1 files changed, 138 insertions, 0 deletions
diff --git a/compiler/rustc_passes/src/lib_features.rs b/compiler/rustc_passes/src/lib_features.rs
new file mode 100644
index 000000000..e05994f13
--- /dev/null
+++ b/compiler/rustc_passes/src/lib_features.rs
@@ -0,0 +1,138 @@
+//! Detecting lib features (i.e., features that are not lang features).
+//!
+//! These are declared using stability attributes (e.g., `#[stable (..)]` and `#[unstable (..)]`),
+//! but are not declared in one single location (unlike lang features), which means we need to
+//! collect them instead.
+
+use rustc_ast::{Attribute, MetaItemKind};
+use rustc_errors::struct_span_err;
+use rustc_hir::intravisit::Visitor;
+use rustc_middle::hir::nested_filter;
+use rustc_middle::middle::lib_features::LibFeatures;
+use rustc_middle::ty::query::Providers;
+use rustc_middle::ty::TyCtxt;
+use rustc_span::symbol::Symbol;
+use rustc_span::{sym, Span};
+
+fn new_lib_features() -> LibFeatures {
+    LibFeatures { stable: Default::default(), unstable: Default::default() }
+}
+
+pub struct LibFeatureCollector<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    lib_features: LibFeatures,
+}
+
+impl<'tcx> LibFeatureCollector<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>) -> LibFeatureCollector<'tcx> {
+        LibFeatureCollector { tcx, lib_features: new_lib_features() }
+    }
+
+    fn extract(&self, attr: &Attribute) -> Option<(Symbol, Option<Symbol>, Span)> {
+        let stab_attrs =
+            [sym::stable, sym::unstable, sym::rustc_const_stable, sym::rustc_const_unstable];
+
+        // Find a stability attribute: one of #[stable(…)], #[unstable(…)],
+        // #[rustc_const_stable(…)], or #[rustc_const_unstable(…)].
+        if let Some(stab_attr) = stab_attrs.iter().find(|stab_attr| attr.has_name(**stab_attr)) {
+            let meta_kind = attr.meta_kind();
+            if let Some(MetaItemKind::List(ref metas)) = meta_kind {
+                let mut feature = None;
+                let mut since = None;
+                for meta in metas {
+                    if let Some(mi) = meta.meta_item() {
+                        // Find the `feature = ".."` meta-item.
+                        match (mi.name_or_empty(), mi.value_str()) {
+                            (sym::feature, val) => feature = val,
+                            (sym::since, val) => since = val,
+                            _ => {}
+                        }
+                    }
+                }
+                if let Some(feature) = feature {
+                    // This additional check for stability is to make sure we
+                    // don't emit additional, irrelevant errors for malformed
+                    // attributes.
+                    let is_unstable =
+                        matches!(*stab_attr, sym::unstable | sym::rustc_const_unstable);
+                    if since.is_some() || is_unstable {
+                        return Some((feature, since, attr.span));
+                    }
+                }
+                // We need to iterate over the other attributes, because
+                // `rustc_const_unstable` is not mutually exclusive with
+                // the other stability attributes, so we can't just `break`
+                // here.
+            }
+        }
+
+        None
+    }
+
+    fn collect_feature(&mut self, feature: Symbol, since: Option<Symbol>, span: Span) {
+        let already_in_stable = self.lib_features.stable.contains_key(&feature);
+        let already_in_unstable = self.lib_features.unstable.contains_key(&feature);
+
+        match (since, already_in_stable, already_in_unstable) {
+            (Some(since), _, false) => {
+                if let Some((prev_since, _)) = self.lib_features.stable.get(&feature) {
+                    if *prev_since != since {
+                        self.span_feature_error(
+                            span,
+                            &format!(
+                                "feature `{}` is declared stable since {}, \
+                                 but was previously declared stable since {}",
+                                feature, since, prev_since,
+                            ),
+                        );
+                        return;
+                    }
+                }
+
+                self.lib_features.stable.insert(feature, (since, span));
+            }
+            (None, false, _) => {
+                self.lib_features.unstable.insert(feature, span);
+            }
+            (Some(_), _, true) | (None, true, _) => {
+                self.span_feature_error(
+                    span,
+                    &format!(
+                        "feature `{}` is declared {}, but was previously declared {}",
+                        feature,
+                        if since.is_some() { "stable" } else { "unstable" },
+                        if since.is_none() { "stable" } else { "unstable" },
+                    ),
+                );
+            }
+        }
+    }
+
+    fn span_feature_error(&self, span: Span, msg: &str) {
+        struct_span_err!(self.tcx.sess, span, E0711, "{}", &msg).emit();
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for LibFeatureCollector<'tcx> {
+    type NestedFilter = nested_filter::All;
+
+    fn nested_visit_map(&mut self) -> Self::Map {
+        self.tcx.hir()
+    }
+
+    fn visit_attribute(&mut self, attr: &'tcx Attribute) {
+        if let Some((feature, stable, span)) = self.extract(attr) {
+            self.collect_feature(feature, stable, span);
+        }
+    }
+}
+
+fn lib_features(tcx: TyCtxt<'_>, (): ()) -> LibFeatures {
+    let mut collector = LibFeatureCollector::new(tcx);
+    tcx.hir().walk_attributes(&mut collector);
+    collector.lib_features
+}
+
+pub fn provide(providers: &mut Providers) {
+    providers.lib_features = lib_features;
+}