Auto merge of #63824 - Centril:split-feature_gate, r=oli-obk

Refactor `feature_gate.rs` into modules & cleanup

Split `src/libsyntax/feature_gate.rs` into `src/libsyntax/feature_gate/` with files:
- `accepted.rs` (accepted feature gates)
- `removed.rs` (...)
- `active.rs` (...)
- `builtin_attrs.rs` (definition of builtin attributes and their gates as well as gating `cfg` flags)
- `check.rs` (post expansion checking of feature gates)
- `mod.rs` (just reexports)

Additionally, `tidy.rs` is adjusted to respect the new scheme.

Also, `builtin_attrs.rs` sees some cleanup, organization, and DSL-ification to reduce repetition.

This is probably best read commit-by-commit I think.

r? @oli-obk

src/libsyntax/feature_gate/accepted.rs

@@ -0,0 +1,236 @@
+//! List of the accepted feature gates.
+
+use crate::symbol::{Symbol, sym};
+
+macro_rules! declare_features {
+    ($((accepted, $feature: ident, $ver: expr, $issue: expr, None),)+) => {
+        /// Those language feature has since been Accepted (it was once Active)
+        pub const ACCEPTED_FEATURES: &[(Symbol, &str, Option<u32>, Option<&str>)] = &[
+            $((sym::$feature, $ver, $issue, None)),+
+        ];
+    }
+}
+
+declare_features! (
+    // -------------------------------------------------------------------------
+    // feature-group-start: for testing purposes
+    // -------------------------------------------------------------------------
+
+    // A temporary feature gate used to enable parser extensions needed
+    // to bootstrap fix for #5723.
+    (accepted, issue_5723_bootstrap, "1.0.0", None, None),
+    // These are used to test this portion of the compiler,
+    // they don't actually mean anything.
+    (accepted, test_accepted_feature, "1.0.0", None, None),
+
+    // -------------------------------------------------------------------------
+    // feature-group-end: for testing purposes
+    // -------------------------------------------------------------------------
+
+    // -------------------------------------------------------------------------
+    // feature-group-start: accepted features
+    // -------------------------------------------------------------------------
+
+    // Allows using associated `type`s in `trait`s.
+    (accepted, associated_types, "1.0.0", None, None),
+    // Allows using assigning a default type to type parameters in algebraic data type definitions.
+    (accepted, default_type_params, "1.0.0", None, None),
+    // FIXME: explain `globs`.
+    (accepted, globs, "1.0.0", None, None),
+    // Allows `macro_rules!` items.
+    (accepted, macro_rules, "1.0.0", None, None),
+    // Allows use of `&foo[a..b]` as a slicing syntax.
+    (accepted, slicing_syntax, "1.0.0", None, None),
+    // Allows struct variants `Foo { baz: u8, .. }` in enums (RFC 418).
+    (accepted, struct_variant, "1.0.0", None, None),
+    // Allows indexing tuples.
+    (accepted, tuple_indexing, "1.0.0", None, None),
+    // Allows the use of `if let` expressions.
+    (accepted, if_let, "1.0.0", None, None),
+    // Allows the use of `while let` expressions.
+    (accepted, while_let, "1.0.0", None, None),
+    // Allows using `#![no_std]`.
+    (accepted, no_std, "1.6.0", None, None),
+    // Allows overloading augmented assignment operations like `a += b`.
+    (accepted, augmented_assignments, "1.8.0", Some(28235), None),
+    // Allows empty structs and enum variants with braces.
+    (accepted, braced_empty_structs, "1.8.0", Some(29720), None),
+    // Allows `#[deprecated]` attribute.
+    (accepted, deprecated, "1.9.0", Some(29935), None),
+    // Allows macros to appear in the type position.
+    (accepted, type_macros, "1.13.0", Some(27245), None),
