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Call of Testing

Also include anyone is following this issue and missed it, we put out a call of testing of the new v2 build-dir layout in a blog post here
See the blog post for details and testing instructions

Implementation:

• feat(unstable): Added -Zbuild-dir-new-layout unstable feature #15848
• feat(build-dir): Reorganize build-dir layout #15947
• fix(layout): Remove hashes from bins in new layout  #16351

Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#build-dir-new-layout

Resolves

• Shadowing binaries on Windows (Workspace binary artifacts are added to target/debug/deps on Windows #7919)
• Collisions between intermediate build artifacts (Output filename collision with dylib on windows release mode #8794, output filename collision with panic = "abort" #16673)
• Inconsistent use of -Cextra-filename (Consider changing hashing behavior for executables. #8332)
• Slow performance when intermediate artifacts accumulate (Presence of old builds can dramatically raise incremental build time, via deps dir size growth #16665)

Past efforts

• Unify build directories. #6668

Overview

• .fingerprint, deps/, build-script, and build-script-execution/ move under build/<pkgname>/<unit-id>
  • -L params to rustc were updated
  • cargo clean -p was updated
  • incremental/ is untouched
• -Cextra-filename is removed from all bins (fix(layout): Remove hashes from bins in new layout  #16351)

Known issues

• As a side effect, we pass a lot more parameters to rustc, likely making cargo -vv more annoying, similar to Reduce --verbose output of --check-cfg args #13941
• It also makes impossible to just copy paste commands into your shell if the limit is exceeded. See bjorn3's comment.
• This will break people relying on build-dir internals
  • See #t-cargo > build-dir tool compatibility @ 💬
• Increases the need for solutions for when people rely on build-dir internals
  • Access to compiler artifact notifications messages for custom --emit flags #13672
  • cargo ./target fills with outdated artifacts as toolchains are updated/changed #5026
  • Allow build scripts to stage final artifacts #13663
  • RFC: cargo-sbom rfcs#3553 (more precise determination of what is built than cargo metadata)
• cargo clean -p only cleans up files for the current layout rather than supporting both

Unresolved issues

Open questions

• Transition plan: while build-dir isn't stable, enough tools rely on the layout that we'd want to setup a transition plan so they can have time to test against the new layout and work to support both
• What do we call the directory? I said build/ as its all encompassing
  • #t-cargo > directory naming with new build layout @ 💬
• Can the old build/ and deps/ content live in the same place?
  • Seems to work
• How should we handle incremental/?
  • rustc loads incremental artifacts only for local crates. cross-crates is from rmeta. Therefore per-crate incremental artifacts should be fine.
  • Given incremental directory has its own flock mechanism, we don't need to add flock for that directory.
  • See #t-compiler > ✔ Cargo switching to one `-C incremental` directory per c...
• Under the new layout, should cargo clean -p also cleans old layout paths? See feat(build-dir): Reorganize build-dir layout #15947 (comment)
• Is the current handling of build scripts sufficient or should we explicitly split them into separate entries. We separated build script build and execution

Future extensions

• More granular locking in cargo_rustc #4282
• Per-user compiled artifact cache #5931
• cargo ./target fills with outdated artifacts as toolchains are updated/changed #5026

Deferred layout changes

• Can we remove -Cextra-filename completely as it is redundant with the build-unit's <name>/<hash>
  • It may not be possible to remove -Cextra-filename for rlibs. rustc relies on it. See feat(build-dir): Reorganize build-dir layout #15947 (comment)
• Can we remove <tuple> from the parent path?
  • In addition to reducing path length pressure on Windows, could allow sharing of host builds between platforms
  • Blocked on More granular locking in cargo_rustc #4282
• Can we remove <profile> from the parent path?
  • In addition to reducing path length pressure on Windows, could allow sharing when build units have the same settings across profiles
  • <hash> is the -C extra-filename hash and doesn't encompass all of fingerprinting, so we'd need to audit if there are cases that don't change the hash that we'd still need per-profile
  • Changing of local source is one example, so at least local packages still need to be scoped by profile
  • Blocked on More granular locking in cargo_rustc #4282
• Can we simplify how fingerprints are stored, reducing pressure on path lengths
• If Windows filesystem perf issues crop up with <pkgname>/<hash>, we can go with prefixes to reduce the number of items within a directory. See bjorn3's comment.
  • if deps/ hasn't been a problem, this is unlikely to be a problem

About tracking issues

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They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Original

note: I specify build-dir to clarify which half of #14125 I'm referring to. The files and layout of build-dir does not have compatibility guarantees (source).

Currently, build-dir is laid out like

• target/
  • <target-platform>/?
    • <profile>/
      • incremental/
      • build/
        • <package>-<hash>/
          • build*script-*
      • deps/
        • <package>-<hash>*

Currently,

• cargo clean -p <package> will operate on everything for <package>

In the future, we could have

• GC will track and operate on everything for <package>-<hash> (cargo ./target fills with outdated artifacts as toolchains are updated/changed #5026)
• Change the locking so only overlapping <package>-<hash> that are being built block (More granular locking in cargo_rustc #4282)
• Centrally cache <package>-<hash> artifacts across all projects (Per-user compiled artifact cache #5931)
• Reduced overhead on systems that are slow when there are a lot of files within a directory (see also Adding tests has a high friction #15691)

These could be aided by re-arranging the build-dir to be organized around <package>-<hash>, like

• target/
  • <target-platform>/?
    • <profile>/
      • incremental/
      • build/
        • <package>-<hash>/
          • build*script-*
          • *.d

Side effects

• We'll have to change how we invoke rustc which will increase the length of the command-line
  • Currently, we blindly point rustc at deps/ and rustc finds the files it needs. We'll instead need to point to each individual artifact rustc may need.