feature: replace `lazy_static` by `SyncLazy` from std

Cargo.lock

@@ -3513,7 +3513,6 @@ dependencies = [
 name = "rustc_feature"
 version = "0.0.0"
 dependencies = [
- "lazy_static",
  "rustc_data_structures",
  "rustc_span",
 ]

compiler/rustc_feature/Cargo.toml

@@ -9,5 +9,4 @@ doctest = false
 
 [dependencies]
 rustc_data_structures = { path = "../rustc_data_structures" }
-lazy_static = "1.0.0"
 rustc_span = { path = "../rustc_span" }

compiler/rustc_feature/src/builtin_attrs.rs

@@ -5,10 +5,11 @@ use AttributeType::*;
 
 use crate::{Features, Stability};
 
-use lazy_static::lazy_static;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_span::symbol::{sym, Symbol};
 
+use std::lazy::SyncLazy;
+
 type GateFn = fn(&Features) -> bool;
 
 macro_rules! cfg_fn {
@@ -589,14 +590,12 @@ pub fn is_builtin_attr_name(name: Symbol) -> bool {
     BUILTIN_ATTRIBUTE_MAP.get(&name).is_some()
 }
 
-lazy_static! {
-    pub static ref BUILTIN_ATTRIBUTE_MAP: FxHashMap<Symbol, &'static BuiltinAttribute> = {
-        let mut map = FxHashMap::default();
-        for attr in BUILTIN_ATTRIBUTES.iter() {
-            if map.insert(attr.0, attr).is_some() {
-                panic!("duplicate builtin attribute `{}`", attr.0);
-            }
+pub static BUILTIN_ATTRIBUTE_MAP: SyncLazy<FxHashMap<Symbol, &'static BuiltinAttribute>> = SyncLazy::new(|| {
+    let mut map = FxHashMap::default();
+    for attr in BUILTIN_ATTRIBUTES.iter() {
+        if map.insert(attr.0, attr).is_some() {
+            panic!("duplicate builtin attribute `{}`", attr.0);
         }
-        map
-    };
-}
+    }
+    map
+});

compiler/rustc_feature/src/lib.rs

@@ -11,6 +11,8 @@
 //! even if it is stabilized or removed, *do not remove it*. Instead, move the
 //! symbol to the `accepted` or `removed` modules respectively.
 
+#![feature(once_cell)]
+
 mod accepted;
 mod active;
 mod builtin_attrs;