+    // Allows use of the postfix `?` operator in expressions.
+    (accepted, question_mark, "1.13.0", Some(31436), None),
+    // Allows `..` in tuple (struct) patterns.
+    (accepted, dotdot_in_tuple_patterns, "1.14.0", Some(33627), None),
+    // Allows some increased flexibility in the name resolution rules,
+    // especially around globs and shadowing (RFC 1560).
+    (accepted, item_like_imports, "1.15.0", Some(35120), None),
+    // Allows using `Self` and associated types in struct expressions and patterns.
+    (accepted, more_struct_aliases, "1.16.0", Some(37544), None),
+    // Allows elision of `'static` lifetimes in `static`s and `const`s.
+    (accepted, static_in_const, "1.17.0", Some(35897), None),
+    // Allows field shorthands (`x` meaning `x: x`) in struct literal expressions.
+    (accepted, field_init_shorthand, "1.17.0", Some(37340), None),
+    // Allows the definition recursive static items.
+    (accepted, static_recursion, "1.17.0", Some(29719), None),
+    // Allows `pub(restricted)` visibilities (RFC 1422).
+    (accepted, pub_restricted, "1.18.0", Some(32409), None),
+    // Allows `#![windows_subsystem]`.
+    (accepted, windows_subsystem, "1.18.0", Some(37499), None),
+    // Allows `break {expr}` with a value inside `loop`s.
+    (accepted, loop_break_value, "1.19.0", Some(37339), None),
+    // Allows numeric fields in struct expressions and patterns.
+    (accepted, relaxed_adts, "1.19.0", Some(35626), None),
+    // Allows coercing non capturing closures to function pointers.
+    (accepted, closure_to_fn_coercion, "1.19.0", Some(39817), None),
+    // Allows attributes on struct literal fields.
+    (accepted, struct_field_attributes, "1.20.0", Some(38814), None),
+    // Allows the definition of associated constants in `trait` or `impl` blocks.
+    (accepted, associated_consts, "1.20.0", Some(29646), None),
+    // Allows usage of the `compile_error!` macro.
+    (accepted, compile_error, "1.20.0", Some(40872), None),
+    // Allows code like `let x: &'static u32 = &42` to work (RFC 1414).
+    (accepted, rvalue_static_promotion, "1.21.0", Some(38865), None),
+    // Allows `Drop` types in constants (RFC 1440).
+    (accepted, drop_types_in_const, "1.22.0", Some(33156), None),
+    // Allows the sysV64 ABI to be specified on all platforms
+    // instead of just the platforms on which it is the C ABI.
+    (accepted, abi_sysv64, "1.24.0", Some(36167), None),
+    // Allows `repr(align(16))` struct attribute (RFC 1358).
+    (accepted, repr_align, "1.25.0", Some(33626), None),
+    // Allows '|' at beginning of match arms (RFC 1925).
+    (accepted, match_beginning_vert, "1.25.0", Some(44101), None),
+    // Allows nested groups in `use` items (RFC 2128).
+    (accepted, use_nested_groups, "1.25.0", Some(44494), None),
+    // Allows indexing into constant arrays.
+    (accepted, const_indexing, "1.26.0", Some(29947), None),
+    // Allows using `a..=b` and `..=b` as inclusive range syntaxes.
+    (accepted, inclusive_range_syntax, "1.26.0", Some(28237), None),
+    // Allows `..=` in patterns (RFC 1192).
+    (accepted, dotdoteq_in_patterns, "1.26.0", Some(28237), None),
+    // Allows `fn main()` with return types which implements `Termination` (RFC 1937).
+    (accepted, termination_trait, "1.26.0", Some(43301), None),
+    // Allows implementing `Clone` for closures where possible (RFC 2132).
+    (accepted, clone_closures, "1.26.0", Some(44490), None),
+    // Allows implementing `Copy` for closures where possible (RFC 2132).
+    (accepted, copy_closures, "1.26.0", Some(44490), None),
+    // Allows `impl Trait` in function arguments.
+    (accepted, universal_impl_trait, "1.26.0", Some(34511), None),
