RFC: Add `required-targets` for workspace package selection by Sakib25800 · Pull Request #4013 · rust-lang/rfcs

Sakib25800 · 2026-10-05T09:30:28Z

Summary

Add a required-targets field to the [package] table of Cargo.toml so packages can declare target requirements using cfg syntax.

This builds on #3759 by @carloskiki, and addresses feedback from that proposal.

Rendered

FCP

Tracking issue: rust-lang/cargo#17671

Per: - rust-lang#3759 (comment)

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment)

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment)

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment)

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment)

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment)

WhyNotHugo · 2026-10-06T19:36:10Z

+
+## Target-specific dependency resolution
+
+`Cargo.lock`, and by extension, `cargo vendor`, must assume that a package may be built on any platform that has or will exist.  This means that if a transitive dependency pulls in Windows-specific dependencies, `cargo vendor` will include them when run on a Linux-only application.  Being able to tell `cargo vendor` what platforms to care about can reduce the space used in a repo and reduce churn.


Currently, cargo vendor for lot of projects which have ~3MB in dependencies download ~200MB in additional dependencies for unsupported platforms. A huge interest in being able to declare supported targets is to make cargo vendor useful.

Why does this RFC specifically spell out that this misbehaviour must continue?

View changes since the review

That problem is related but as we discussed in #3759, the solutions will end up being very different. This section does talk a lot about that design (a resolver.targets in .cargo/config.toml). See also my comment at #4013 (comment)

Since these can be different RFCs they should so we can have more focused conversations and move things along more smoothly. See also https://epage.github.io/dev/pr-style/#c-isolate

Yeah, I kept that from the original RFC. I've now reduced resolver.targets to a quick mention under Related work and removed the design details.

The original RFC has essentially split into two proposals: required-targets for workspace package selection (this one) and resolver.targets for targeted dependency resolution and vendoring, to be covered in another RFC.

epage · 2026-10-06T20:42:03Z

@Sakib25800 as I'm assuming this is your first RFC, I would recommend

Never edit history
Make edits in response to feedback in its own commit
Reply to the feedback with the commit hash.
Only resolve a thread if you are very sure you understood the feedback and handled it as the person expected.

Per rust-lang#4013 (comment)

Per rust-lang#4013 (comment) Co-authored-by: Ed Page <eopage@gmail.com>

epage · 2026-10-14T21:47:16Z

@rfcbot concern publish

See https://github.com/rust-lang/rfcs/pull/4013/changes#r4276615204

epage · 2026-10-14T21:50:11Z

@rfcbot resolve dependency
@rfcbot resolve no-resolver-rationale
@rfcbot resolve strip-on-publish
@rfcbot resolve supported-cfgs

Per rust-lang#4013 (comment) Co-authored-by: Ed Page <eopage@gmail.com>

Per rust-lang#4013 (comment)

epage · 2026-10-16T13:55:55Z

@rfcbot resolve publish

weihanglo

@rfcbot reviewed

View changes since this review

rust-rfcbot · 2026-10-22T08:00:53Z

🔔 This is now entering its final comment period, as per the review above. 🔔

rust-rfcbot · 2026-11-01T09:01:12Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

weihanglo

Thanks folks for your participation! This has passed FCP. Merging right now.

Further discussion and progress update would happen in rust-lang/cargo#17671.

View changes since this review

carloskiki and others added 9 commits October 4, 2026 14:25

Initial commit from rust-lang#3759

e507264

chore: narrow to required-targets

338438b

Per: - rust-lang#3759 (comment)

fix: clarify local workspace package selection

76885a5

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment)

docs: clarify alternatives

6747e4f

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment)

docs: separate target-specific dependency resolution

78c8fa3

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment)

docs: describe complementary target-selection features

12c2d51

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment)

fix: clarify future dependency compatibility checks

d36489a

Per: - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment) - rust-lang#3759 (comment)

docs: polish wording and structure

2abbce4

docs: update PR link

bb0a423

ranger-ross added the T-cargo Relevant to the Cargo team, which will review and decide on the RFC. label Oct 6, 2026

github-project-automation Bot added this to Cargo status tracker Oct 6, 2026

epage reviewed Oct 6, 2026

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Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 6, 2026

