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Suggest removing #![feature] for library features that have been stabilized #89012

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Sep 17, 2021
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Suggest removing #![feature] for library features that have been stabilized #89012

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6 changes: 6 additions & 0 deletions compiler/rustc_ast_passes/src/feature_gate.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -702,10 +702,16 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
}

fn maybe_stage_features(sess: &Session, krate: &ast::Crate) {
// checks if `#![feature]` has been used to enable any lang feature
// does not check the same for lib features unless there's at least one
// declared lang feature
use rustc_errors::Applicability;

if !sess.opts.unstable_features.is_nightly_build() {
let lang_features = &sess.features_untracked().declared_lang_features;
if lang_features.len() == 0 {
return;
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What is this check doing? It should never be hit since there will always be some features built-in, but it also doesn't seem to have anything to do with the rest of the code.

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The lang_features vector contains the declared_lang_features in the session. So only the ones which have been enabled show up here.
I've added this check to skip emitting an error if only lib_features have been enabled and no lang_features

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Ok. That still seems like the wrong check, though - if there's one language and one library feature, it should still suggest removing the stable library feature.

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How will that be possible since the suggestion can be generated only after TyCtxt is generated? And we will be throwing an error at the parsing stage if a lang feature has been enabled, the compiler will abort and won't go to the next stage

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And we will be throwing an error at the parsing stage if a lang feature has been enabled, the compiler will abort and won't go to the next stage

Oh hmm, good point. Ok, this seems fine then - if the language feature wasn't stable removing the library feature wouldn't have helped anyway.

}
for attr in krate.attrs.iter().filter(|attr| attr.has_name(sym::feature)) {
let mut err = struct_span_err!(
sess.parse_sess.span_diagnostic,
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10 changes: 10 additions & 0 deletions compiler/rustc_passes/src/stability.rs
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Expand Up @@ -929,6 +929,16 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
let declared_lib_features = &tcx.features().declared_lib_features;
let mut remaining_lib_features = FxHashMap::default();
for (feature, span) in declared_lib_features {
if !tcx.sess.opts.unstable_features.is_nightly_build() {
struct_span_err!(
tcx.sess,
*span,
E0554,
"`#![feature]` may not be used on the {} release channel",
env!("CFG_RELEASE_CHANNEL")
)
.emit();
}
if remaining_lib_features.contains_key(&feature) {
// Warn if the user enables a lib feature multiple times.
duplicate_feature_err(tcx.sess, *span, *feature);
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