Implement arbitrary_enum_discriminant

src/doc/unstable-book/src/language-features/arbitrary-enum-discriminant.md

@@ -0,0 +1,37 @@
+# `arbitrary_enum_discriminant`
+
+The tracking issue for this feature is: [#60553]
+
+[#60553]: https://github.com/rust-lang/rust/issues/60553
+
+------------------------
+
+The `arbitrary_enum_discriminant` feature permits tuple-like and
+struct-like enum variants with `#[repr(<int-type>)]` to have explicit discriminants.
+
+## Examples
+
+```rust
+#![feature(arbitrary_enum_discriminant)]
+
+#[allow(dead_code)]
+#[repr(u8)]
+enum Enum {
+    Unit = 3,
+    Tuple(u16) = 2,
+    Struct {
+        a: u8,
+        b: u16,
+    } = 1,
+}
+
+impl Enum {
+    fn tag(&self) -> u8 {
+        unsafe { *(self as *const Self as *const u8) }
+    }
+}
+
+assert_eq!(3, Enum::Unit.tag());
+assert_eq!(2, Enum::Tuple(5).tag());
+assert_eq!(1, Enum::Struct{a: 7, b: 11}.tag());
+```

src/librustc_typeck/check/mod.rs

@@ -1936,6 +1936,25 @@ pub fn check_enum<'tcx>(tcx: TyCtxt<'tcx>, sp: Span, vs: &'tcx [hir::Variant], i
         }
     }
 
+    if tcx.adt_def(def_id).repr.int.is_none() && tcx.features().arbitrary_enum_discriminant {
+        let is_unit =
+            |var: &hir::Variant| match var.node.data {
+                hir::VariantData::Unit(..) => true,
+                _ => false
+            };
+
+        let has_disr = |var: &hir::Variant| var.node.disr_expr.is_some();
+        let has_non_units = vs.iter().any(|var| !is_unit(var));
+        let disr_units = vs.iter().any(|var| is_unit(&var) && has_disr(&var));
+        let disr_non_unit = vs.iter().any(|var| !is_unit(&var) && has_disr(&var));
+
+        if disr_non_unit || (disr_units && has_non_units) {
+            let mut err = struct_span_err!(tcx.sess, sp, E0732,
+                                           "`#[repr(inttype)]` must be specified");
+            err.emit();
+        }
+    }
+
     let mut disr_vals: Vec<Discr<'tcx>> = Vec::with_capacity(vs.len());
     for ((_, discr), v) in def.discriminants(tcx).zip(vs) {
         // Check for duplicate discriminant values

src/librustc_typeck/error_codes.rs

@@ -4733,6 +4733,38 @@ if there are multiple variants, it is not clear how the enum should be
 represented.
 "##,
 
+E0732: r##"
+An `enum` with a discriminant must specify a `#[repr(inttype)]`.
+
+A `#[repr(inttype)]` must be provided on an `enum` if it has a non-unit
+variant with a discriminant, or where there are both unit variants with
+discriminants and non-unit variants. This restriction ensures that there
+is a well-defined way to extract a variant's discriminant from a value;
+for instance:
+
+```
+#![feature(arbitrary_enum_discriminant)]
+
+#[repr(u8)]
+enum Enum {
+    Unit = 3,
+    Tuple(u16) = 2,
+    Struct {
+        a: u8,
+        b: u16,
+    } = 1,
+}
+
+fn discriminant(v : &Enum) -> u8 {
+    unsafe { *(v as *const Enum as *const u8) }
+}
+
+assert_eq!(3, discriminant(&Enum::Unit));
+assert_eq!(2, discriminant(&Enum::Tuple(5)));
+assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));
+```
+"##,
+
 }
 
 register_diagnostics! {

src/libsyntax/feature_gate.rs

@@ -25,13 +25,14 @@ use crate::source_map::Spanned;
 use crate::edition::{ALL_EDITIONS, Edition};
 use crate::visit::{self, FnKind, Visitor};
 use crate::parse::{token, ParseSess};
+use crate::parse::parser::Parser;
 use crate::symbol::{Symbol, sym};
 use crate::tokenstream::TokenTree;
 
 use errors::{Applicability, DiagnosticBuilder, Handler};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_target::spec::abi::Abi;
-use syntax_pos::{Span, DUMMY_SP};
+use syntax_pos::{Span, DUMMY_SP, MultiSpan};
 use log::debug;
 use lazy_static::lazy_static;
 