+    // Allows `impl Trait` in function return types.
+    (accepted, conservative_impl_trait, "1.26.0", Some(34511), None),
+    // Allows using the `u128` and `i128` types.
+    (accepted, i128_type, "1.26.0", Some(35118), None),
+    // Allows default match binding modes (RFC 2005).
+    (accepted, match_default_bindings, "1.26.0", Some(42640), None),
+    // Allows `'_` placeholder lifetimes.
+    (accepted, underscore_lifetimes, "1.26.0", Some(44524), None),
+    // Allows attributes on lifetime/type formal parameters in generics (RFC 1327).
+    (accepted, generic_param_attrs, "1.27.0", Some(48848), None),
+    // Allows `cfg(target_feature = "...")`.
+    (accepted, cfg_target_feature, "1.27.0", Some(29717), None),
+    // Allows `#[target_feature(...)]`.
+    (accepted, target_feature, "1.27.0", None, None),
+    // Allows using `dyn Trait` as a syntax for trait objects.
+    (accepted, dyn_trait, "1.27.0", Some(44662), None),
+    // Allows `#[must_use]` on functions, and introduces must-use operators (RFC 1940).
+    (accepted, fn_must_use, "1.27.0", Some(43302), None),
+    // Allows use of the `:lifetime` macro fragment specifier.
+    (accepted, macro_lifetime_matcher, "1.27.0", Some(34303), None),
+    // Allows `#[test]` functions where the return type implements `Termination` (RFC 1937).
+    (accepted, termination_trait_test, "1.27.0", Some(48854), None),
+    // Allows the `#[global_allocator]` attribute.
+    (accepted, global_allocator, "1.28.0", Some(27389), None),
+    // Allows `#[repr(transparent)]` attribute on newtype structs.
+    (accepted, repr_transparent, "1.28.0", Some(43036), None),
+    // Allows procedural macros in `proc-macro` crates.
+    (accepted, proc_macro, "1.29.0", Some(38356), None),
+    // Allows `foo.rs` as an alternative to `foo/mod.rs`.
+    (accepted, non_modrs_mods, "1.30.0", Some(44660), None),
+    // Allows use of the `:vis` macro fragment specifier
+    (accepted, macro_vis_matcher, "1.30.0", Some(41022), None),
+    // Allows importing and reexporting macros with `use`,
+    // enables macro modularization in general.
+    (accepted, use_extern_macros, "1.30.0", Some(35896), None),
+    // Allows keywords to be escaped for use as identifiers.
+    (accepted, raw_identifiers, "1.30.0", Some(48589), None),
+    // Allows attributes scoped to tools.
+    (accepted, tool_attributes, "1.30.0", Some(44690), None),
+    // Allows multi-segment paths in attributes and derives.
+    (accepted, proc_macro_path_invoc, "1.30.0", Some(38356), None),
+    // Allows all literals in attribute lists and values of key-value pairs.
+    (accepted, attr_literals, "1.30.0", Some(34981), None),
+    // Allows inferring outlives requirements (RFC 2093).
+    (accepted, infer_outlives_requirements, "1.30.0", Some(44493), None),
+    // Allows annotating functions conforming to `fn(&PanicInfo) -> !` with `#[panic_handler]`.
+    // This defines the behavior of panics.
+    (accepted, panic_handler, "1.30.0", Some(44489), None),
+    // Allows `#[used]` to preserve symbols (see llvm.used).
+    (accepted, used, "1.30.0", Some(40289), None),
+    // Allows `crate` in paths.
+    (accepted, crate_in_paths, "1.30.0", Some(45477), None),
+    // Allows resolving absolute paths as paths from other crates.
+    (accepted, extern_absolute_paths, "1.30.0", Some(44660), None),
+    // Allows access to crate names passed via `--extern` through prelude.
+    (accepted, extern_prelude, "1.30.0", Some(44660), None),
+    // Allows parentheses in patterns.
+    (accepted, pattern_parentheses, "1.31.0", Some(51087), None),
+    // Allows the definition of `const fn` functions.