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Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 6, 2026

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Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 6, 2026

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Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 6, 2026

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Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 6, 2026

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Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 6, 2026

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Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 6, 2026

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Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 6, 2026

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Comment thread text/0000-cargo-required-targets.md Resolved

carloskiki mentioned this pull request Oct 6, 2026

RFC: Allow packages to specify a set of supported targets #3759

Closed

WhyNotHugo reviewed Oct 6, 2026

View reviewed changes

Sakib25800 added 7 commits October 7, 2026 09:55

fix: apply required-targets to fetch and tree

0b301bd

Per rust-lang#4013 (comment)

docs: add target tuples as a future possibility

ef92f67

Per rust-lang#4013 (comment)

docs: add removing unstable forced-target to future possibilities

5a62e4a

Per rust-lang#4013 (comment)

docs: move forced-target comparison to alternatives

00fa993

Per rust-lang#4013 (comment)

docs: describe manual package selection as an alternative

4f0901a

Per rust-lang#4013 (comment)

docs: describe documentation and compile-time errors as alternatives

fa0d727

Per rust-lang#4013 (comment)

docs: compare platform compatibility prior art

2de51a0

Per rust-lang#4013 (comment)

Sakib25800 and others added 3 commits October 14, 2026 22:05

docs: group feature-unification discussion

bb41f06

Per rust-lang#4013 (comment) Co-authored-by: Ed Page <eopage@gmail.com>

docs: label proc-macro scenarios as an example

d4b09b3

Per rust-lang#4013 (comment) Co-authored-by: Ed Page <eopage@gmail.com>

docs: make proc-macro example easier to scan

223ff0b

Per rust-lang#4013 (comment) Co-authored-by: Ed Page <eopage@gmail.com>

epage reviewed Oct 14, 2026

View reviewed changes

Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 14, 2026

View reviewed changes

Comment thread text/0000-cargo-required-targets.md Resolved

epage reviewed Oct 14, 2026

View reviewed changes

Comment thread text/0000-cargo-required-targets.md Resolved

Sakib25800 and others added 4 commits October 15, 2026 09:28

docs: clarify required-targets checks during package verification

d8eea17

Per rust-lang#4013 (comment) Co-authored-by: Ed Page <eopage@gmail.com>

docs: expand rationale for retaining required-targets on publication

affe719

Per rust-lang#4013 (comment)

docs: remove forced-target caveat from proc-macro example

b56379d

Per rust-lang#4013 (comment)

docs: filter package verification by required targets

c13138e

Per rust-lang#4013 (comment)

weihanglo reviewed Oct 20, 2026

View reviewed changes

weihanglo moved this to FCP merge in Cargo status tracker Oct 20, 2026

rust-rfcbot added final-comment-period Will be merged/postponed/closed in ~10 calendar days unless new substational objections are raised. and removed proposed-final-comment-period Currently awaiting signoff of all team members in order to enter the final comment period. labels Oct 22, 2026

weihanglo mentioned this pull request Nov 1, 2026

Tracking Issue for required-targets rust-lang/cargo#17671

Open

1 task

weihanglo added 4 commits November 1, 2026 09:06

fix(4013): grammar and typos

c0b6ba7

fix(4013): rename required-targets RFC to 4013

8d4767f

fix(4013): remove empty pre-rfc field

b9aae56

fix(4013): add tracking issue link

5bce118

weihanglo approved these changes Nov 1, 2026

View reviewed changes

weihanglo merged commit 3bdcd22 into rust-lang:main Nov 1, 2026
1 check passed


		## Target-specific dependency resolution

		`Cargo.lock`, and by extension, `cargo vendor`, must assume that a package may be built on any platform that has or will exist. This means that if a transitive dependency pulls in Windows-specific dependencies, `cargo vendor` will include them when run on a Linux-only application. Being able to tell `cargo vendor` what platforms to care about can reduce the space used in a repo and reduce churn.

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