Auto merge of #66945 - Centril:de-macro-gating, r=<try>

de-macro-ize feature gate checking

This PR removes the usage of macros in feature gating preferring to take in symbols instead. The aim is to make feature gating more understandable overall while also reducing some duplication.

To check whether a feature gate is active, `tcx.features().on(sym::my_feature)` can be used.
Inside `PostExpansionVisitor` it's however better to use `self.gate(...)` which provides a nicer API atop of that. Outside of the visitor, if `gate_feature` can be used, it should be preferred over `feature_err`.

As a follow-up, and once #66878 is merged, it might be a good idea to add a method to `Session` for more convenient access to `gate_feature` and `feature_err` respectively.

Originally I intended to go for a `HashSet` representation, but this felt like a smaller change to just expand to a `match` on the symbol => field pairs.

r? @oli-obk

Author: bors

src/librustc/hir/lowering.rs

@@ -834,7 +834,7 @@ impl<'a> LoweringContext<'a> {
             return;
         }
 
-        if !self.sess.features_untracked().in_band_lifetimes {
+        if !self.sess.features_untracked().on(sym::in_band_lifetimes) {
             return;
         }
 
@@ -1394,7 +1394,7 @@ impl<'a> LoweringContext<'a> {
                     }
                     ImplTraitContext::Disallowed(pos) => {
                         let allowed_in = if self.sess.features_untracked()
-                                                .impl_trait_in_bindings {
+                                                .on(sym::impl_trait_in_bindings) {
                             "bindings or function and inherent method return types"
                         } else {
                             "function and inherent method return types"
@@ -2118,7 +2118,7 @@ impl<'a> LoweringContext<'a> {
 
     fn lower_local(&mut self, l: &Local) -> (hir::Local, SmallVec<[NodeId; 1]>) {
         let mut ids = SmallVec::<[NodeId; 1]>::new();
-        if self.sess.features_untracked().impl_trait_in_bindings {
+        if self.sess.features_untracked().on(sym::impl_trait_in_bindings) {
             if let Some(ref ty) = l.ty {
                 let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
                 visitor.visit_ty(ty);
@@ -2130,7 +2130,7 @@ impl<'a> LoweringContext<'a> {
             ty: l.ty
                 .as_ref()
                 .map(|t| self.lower_ty(t,
-                    if self.sess.features_untracked().impl_trait_in_bindings {
+                    if self.sess.features_untracked().on(sym::impl_trait_in_bindings) {
                         ImplTraitContext::OpaqueTy(Some(parent_def_id))
                     } else {
                         ImplTraitContext::Disallowed(ImplTraitPosition::Binding)

src/librustc/hir/lowering/item.rs

@@ -181,15 +181,15 @@ impl LoweringContext<'_> {
             ItemKind::Impl(.., None, _, _) => smallvec![i.id],
             ItemKind::Static(ref ty, ..) => {
                 let mut ids = smallvec![i.id];
-                if self.sess.features_untracked().impl_trait_in_bindings {
+                if self.sess.features_untracked().on(sym::impl_trait_in_bindings) {
                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
                     visitor.visit_ty(ty);
                 }
                 ids
             },
             ItemKind::Const(ref ty, ..) => {
                 let mut ids = smallvec![i.id];
-                if self.sess.features_untracked().impl_trait_in_bindings {
+                if self.sess.features_untracked().on(sym::impl_trait_in_bindings) {
                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
                     visitor.visit_ty(ty);
                 }
@@ -285,7 +285,7 @@ impl LoweringContext<'_> {
                 hir::ItemKind::Static(
                     self.lower_ty(
                         t,
-                        if self.sess.features_untracked().impl_trait_in_bindings {
+                        if self.sess.features_untracked().on(sym::impl_trait_in_bindings) {
                             ImplTraitContext::OpaqueTy(None)
                         } else {
                             ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
@@ -299,7 +299,7 @@ impl LoweringContext<'_> {
                 hir::ItemKind::Const(
                     self.lower_ty(
                         t,
-                        if self.sess.features_untracked().impl_trait_in_bindings {
+                        if self.sess.features_untracked().on(sym::impl_trait_in_bindings) {
                             ImplTraitContext::OpaqueTy(None)
                         } else {
                             ImplTraitContext::Disallowed(ImplTraitPosition::Binding)