+    (accepted, min_const_fn, "1.31.0", Some(53555), None),
+    // Allows scoped lints.
+    (accepted, tool_lints, "1.31.0", Some(44690), None),
+    // Allows lifetime elision in `impl` headers. For example:
+    // + `impl<I:Iterator> Iterator for &mut Iterator`
+    // + `impl Debug for Foo<'_>`
+    (accepted, impl_header_lifetime_elision, "1.31.0", Some(15872), None),
+    // Allows `extern crate foo as bar;`. This puts `bar` into extern prelude.
+    (accepted, extern_crate_item_prelude, "1.31.0", Some(55599), None),
+    // Allows use of the `:literal` macro fragment specifier (RFC 1576).
+    (accepted, macro_literal_matcher, "1.32.0", Some(35625), None),
+    // Allows use of `?` as the Kleene "at most one" operator in macros.
+    (accepted, macro_at_most_once_rep, "1.32.0", Some(48075), None),
+    // Allows `Self` struct constructor (RFC 2302).
+    (accepted, self_struct_ctor, "1.32.0", Some(51994), None),
+    // Allows `Self` in type definitions (RFC 2300).
+    (accepted, self_in_typedefs, "1.32.0", Some(49303), None),
+    // Allows `use x::y;` to search `x` in the current scope.
+    (accepted, uniform_paths, "1.32.0", Some(53130), None),
+    // Allows integer match exhaustiveness checking (RFC 2591).
+    (accepted, exhaustive_integer_patterns, "1.33.0", Some(50907), None),
+    // Allows `use path as _;` and `extern crate c as _;`.
+    (accepted, underscore_imports, "1.33.0", Some(48216), None),
+    // Allows `#[repr(packed(N))]` attribute on structs.
+    (accepted, repr_packed, "1.33.0", Some(33158), None),
+    // Allows irrefutable patterns in `if let` and `while let` statements (RFC 2086).
+    (accepted, irrefutable_let_patterns, "1.33.0", Some(44495), None),
+    // Allows calling `const unsafe fn` inside `unsafe` blocks in `const fn` functions.
+    (accepted, min_const_unsafe_fn, "1.33.0", Some(55607), None),
+    // Allows let bindings, assignments and destructuring in `const` functions and constants.
+    // As long as control flow is not implemented in const eval, `&&` and `||` may not be used
+    // at the same time as let bindings.
+    (accepted, const_let, "1.33.0", Some(48821), None),
+    // Allows `#[cfg_attr(predicate, multiple, attributes, here)]`.
+    (accepted, cfg_attr_multi, "1.33.0", Some(54881), None),
+    // Allows top level or-patterns (`p | q`) in `if let` and `while let`.
+    (accepted, if_while_or_patterns, "1.33.0", Some(48215), None),
+    // Allows `cfg(target_vendor = "...")`.
+    (accepted, cfg_target_vendor, "1.33.0", Some(29718), None),
+    // Allows `extern crate self as foo;`.
+    // This puts local crate root into extern prelude under name `foo`.
+    (accepted, extern_crate_self, "1.34.0", Some(56409), None),
+    // Allows arbitrary delimited token streams in non-macro attributes.
+    (accepted, unrestricted_attribute_tokens, "1.34.0", Some(55208), None),
+    // Allows paths to enum variants on type aliases including `Self`.
+    (accepted, type_alias_enum_variants, "1.37.0", Some(49683), None),
+    // Allows using `#[repr(align(X))]` on enums with equivalent semantics
+    // to wrapping an enum in a wrapper struct with `#[repr(align(X))]`.
+    (accepted, repr_align_enum, "1.37.0", Some(57996), None),
+    // Allows `const _: TYPE = VALUE`.
+    (accepted, underscore_const_names, "1.37.0", Some(54912), None),
+    // Allows free and inherent `async fn`s, `async` blocks, and `<expr>.await` expressions.
+    (accepted, async_await, "1.39.0", Some(50547), None),
+
+    // -------------------------------------------------------------------------
+    // feature-group-end: accepted features
+    // -------------------------------------------------------------------------
+);