Add special support for libc

src/librustc/middle/lib_features.rs

@@ -62,8 +62,6 @@ impl<'a, 'tcx> LibFeatureCollector<'a, 'tcx> {
         let mut features = vec![];
 
         for attr in attrs {
-            // FIXME(varkor): the stability attribute might be behind a `#[cfg]` attribute.
-
             // Find a stability attribute (i.e. `#[stable (..)]`, `#[unstable (..)]`,
             // `#[rustc_const_unstable (..)]`).
             if let Some(stab_attr) = stab_attrs.iter().find(|stab_attr| {

src/librustc/middle/stability.rs

@@ -813,7 +813,6 @@ pub fn check_unused_or_stable_features<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
         krate.visit_all_item_likes(&mut missing.as_deep_visitor());
     }
 
-
     let declared_lang_features = &tcx.features().declared_lang_features;
     let mut lang_features = FxHashSet();
     for &(feature, span, since) in declared_lang_features {
@@ -837,8 +836,9 @@ pub fn check_unused_or_stable_features<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
         }
         remaining_lib_features.insert(feature, span.clone());
     }
-    // FIXME(varkor): we don't properly handle lib features behind `cfg` attributes yet,
-    // but it happens just to affect `libc`, so we're just going to hard-code it for now.
+    // `stdbuild` has special handling for `libc`, so we need to
+    // recognise the feature when building std.
+    // FIXME: only remove `libc` when `stdbuild` is active.
     remaining_lib_features.remove(&Symbol::intern("libc"));
 
     for (feature, stable) in tcx.lib_features().iter() {

src/libsyntax/feature_gate.rs

@@ -321,7 +321,6 @@ declare_features! (
     // `extern "x86-interrupt" fn()`
     (active, abi_x86_interrupt, "1.17.0", Some(40180), None),
 
-
     // Allows the `catch {...}` expression
     (active, catch_expr, "1.17.0", Some(31436), Some(Edition::Edition2018)),