Rollup merge of #66895 - Centril:rustc_feature, r=oli-obk

Feature gating *declarations* => new crate `rustc_feature`

This PR moves the data-oriented parts of feature gating into its own crate, `rustc_feature`.
The parts consist of some data types as well as `accepted`, `active`, `removed`, and `builtin_attrs`.

Feature gate checking itself remains in `syntax::feature_gate::check`. The parts which define how to emit feature gate errors could probably be moved to `rustc_errors` or to the new `rustc_session` crate introduced in #66878. The visitor itself could probably be moved as a pass in `rustc_passes` depending on how the dependency edges work out.

The PR also contains some drive-by cleanup of feature gate checking. As such, the PR probably best read commit-by-commit.

r? @oli-obk
cc @petrochenkov
cc @Mark-Simulacrum

3 files changed, 3 insertions, 2 deletions

diff --git a/src/libsyntax_ext/Cargo.toml b/src/libsyntax_ext/Cargo.toml
index 2ebdac8ef7c..d73a9ea6cdb 100644
--- a/src/libsyntax_ext/Cargo.toml
+++ b/src/libsyntax_ext/Cargo.toml
@@ -14,6 +14,7 @@ errors = { path = "../librustc_errors", package = "rustc_errors" }
 fmt_macros = { path = "../libfmt_macros" }
 log = "0.4"
 rustc_data_structures = { path = "../librustc_data_structures" }
+rustc_feature = { path = "../librustc_feature" }
 rustc_parse = { path = "../librustc_parse" }
 rustc_target = { path = "../librustc_target" }
 smallvec = { version = "1.0", features = ["union", "may_dangle"] }
diff --git a/src/libsyntax_ext/test_harness.rs b/src/libsyntax_ext/test_harness.rs
index 659780d7a43..4c1eec38c6e 100644
--- a/src/libsyntax_ext/test_harness.rs
+++ b/src/libsyntax_ext/test_harness.rs
@@ -2,13 +2,13 @@
 
 use log::debug;
 use smallvec::{smallvec, SmallVec};
+use rustc_feature::Features;
 use rustc_target::spec::PanicStrategy;
 use syntax::ast::{self, Ident};
 use syntax::attr;
 use syntax::entry::{self, EntryPointType};
 use syntax_expand::base::{ExtCtxt, Resolver};
 use syntax_expand::expand::{AstFragment, ExpansionConfig};
-use syntax::feature_gate::Features;
 use syntax::mut_visit::{*, ExpectOne};
 use syntax::ptr::P;
 use syntax::sess::ParseSess;
diff --git a/src/libsyntax_ext/util.rs b/src/libsyntax_ext/util.rs
index e59daab1770..f7bd9a05604 100644
--- a/src/libsyntax_ext/util.rs
+++ b/src/libsyntax_ext/util.rs
@@ -1,7 +1,7 @@
 use rustc_parse::validate_attr;
+use rustc_feature::AttributeTemplate;
 use syntax_pos::Symbol;
 use syntax::ast::MetaItem;
-use syntax::attr::AttributeTemplate;
 use syntax_expand::base::ExtCtxt;
 
 pub fn check_builtin_macro_attribute(ecx: &ExtCtxt<'_>, meta_item: &MetaItem, name: Symbol) {