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Add -C hint-mostly-unused
option
#829
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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. Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:
Concerns can be lifted with:
See documentation at https://forge.rust-lang.org cc @rust-lang/compiler |
-C defer-codegen
option-C hint-mostly-unused
option
Implementation at rust-lang/rust#135656 |
@rustbot concern prior-art-does-not-motivate-fast-stabilization |
Based on discussion at RustWeek, I'm closing this MCP, and opening one to add an unstable option |
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Proposal
Some libraries are extremely large, and users use only a small fraction of their surface area. Crate features aren't ideal for just reducing the size of a very large library; that would require the crate maintainer to predict and provide semantic subsets that people will use, and commit to maintaining those subsets indefinitely as a stable API. That also doesn't scale well to crates with thousands of functions or structures; having thousands of features is a problem for maintenance, and for usability (carefully enabling the needed subset).
Introduce an option,
-C hint-mostly-unused
, which hints to rustc that most of a crate's API surface area may go unused.Currently, this will make a crate provide all its code as MIR, to be compiled only when referenced. This functionality already exists as
-Zcross-crate-inline-threshold=always
. I'm proposing to provide a stablehint
option to access the same underlying functionality.I've tested this on some crates with very large dependencies. On one project, adding this option to just the three largest dependencies reduced total compilation time from 10m30s to 6m12s.
(No attachment to the specific option name, happy to change it.)
I also intend to propose a corresponding option in Cargo, which can be enabled on a per-dependency basis. (And, later, we can add a way for large crates to hint that they should have this option enabled by default unless overridden.)
Inspired by @saethlin's MIR-only rlibs work at rust-lang/rust#119017 .
Notes
#[inline(never)]
.Process
The main points of the Major Change Process are as follows:
@rustbot second
.-C flag
, then full team check-off is required.@rfcbot fcp merge
on either the MCP or the PR.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.
Caution
Concerns (1 active)
Managed by
@rustbot
—see help for details.The text was updated successfully, but these errors were encountered: