Tracking issue for migrating all remaining post-expansion feature gates to pre-expansion ones

Tip

If you've arrived here because you encountered a feature gate warning for unstable syntax behind #[cfg(…)], here's how you can transform your code while keeping the unstable syntax cfg-conditional:

Before (triggers a feature gate warning):

#[cfg(feature = "nightly")] // or a different cfg-condition
impl !Trait for Type {}     // or a different unstable syntax

After (first approach) (idiomatic, doesn't trigger a warning):

macro_rules! generate { ($( $tt:tt )*) => { $( $tt )* } }

#[cfg(feature = "nightly")]
generate! { impl !Send for NotSend {} }

After (second approach) (idiomatic, doesn't trigger a warning):

macro_rules! generate {
    // Of course, you can use macro metavariables to avoid code duplication etc.
    () => { impl !Trait for Type {} }
}

#[cfg(feature = "nightly")]
generate!();

If you'd like to see a more realistic example, head on over to https://github.com/arcnmx/packed-rs/pull/1/changes.

Steps

Find the most recent relevant crater runs and manually reaudit affected crates
- check if at least some crates have been fixed downstream by now
Collect all features where the crater run has happened a long time ago & issue a single crater run for all of them at once
~~For all the other ones that also don't emit a pre-expansion warning yet, add warnings immediately~~ (done)
- ~~no further input from T-compiler/T-lang should be needed since it's already legitimized by MCP 535.~~

Affected Syntactic Constructs

item modifier default (feature specialization or min_specialization for fns only)
- we potentially want to drop this feature in favor of a different approach...
- ...so a syntax gate would make it easier to phase out the syntax later on
- issue a pre-expansion feature gate warning (legitimized by MCP 535)
- turn the warning into a hard error (needs crater, T-lang FCP)
trait impl modifier ! (feature negative_impls)
- likely to be stabilized in 1–3 years, so a syntax gate isn't super pressing
- issue a pre-expansion feature gate warning (legitimized by MCP 535)
- turn the warning into a hard error (needs crater, T-lang FCP)
expression try { … } (feature try_blocks)
- already has a pre-expansion feature gate warning
- incredibly likely to be stabilized this year...
- ...so a syntax gate would be counterproductive / harmful (!)
- turn the warning into a hard error (needs crater, T-lang FCP)
pattern box $pat (feature box_patterns)
- already has a pre-expansion feature gate warning
- incredibly likely to be replaced with explicit or implicit deref patterns...
- ...so a syntax gate would make it easier to phase out the syntax later on
- ~~turn the warning into a hard error~~ (needs crater, T-lang FCP)
- remove the feature entirely instead (cratered, fcp'ed): remove box_patterns #156749
pattern $patpath { box $ident }, pseudo binding mode (feature box_patterns)
- incredibly likely to be replaced with explicit or implicit deref patterns...
- ...so a syntax gate would make it easier to phase out the syntax later on
- issue a pre-expansion feature gate warning (legitimized by MCP 535)
- ~~turn the warning into a hard error~~ (needs crater, T-lang FCP)
- remove the feature entirely instead (cratered, fcp'ed): remove box_patterns #156749
item trait $ident $genericparams = $bounds; (feature trait_alias)
- already has a pre-expansion feature gate warning
- turn the warning into a hard error (needs crater, T-lang FCP)
trait item modifier auto (feature auto_traits)
- already has a pre-expansion feature gate warning
- turn the warning into a hard error (needs crater, T-lang FCP)
(not fitting 100%) syntax impl $traitref for .. {} (early version of a now removed predecessor of feature auto_traits)
- most recent crater run: PR RUST-121072
- issue a pre-expansion feature gate warning (legitimized by MCP 535, maybe?)
- reevaluate RUST-121072's crater report & send downstream patches
- turn the warning into a hard error (needs T-lang FCP, maybe another crater run)
item macro $ident$macroparams { $ttstar } (feature decl_macro)
- already has a pre-expansion feature gate warning
- turn the warning into a hard error (needs crater, T-lang FCP)
(not fitting 100%) predicates $ty = $ty and $ty == $ty (no corresp. feature was ever added (cc RUST-22074, RUST-87471))
- straight up hard error (crater: 0 regressions, needs T-lang FCP): PR RUST-153513
attribute in certain expression contexts (feature stmt_expr_attributes)
- META: FIXME: elaborate
- see also Stabilize attributes on closure and method call expressions #159581

Pre-History (Incomplete)

Implementation History

Downstream Patches

Incoming breakage: Use new auto trait syntax arcnmx/packed-rs#1

For why pre-expansion feature gates are superior to post-expansion ones, read my very recent comment: #153513 (comment).

In case anybody is wondering where the supposed future-incompat lint is that's mentioned in MCP rust-lang/compiler-team#535 and enabled in PR #99935, it's called unstable_syntax_pre_expansion (see also its entry in the rustc book) but (1) it's not advertised in the diagnostic (2) lint control attrs/flags (allow, warn, deny, forbid, expect) don't work for it (incl. -Dwarnings). I haven't found any reasoning behind that.

Moreover, the diagnostic links to issue #65860 which I have closed in favor of this issue. In any case, it wasn't a proper C-future-incompatibility Category: Future-incompatibility lints C-tracking-issue Category: An issue tracking the progress of sth. like the implementation of an RFC anyway.

I'm currently leaning towards removing that lint entirely (turning the improper lint warning into a proper non-lint warning) (not even keeping a removed entry since it isn't advertised anywhere & can't be controlled) however I do acknowledge that future-incompat lints allow for things like no-warn-in-deps vs. warn-in-deps, so that idea might not work out.

Once all gates have been turned into proper pre-expansion ones / the corresp. features have been stabilized, we can easily remove this hidden semi-functioning lint without any traces.

Rollup merge of rust-lang#154475 - fmease:soft-gate-box-struct-field-…

Unrolled build for #154475

Rollup merge of rust-lang#154527 - fmease:more-soft-gates, r=nnethercote

Rollup merge of rust-lang#155017 - fmease:update-issue-number-unstabl…

Unrolled build for #155017

Thanks a lot for the explanation and the guidance with the macro indirection, this fixes it for me! As long as there's a way to remediate the warning and to publish a new patch release on crates.io my concerns are resolved :)

Wrap nightly syntax inside a macro to avoid warnings and future errors.

Use pre-expansion cfg for feature spec

Fix unstable syntax warnings on recent nightly compilers

stmt_expr_attributes is also supposed to be a pre-expansion feature gate, but it's not.

Rollup merge of rust-lang#156749 - cyrgani:unbox-full, r=Kivooeo,fmease

Unrolled build for #156749 in rollup 161731

Rollup merge of #156749 - cyrgani:unbox-full, r=Kivooeo,fmease

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Tracking issue for migrating all remaining post-expansion feature gates to pre-expansion ones #154045

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Affected Syntactic Constructs

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Tracking issue for migrating all remaining post-expansion feature gates to pre-expansion ones #154045

Description

Steps

Affected Syntactic Constructs

Pre-History (Incomplete)

Implementation History

Downstream Patches

Activity

fmease commented on Mar 18, 2026

fmease commented on Mar 27, 2026

8 remaining items

gendx commented on Apr 9, 2026

petrochenkov commented on Jul 27, 2026

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