10 @@
 //!
 //! [`check_filesystem`]: Fingerprint::check_filesystem
 //! [`Metadata`]: crate::core::compiler::Metadata
+//! [`Metadata::unit_id`]: crate::core::compiler::Metadata::unit_id
+//! [`Metadata::c_metadata`]: crate::core::compiler::Metadata::c_metadata
+//! [`Metadata::c_extra_filename`]: crate::core::compiler::Metadata::c_extra_filename
+//! [`UnitHash`]: crate::core::compiler::UnitHash
 //! [`Profile`]: crate::core::profiles::Profile
 //! [`CompileMode`]: crate::core::compiler::CompileMode
 //! [`Lto`]: crate::core::compiler::Lto

src/cargo/util/hasher.rs

@@ -6,6 +6,7 @@
 
 use std::hash::{Hasher, SipHasher};
 
+#[derive(Clone)]
 pub struct StableHasher(SipHasher);
 
 impl StableHasher {

tests/testsuite/config_include.rs

@@ -244,7 +244,6 @@ fn works_with_cli() {
     p.cargo("check -v -Z config-include")
         .masquerade_as_nightly_cargo(&["config-include"])
         .with_stderr_data(str![[r#"
-[DIRTY] foo v0.0.1 ([ROOT]/foo): the rustflags changed
 [CHECKING] foo v0.0.1 ([ROOT]/foo)
 [RUNNING] `rustc [..]-W unsafe-code -W unused`
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s