@@ -566,6 +567,9 @@ declare_features! (
     // #[repr(transparent)] on unions.
     (active, transparent_unions, "1.37.0", Some(60405), None),
 
+    // Allows explicit discriminants on non-unit enum variants.
+    (active, arbitrary_enum_discriminant, "1.37.0", Some(60553), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -1705,20 +1709,20 @@ pub fn emit_feature_err(
     feature_err(sess, feature, span, issue, explain).emit();
 }
 
-pub fn feature_err<'a>(
+pub fn feature_err<'a, S: Into<MultiSpan>>(
     sess: &'a ParseSess,
     feature: Symbol,
-    span: Span,
+    span: S,
     issue: GateIssue,
     explain: &str,
 ) -> DiagnosticBuilder<'a> {
     leveled_feature_err(sess, feature, span, issue, explain, GateStrength::Hard)
 }
 
-fn leveled_feature_err<'a>(
+fn leveled_feature_err<'a, S: Into<MultiSpan>>(
     sess: &'a ParseSess,
     feature: Symbol,
-    span: Span,
+    span: S,
     issue: GateIssue,
     explain: &str,
     level: GateStrength,
@@ -2033,6 +2037,29 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
             }
 
+            ast::ItemKind::Enum(ast::EnumDef{ref variants, ..}, ..) => {
+                for variant in variants {
+                    match (&variant.node.data, &variant.node.disr_expr) {
+                        (ast::VariantData::Unit(..), _) => {},
+                        (_, Some(disr_expr)) =>
+                            gate_feature_post!(
+                                &self,
+                                arbitrary_enum_discriminant,
+                                disr_expr.value.span,
+                                "discriminants on non-unit variants are experimental"),
+                        _ => {},
+                    }
+                }
+
+                let has_feature = self.context.features.arbitrary_enum_discriminant;
+                if !has_feature && !i.span.allows_unstable(sym::arbitrary_enum_discriminant) {
+                    Parser::maybe_report_invalid_custom_discriminants(
+                        self.context.parse_sess,
+                        &variants,
+                    );
+                }
+            }
+
             ast::ItemKind::Impl(_, polarity, defaultness, _, _, _, _) => {
                 if polarity == ast::ImplPolarity::Negative {
                     gate_feature_post!(&self, optin_builtin_traits,

src/libsyntax/parse/diagnostics.rs

@@ -2,7 +2,7 @@ use crate::ast::{
     self, Arg, BinOpKind, BindingMode, BlockCheckMode, Expr, ExprKind, Ident, Item, ItemKind,
     Mutability, Pat, PatKind, PathSegment, QSelf, Ty, TyKind, VariantData,
 };
-use crate::parse::{SeqSep, PResult, Parser};
+use crate::parse::{SeqSep, PResult, Parser, ParseSess};
 use crate::parse::parser::{BlockMode, PathStyle, SemiColonMode, TokenType, TokenExpectType};
 use crate::parse::token::{self, TokenKind};
 use crate::print::pprust;
@@ -539,37 +539,47 @@ impl<'a> Parser<'a> {
     }
 
     crate fn maybe_report_invalid_custom_discriminants(
-        &mut self,
-        discriminant_spans: Vec<Span>,
+        sess: &ParseSess,
         variants: &[Spanned<ast::Variant_>],
     ) {
         let has_fields = variants.iter().any(|variant| match variant.node.data {
             VariantData::Tuple(..) | VariantData::Struct(..) => true,
             VariantData::Unit(..) => false,
         });
 
