Auto merge of #53444 - varkor:lib_features-conditional, r=<try>

[WIP] Only fetch lib_features when there are unknown feature attributes

An attempt to win back some of the performance lost in #52644 (comment).

cc @nnethercote

src/librustc/middle/lib_features.rs

@@ -18,6 +18,7 @@ use ty::TyCtxt;
 use syntax::symbol::Symbol;
 use syntax::ast::{Attribute, MetaItem, MetaItemKind};
 use syntax_pos::{Span, DUMMY_SP};
+use hir::{Item, ItemKind};
 use hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_data_structures::fx::{FxHashSet, FxHashMap};
 use errors::DiagnosticId;
@@ -143,6 +144,26 @@ impl<'a, 'tcx> Visitor<'tcx> for LibFeatureCollector<'a, 'tcx> {
         NestedVisitorMap::All(&self.tcx.hir)
     }
 
+    fn visit_item(&mut self, item: &'tcx Item) {
+        match item.node {
+            ItemKind::ExternCrate(_) => {
+                let def_id = self.tcx.hir.local_def_id(item.id);
+                let cnum = match self.tcx.extern_mod_stmt_cnum(def_id) {
+                    Some(cnum) => cnum,
+                    None => return,
+                };
+
+                for &(feature, since) in self.tcx.defined_lib_features(cnum).iter() {
+                    self.collect_feature(feature, since, DUMMY_SP);
+                }
+            }
+
+            _ => {}
+        }
+
+        intravisit::walk_item(self, item);
+    }
+
     fn visit_attribute(&mut self, attr: &'tcx Attribute) {
         if let Some((feature, stable, span)) = self.extract(attr) {
             self.collect_feature(feature, stable, span);
@@ -152,11 +173,6 @@ impl<'a, 'tcx> Visitor<'tcx> for LibFeatureCollector<'a, 'tcx> {
 
 pub fn collect<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> LibFeatures {
     let mut collector = LibFeatureCollector::new(tcx);
-    for &cnum in tcx.crates().iter() {
-        for &(feature, since) in tcx.defined_lib_features(cnum).iter() {
-            collector.collect_feature(feature, since, DUMMY_SP);
-        }
-    }
     intravisit::walk_crate(&mut collector, tcx.hir.krate());
     collector.lib_features
 }

src/librustc/middle/stability.rs

@@ -846,18 +846,20 @@ pub fn check_unused_or_stable_features<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     remaining_lib_features.remove(&Symbol::intern("libc"));
     remaining_lib_features.remove(&Symbol::intern("test"));
 
-    for (feature, stable) in tcx.lib_features().to_vec() {
-        if let Some(since) = stable {
-            if let Some(span) = remaining_lib_features.get(&feature) {
-                // Warn if the user has enabled an already-stable lib feature.
-                unnecessary_stable_feature_lint(tcx, *span, feature, since);
+    if !remaining_lib_features.is_empty() {
+        for (feature, stable) in tcx.lib_features().to_vec() {
+            if let Some(since) = stable {
+                if let Some(span) = remaining_lib_features.get(&feature) {
+                    // Warn if the user has enabled an already-stable lib feature.
+                    unnecessary_stable_feature_lint(tcx, *span, feature, since);
+                }
             }
+            remaining_lib_features.remove(&feature);
         }
-        remaining_lib_features.remove(&feature);
-    }
 
-    for (feature, span) in remaining_lib_features {
-        struct_span_err!(tcx.sess, span, E0635, "unknown feature `{}`", feature).emit();
+        for (feature, span) in remaining_lib_features {
+            struct_span_err!(tcx.sess, span, E0635, "unknown feature `{}`", feature).emit();
+        }
     }
 
     // FIXME(#44232): the `used_features` table no longer exists, so we

src/test/ui/feature-gate/stability-attribute-consistency.rs

@@ -11,6 +11,12 @@
 #![stable(feature = "stable_test_feature", since = "1.0.0")]
 
 #![feature(staged_api)]
+// Right now, stability attributes are only checked for consistency if we're
+// collecting lib feature attributes, for performance. We collect lib
+// feature attributes if there are non-lang feature attributes, or a crate
+// that depends on the current one has non-lang feature attributes. Thus,
+// we're enabling an arbitrary lib feature to force the check to kick in.
+#![feature(rustc_private)]
 
 #[stable(feature = "foo", since = "1.0.0")]
 fn foo_stable_1_0_0() {}

src/test/ui/feature-gate/stability-attribute-consistency.stderr

@@ -1,11 +1,11 @@
 error[E0711]: feature `foo` is declared stable since 1.29.0, but was previously declared stable since 1.0.0
-  --> $DIR/stability-attribute-consistency.rs:18:1
+  --> $DIR/stability-attribute-consistency.rs:24:1
    |
 LL | #[stable(feature = "foo", since = "1.29.0")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0711]: feature `foo` is declared unstable, but was previously declared stable
-  --> $DIR/stability-attribute-consistency.rs:22:1
+  --> $DIR/stability-attribute-consistency.rs:28:1
    |
 LL | #[unstable(feature = "foo", issue = "0")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^