A --temps-dir option for specifying where the intermediate files are written

[torhovland]

Proposal

Currently, the compiler puts intermediate files in the output directory, temporarily or permanently, depending on the -C save-temps option. This is a problem if more than one invocation is run in parallel, e.g. for producing different crate types, because the invocations will overwrite each other's intermediate files. This issue is described in rust-lang/rust#10971.

A --temps-dir option for specifying where the intermediate files are written is implemented in rust-lang/rust#83846.

If additional files are specifically requested using --emit asm,llvm-bc,llvm-ir,obj,metadata,link,dep-info,mir, these are considered output files, and will be put in the output directory rather than in the intermediate directory.

This is a backward-compatible change, i.e. if --temps-dir is not specified, the behavior is the same as before.

The name of the option has been chosen to be consistent with -C save-temps as well as with --out-dir.

Precedent

D provides an -od option which does the same as --temps-dir:

ldc2 -relocation-model=pic --shared lib.d -od=shared_lib_objects & ldc2 --lib lib.d -od=static_lib_objects

gcc provides the option to specify a path where each intermediate file goes:

gcc -fPIC -c lib.c -o lib_shared/lib.o & gcc -c lib.c -o lib_static/lib.o

Alternatives

Issue rust-lang/rust#10971 could also have been solved by hashing the command line arguments and adding the hash to the name of the intermediate files.

Mentors or Reviewers

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Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[pnkfelix]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted