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Add comments to config::features.

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I found this function very confusing, and it took me quite some time to
work out what it was doing. These comments capture that hard-earned
knowledge.
This commit is contained in:
Nicholas Nethercote 2023-10-05 10:17:42 +11:00
parent 1ddb2872dd
commit e24f394404
1 changed files with 30 additions and 5 deletions
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35
compiler/rustc_expand/src/config.rs
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@ -58,21 +58,28 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
let mut edition_enabled_features = FxHashMap::default();
let crate_edition = sess.edition();
// Enable edition umbrella feature-gates based on the crate edition.
// - enable `rust_2015_preview` always
// - enable `rust_2018_preview` if the crate edition is 2018 or higher
// - enable `rust_2021_preview` if the crate edition is 2021 or higher
// - etc.
for &edition in ALL_EDITIONS {
if edition <= crate_edition {
// The `crate_edition` implies its respective umbrella feature-gate
// (i.e., `#![feature(rust_20XX_preview)]` isn't needed on edition 20XX).
edition_enabled_features.insert(edition.feature_name(), edition);
}
}
// Enable edition-dependent features based on the crate edition.
// - E.g. `test_2018_feature` if the crate edition is 2018 or higher
for feature in active_features_up_to(crate_edition) {
feature.set(&mut features);
edition_enabled_features.insert(feature.name, crate_edition);
}
// Process the edition umbrella feature-gates first, to ensure
// `edition_enabled_features` is completed before it's queried.
// Enable edition umbrella feature-gates that are declared in the code. If
// present, enable edition-specific features based on that.
// - E.g. enable `test_2018_feature` if the crate edition is 2015 but
// `rust_2018_preview` is present
for attr in krate_attrs {
if !attr.has_name(sym::feature) {
continue;
@ -105,6 +112,7 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
}
}
// Process all features declared in the code.
for attr in krate_attrs {
if !attr.has_name(sym::feature) {
continue;
@ -136,6 +144,13 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
}
};
// If the declared feature is edition-specific and already enabled
// due to the crate edition or a declared edition umbrella
// feature-gate, give a warning.
// - E.g. warn if `test_2018_feature` is declared when the crate
// edition is 2018 or higher
// - E.g. warn if `test_2018_feature` is declared when
// `rust_2018_preview` or higher is declared.
if let Some(&edition) = edition_enabled_features.get(&name) {
sess.emit_warning(FeatureIncludedInEdition {
span: mi.span(),
@ -145,11 +160,14 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
continue;
}
// If the declared feature is an edition umbrella feature-gate,
// ignore it, because it was already handled above.
// - E.g. `rust_20XX_preview`
if ALL_EDITIONS.iter().any(|e| name == e.feature_name()) {
// Handled in the separate loop above.
continue;
}
// If the declared feature is removed, issue an error.
let removed = REMOVED_FEATURES.iter().find(|f| name == f.name);
let stable_removed = STABLE_REMOVED_FEATURES.iter().find(|f| name == f.name);
if let Some(Feature { state, .. }) = removed.or(stable_removed) {
@ -161,6 +179,7 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
}
}
// If the declared feature is stable, record it.
if let Some(Feature { since, .. }) = ACCEPTED_FEATURES.iter().find(|f| name == f.name) {
let since = Some(Symbol::intern(since));
features.declared_lang_features.push((name, mi.span(), since));
@ -168,6 +187,9 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
continue;
}
// If `-Z allow-features` is used and the declared feature is
// unstable and not also listed as one of the allowed features,
// issue an error.
if let Some(allowed) = sess.opts.unstable_opts.allow_features.as_ref() {
if allowed.iter().all(|f| name.as_str() != f) {
sess.emit_err(FeatureNotAllowed { span: mi.span(), name });
@ -175,6 +197,7 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
}
}
// If the declared feature is unstable, record it.
if let Some(f) = ACTIVE_FEATURES.iter().find(|f| name == f.name) {
f.set(&mut features);
features.declared_lang_features.push((name, mi.span(), None));
@ -182,6 +205,8 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
continue;
}
// Otherwise, the feature is unknown. Record it at a lib feature.
// It will be checked later.
features.declared_lib_features.push((name, mi.span()));
features.active_features.insert(name);
}