+        let discriminant_spans = variants.iter().filter(|variant| match variant.node.data {
+            VariantData::Tuple(..) | VariantData::Struct(..) => false,
+            VariantData::Unit(..) => true,
+        })
+        .filter_map(|variant| variant.node.disr_expr.as_ref().map(|c| c.value.span))
+        .collect::<Vec<_>>();
+
         if !discriminant_spans.is_empty() && has_fields {
-            let mut err = self.struct_span_err(
+            let mut err = crate::feature_gate::feature_err(
+                sess,
+                sym::arbitrary_enum_discriminant,
                 discriminant_spans.clone(),
-                "custom discriminant values are not allowed in enums with fields",
+                crate::feature_gate::GateIssue::Language,
+                "custom discriminant values are not allowed in enums with tuple or struct variants",
             );
             for sp in discriminant_spans {
-                err.span_label(sp, "invalid custom discriminant");
+                err.span_label(sp, "disallowed custom discriminant");
             }
             for variant in variants.iter() {
-                if let VariantData::Struct(fields, ..) | VariantData::Tuple(fields, ..) =
-                    &variant.node.data
-                {
-                    let fields = if fields.len() > 1 {
-                        "fields"
-                    } else {
-                        "a field"
-                    };
-                    err.span_label(
-                        variant.span,
-                        &format!("variant with {fields} defined here", fields = fields),
-                    );
-
+                match &variant.node.data {
+                    VariantData::Struct(..) => {
+                        err.span_label(
+                            variant.span,
+                            "struct variant defined here",
+                        );
+                    }
+                    VariantData::Tuple(..) => {
+                        err.span_label(
+                            variant.span,
+                            "tuple variant defined here",
+                        );
+                    }
+                    VariantData::Unit(..) => {}
                 }
             }
             err.emit();

src/libsyntax/parse/parser.rs

@@ -6963,36 +6963,34 @@ impl<'a> Parser<'a> {
     /// Parses the part of an enum declaration following the `{`.
     fn parse_enum_def(&mut self, _generics: &ast::Generics) -> PResult<'a, EnumDef> {
         let mut variants = Vec::new();
-        let mut any_disr = vec![];
         while self.token != token::CloseDelim(token::Brace) {
             let variant_attrs = self.parse_outer_attributes()?;
             let vlo = self.token.span;
 
-            let struct_def;
-            let mut disr_expr = None;
             self.eat_bad_pub();
             let ident = self.parse_ident()?;
-            if self.check(&token::OpenDelim(token::Brace)) {
+
+            let struct_def = if self.check(&token::OpenDelim(token::Brace)) {
                 // Parse a struct variant.
                 let (fields, recovered) = self.parse_record_struct_body()?;
-                struct_def = VariantData::Struct(fields, recovered);
+                VariantData::Struct(fields, recovered)
             } else if self.check(&token::OpenDelim(token::Paren)) {
-                struct_def = VariantData::Tuple(
+                VariantData::Tuple(
                     self.parse_tuple_struct_body()?,
                     ast::DUMMY_NODE_ID,
-                );
-            } else if self.eat(&token::Eq) {
-                disr_expr = Some(AnonConst {
+                )
+            } else {
+                VariantData::Unit(ast::DUMMY_NODE_ID)
+            };
+
+            let disr_expr = if self.eat(&token::Eq) {
+                Some(AnonConst {
                     id: ast::DUMMY_NODE_ID,
                     value: self.parse_expr()?,
-                });
-                if let Some(sp) = disr_expr.as_ref().map(|c| c.value.span) {
-                    any_disr.push(sp);
-                }
-                struct_def = VariantData::Unit(ast::DUMMY_NODE_ID);
+                })
             } else {
-                struct_def = VariantData::Unit(ast::DUMMY_NODE_ID);
-            }
+                None
+            };
 
             let vr = ast::Variant_ {
                 ident,
@@ -7020,7 +7018,6 @@ impl<'a> Parser<'a> {
             }
         }
         self.expect(&token::CloseDelim(token::Brace))?;
-        self.maybe_report_invalid_custom_discriminants(any_disr, &variants);
 
         Ok(ast::EnumDef { variants })
     }

src/libsyntax_pos/symbol.rs

@@ -135,6 +135,7 @@ symbols! {
         always,
         and,
         any,
+        arbitrary_enum_discriminant,
         arbitrary_self_types,
         Arguments,
         ArgumentV1,

src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.rs

@@ -0,0 +1,9 @@
+#![crate_type="lib"]
+#![feature(arbitrary_enum_discriminant)]
+
+enum Enum {
+//~^ ERROR `#[repr(inttype)]` must be specified
+  Unit = 1,
+  Tuple() = 2,
+  Struct{} = 3,
+}

src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.stderr

@@ -0,0 +1,14 @@
+error[E0732]: `#[repr(inttype)]` must be specified
+  --> $DIR/arbitrary_enum_discriminant-no-repr.rs:4:1
+   |
+LL | / enum Enum {
+LL | |
+LL | |   Unit = 1,
+LL | |   Tuple() = 2,
+LL | |   Struct{} = 3,
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0732`.