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docs(fingerprint): Track each use of Metadata separately
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‎src/cargo/core/compiler/build_runner/compilation_files.rs‎

Lines changed: 20 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -41,24 +41,25 @@ impl fmt::Debug for UnitHash {
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}
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}
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/// The `Metadata` is a hash used to make unique file names for each unit in a
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/// build. It is also used for symbol mangling.
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/// [`Metadata`] tracks several [`UnitHash`]s, including
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/// [`Metadata::unit_id`], [`Metadata::c_metadata`], and [`Metadata::c_extra_filename`].
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///
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/// For example:
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/// We use a hash because it is an easy way to guarantee
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/// that all the inputs can be converted to a valid path.
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///
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/// [`Metadata::unit_id`] is used to uniquely identify a unit in the build graph.
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/// This serves as a similar role as [`Metadata::c_extra_filename`] in that it uniquely identifies output
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/// on the filesystem except that its always present.
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///
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/// [`Metadata::c_extra_filename`] is needed for cases like:
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/// - A project may depend on crate `A` and crate `B`, so the package name must be in the file name.
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/// - Similarly a project may depend on two versions of `A`, so the version must be in the file name.
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///
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/// In general this must include all things that need to be distinguished in different parts of
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/// This also acts as the main layer of caching provided by Cargo
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/// so this must include all things that need to be distinguished in different parts of
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/// the same build. This is absolutely required or we override things before
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/// we get chance to use them.
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///
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/// It is also used for symbol mangling, because if you have two versions of
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/// the same crate linked together, their symbols need to be differentiated.
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///
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/// We use a hash because it is an easy way to guarantee
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/// that all the inputs can be converted to a valid path.
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///
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/// This also acts as the main layer of caching provided by Cargo.
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/// For example, we want to cache `cargo build` and `cargo doc` separately, so that running one
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/// does not invalidate the artifacts for the other. We do this by including [`CompileMode`] in the
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/// hash, thus the artifacts go in different folders and do not override each other.
@@ -73,17 +74,20 @@ impl fmt::Debug for UnitHash {
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/// more space than needed. This makes not including something in `Metadata`
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/// a form of cache invalidation.
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///
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/// You should also avoid anything that would interfere with reproducible
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/// Note that the `Fingerprint` is in charge of tracking everything needed to determine if a
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/// rebuild is needed.
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///
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/// [`Metadata::c_metadata`] is used for symbol mangling, because if you have two versions of
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/// the same crate linked together, their symbols need to be differentiated.
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///
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/// You should avoid anything that would interfere with reproducible
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/// builds. For example, *any* absolute path should be avoided. This is one
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/// reason that `RUSTFLAGS` is not in `Metadata`, because it often has
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/// reason that `RUSTFLAGS` is not in [`Metadata::c_metadata`], because it often has
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/// absolute paths (like `--remap-path-prefix` which is fundamentally used for
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/// reproducible builds and has absolute paths in it). Also, in some cases the
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/// mangled symbols need to be stable between different builds with different
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/// settings. For example, profile-guided optimizations need to swap
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/// `RUSTFLAGS` between runs, but needs to keep the same symbol names.
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///
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/// Note that the `Fingerprint` is in charge of tracking everything needed to determine if a
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/// rebuild is needed.
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#[derive(Copy, Clone, Debug)]
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pub struct Metadata {
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unit_id: UnitHash,

