Auto merge of rust-lang#61682 - Centril:stabilize-type_alias_enum_variants, r=petrochenkov

Stabilize `type_alias_enum_variants` in Rust 1.37.0

Stabilize `#![feature(type_alias_enum_variants)]` which allows type-relative resolution with highest priority to `enum` variants in both expression and pattern contexts. For example, you may now write:

```rust
enum Option<T> {
    None,
    Some(T),
}

type OptAlias<T> = Option<T>;

fn work_on_alias(x: Option<u8>) -> u8 {
    match x {
        OptAlias::Some(y) => y + 1,
        OptAlias::None => 0,
    }
}
```

Closes rust-lang/rfcs#2218
Closes rust-lang#52118

r? @petrochenkov

Author: bors

src/doc/unstable-book/src/language-features/type-alias-enum-variants.md

@@ -6,7 +6,7 @@
 #![feature(label_break_value)]
 #![feature(nll)]
 #![feature(rustc_diagnostic_macros)]
-#![feature(type_alias_enum_variants)]
+#![cfg_attr(bootstrap, feature(type_alias_enum_variants))]
 
 #![recursion_limit="256"]
 

src/librustc_resolve/lib.rs

@@ -34,7 +34,6 @@ use std::collections::BTreeSet;
 use std::iter;
 use std::slice;
 
-use super::{check_type_alias_enum_variants_enabled};
 use rustc_data_structures::fx::FxHashSet;
 
 #[derive(Debug)]
@@ -1595,7 +1594,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 });
                 if let Some(variant_def) = variant_def {
                     if permit_variants {
-                        check_type_alias_enum_variants_enabled(tcx, span);
                         tcx.check_stability(variant_def.def_id, Some(hir_ref_id), span);
                         return Ok((qself_ty, DefKind::Variant, variant_def.def_id));
                     } else {

src/librustc_typeck/astconv.rs

@@ -26,7 +26,6 @@ use rustc::infer::{self, InferOk};
 use syntax::ast;
 use syntax_pos::Span;
 
-use crate::{check_type_alias_enum_variants_enabled};
 use self::probe::{IsSuggestion, ProbeScope};
 
 pub fn provide(providers: &mut ty::query::Providers<'_>) {
@@ -417,8 +416,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     tcx.hygienic_eq(method_name, vd.ident, adt_def.did)
                 });
                 if let Some(variant_def) = variant_def {
-                    check_type_alias_enum_variants_enabled(tcx, span);
-
                     // Braced variants generate unusable names in value namespace (reserved for
                     // possible future use), so variants resolved as associated items may refer to
                     // them as well. It's ok to use the variant's id as a ctor id since an

src/librustc_typeck/check/method/mod.rs

@@ -105,7 +105,7 @@ use rustc::lint;
 use rustc::middle;
 use rustc::session;
 use rustc::util::common::ErrorReported;
-use rustc::session::config::{EntryFnType, nightly_options};
+use rustc::session::config::EntryFnType;
 use rustc::traits::{ObligationCause, ObligationCauseCode, TraitEngine, TraitEngineExt};
 use rustc::ty::subst::SubstsRef;
 use rustc::ty::{self, Ty, TyCtxt};
@@ -124,21 +124,6 @@ pub struct TypeAndSubsts<'tcx> {
     ty: Ty<'tcx>,
 }
 
-fn check_type_alias_enum_variants_enabled<'tcx>(tcx: TyCtxt<'tcx>, span: Span) {
-    if !tcx.features().type_alias_enum_variants {
-        let mut err = tcx.sess.struct_span_err(
-            span,
-            "enum variants on type aliases are experimental"
-        );
-        if nightly_options::is_nightly_build() {
-            help!(&mut err,
-                "add `#![feature(type_alias_enum_variants)]` to the \
-                crate attributes to enable");
-        }
-        err.emit();
-    }
-}
-
 fn require_c_abi_if_c_variadic(tcx: TyCtxt<'_>, decl: &hir::FnDecl, abi: Abi, span: Span) {
     if decl.c_variadic && !(abi == Abi::C || abi == Abi::Cdecl) {
         let mut err = struct_span_err!(tcx.sess, span, E0045,

src/librustc_typeck/lib.rs

@@ -530,9 +530,6 @@ declare_features! (
     // Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.
     (active, lint_reasons, "1.31.0", Some(54503), None),
 
-    // Allows paths to enum variants on type aliases.
-    (active, type_alias_enum_variants, "1.31.0", Some(49683), None),
-
     // Allows exhaustive integer pattern matching on `usize` and `isize`.
     (active, precise_pointer_size_matching, "1.32.0", Some(56354), None),
 
@@ -853,6 +850,8 @@ declare_features! (
     (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),

src/libsyntax/feature_gate.rs

@@ -1,17 +1,22 @@
 // run-pass
 
-#![feature(type_alias_enum_variants)]
+// Check that resolving, in the value namespace, to an `enum` variant
+// through a type alias is well behaved in the presence of generics.
+// We check for situations with:
+// 1. a generic type `Alias<T>`, we can type-apply `Alias` when referring to a variant.
+// 2. a monotype `AliasFixed` of generic `Enum<T>`, we can refer to variants
+//    and the type-application of `T` in `AliasFixed` is kept.
 
 #![allow(irrefutable_let_patterns)]
 
-#[allow(dead_code)]
-enum Enum<T> { TSVariant(T), SVariant { v: T } }
+enum Enum<T> { TSVariant(T), SVariant { v: T }, UVariant }
 type Alias<T> = Enum<T>;
 type AliasFixed = Enum<()>;
 
 macro_rules! is_variant {
     (TSVariant, $expr:expr) => (is_variant!(@check TSVariant, (_), $expr));
     (SVariant, $expr:expr) => (is_variant!(@check SVariant, { v: _ }, $expr));
+    (UVariant, $expr:expr) => (is_variant!(@check UVariant, {}, $expr));
     (@check $variant:ident, $matcher:tt, $expr:expr) => (
         assert!(if let Enum::$variant::<()> $matcher = $expr { true } else { false },
                 "expr does not have correct type");
@@ -40,4 +45,15 @@ fn main() {
     is_variant!(SVariant, Alias::<()>::SVariant { v: () });
 
     is_variant!(SVariant, AliasFixed::SVariant { v: () });
+
+    // Unit variant
+
+    is_variant!(UVariant, Enum::UVariant);
+    is_variant!(UVariant, Enum::UVariant::<()>);
+    is_variant!(UVariant, Enum::<()>::UVariant);
+
+    is_variant!(UVariant, Alias::UVariant);
+    is_variant!(UVariant, Alias::<()>::UVariant);
+
+    is_variant!(UVariant, AliasFixed::UVariant);
 }