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

Lines changed: 36 additions & 30 deletions
Original file line numberDiff line numberDiff line change
@@ -47,46 +47,48 @@
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//! reliable and reproducible builds at the cost of being complex, slow, and
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//! platform-dependent.
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//!
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//! ## Fingerprints and Metadata
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//! ## Fingerprints and [`UnitHash`]s
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//!
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//! [`Metadata`] tracks several [`UnitHash`]s, including
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//! [`Metadata::unit_id`], [`Metadata::c_metadata`], and [`Metadata::c_extra_filename`].
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//! See its documentation for more details.
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//!
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//! The [`Metadata`] hash is a hash added to the output filenames to isolate
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//! each unit. See its documentationfor more details.
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//! NOTE: Not all output files are isolated via filename hashes (like dylibs).
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//! The fingerprint directory uses a hash, but sometimes units share the same
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//! fingerprint directory (when they don't have Metadata) so care should be
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//! taken to handle this!
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//!
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//! Fingerprints and Metadata are similar, and track some of the same things.
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//! The Metadata contains information that is required to keep Units separate.
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//! Fingerprints and [`UnitHash`]s are similar, and track some of the same things.
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//! [`UnitHash`]s contains information that is required to keep Units separate.
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//! The Fingerprint includes additional information that should cause a
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//! recompile, but it is desired to reuse the same filenames. A comparison
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//! of what is tracked:
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//!
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//! Value | Fingerprint | Metadata
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//! -------------------------------------------|-------------|----------
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//! rustc | ✓ | ✓
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//! [`Profile`] | ✓ | ✓
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//! `cargo rustc` extra args | ✓ |
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//! [`CompileMode`] | ✓ | ✓
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//! Target Name | ✓ | ✓
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//! `TargetKind` (bin/lib/etc.) | ✓ | ✓
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//! Enabled Features | ✓ | ✓
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//! Declared Features | ✓ |
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//! Immediate dependency’s hashes | ✓[^1] | ✓
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//! [`CompileKind`] (host/target) | ✓ | ✓
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//! `__CARGO_DEFAULT_LIB_METADATA`[^4] | | ✓
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//! `package_id` | | ✓
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//! authors, description, homepage, repo | ✓ |
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//! Target src path relative to ws | ✓ |
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//! Target flags (test/bench/for_host/edition) | ✓ |
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//! -C incremental=… flag | ✓ |
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//! mtime of sources | ✓[^3] |
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//! RUSTFLAGS/RUSTDOCFLAGS | ✓ |
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//! [`Lto`] flags | ✓ | ✓
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//! config settings[^5] | ✓ |
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//! `is_std` | | ✓
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//! `[lints]` table[^6] | ✓ |
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//! `[lints.rust.unexpected_cfgs.check-cfg]` | ✓ |
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//! Value | Fingerprint | `Metadata::unit_id` | `Metadata::c_metadata` | `Metadata::c_extra_filename`
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//! -------------------------------------------|-------------|---------------------|------------------------|----------
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//! rustc | ✓ | ✓ | ✓ | ✓
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//! [`Profile`] | ✓ | ✓ | ✓ | ✓
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//! `cargo rustc` extra args | ✓ | | |
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//! [`CompileMode`] | ✓ | ✓ | ✓ | ✓
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//! Target Name | ✓ | ✓ | ✓ | ✓
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//! `TargetKind` (bin/lib/etc.) | ✓ | ✓ | ✓ | ✓
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//! Enabled Features | ✓ | ✓ | ✓ | ✓
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//! Declared Features | ✓ | | |
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//! Immediate dependency’s hashes | ✓[^1] | ✓ | ✓ | ✓
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//! [`CompileKind`] (host/target) | ✓ | ✓ | ✓ | ✓
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//! `__CARGO_DEFAULT_LIB_METADATA`[^4] | | ✓ | ✓ | ✓
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//! `package_id` | | ✓ | ✓ | ✓
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//! authors, description, homepage, repo | ✓ | | |
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//! Target src path relative to ws | ✓ | | |
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//! Target flags (test/bench/for_host/edition) | ✓ | | |
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//! -C incremental=… flag | ✓ | | |
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//! mtime of sources | ✓[^3] | | |
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//! RUSTFLAGS/RUSTDOCFLAGS | ✓ | | |
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//! [`Lto`] flags | ✓ | ✓ | ✓ | ✓
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//! config settings[^5] | ✓ | | |
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//! `is_std` | | ✓ | ✓ | ✓
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//! `[lints]` table[^6] | ✓ | | |
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//! `[lints.rust.unexpected_cfgs.check-cfg]` | ✓ | | |
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//!
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//! [^1]: Build script and bin dependencies are not included.
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//!
@@ -348,6 +350,10 @@
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//!
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//! [`check_filesystem`]: Fingerprint::check_filesystem
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//! [`Metadata`]: crate::core::compiler::Metadata
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//! [`Metadata::unit_id`]: crate::core::compiler::Metadata::unit_id
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//! [`Metadata::c_metadata`]: crate::core::compiler::Metadata::c_metadata
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//! [`Metadata::c_extra_filename`]: crate::core::compiler::Metadata::c_extra_filename
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//! [`UnitHash`]: crate::core::compiler::UnitHash
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//! [`Profile`]: crate::core::profiles::Profile
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//! [`CompileMode`]: crate::core::compiler::CompileMode
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//! [`Lto`]: crate::core::compiler::Lto

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