Implement const expressions and patterns (RFC 2920)

compiler/rustc_ast/src/ast.rs

@@ -1152,6 +1152,7 @@ impl Expr {
         match self.kind {
             ExprKind::Box(_) => ExprPrecedence::Box,
             ExprKind::Array(_) => ExprPrecedence::Array,
+            ExprKind::ConstBlock(_) => ExprPrecedence::ConstBlock,
             ExprKind::Call(..) => ExprPrecedence::Call,
             ExprKind::MethodCall(..) => ExprPrecedence::MethodCall,
             ExprKind::Tup(_) => ExprPrecedence::Tup,
@@ -1207,6 +1208,8 @@ pub enum ExprKind {
     Box(P<Expr>),
     /// An array (`[a, b, c, d]`)
     Array(Vec<P<Expr>>),
+    /// Allow anonymous constants from an inline `const` block
+    ConstBlock(AnonConst),
     /// A function call
     ///
     /// The first field resolves to the function itself,

compiler/rustc_ast/src/mut_visit.rs

@@ -1106,6 +1106,9 @@ pub fn noop_visit_expr<T: MutVisitor>(
     match kind {
         ExprKind::Box(expr) => vis.visit_expr(expr),
         ExprKind::Array(exprs) => visit_exprs(exprs, vis),
+        ExprKind::ConstBlock(anon_const) => {
+            vis.visit_anon_const(anon_const);
+        }
         ExprKind::Repeat(expr, count) => {
             vis.visit_expr(expr);
             vis.visit_anon_const(count);

compiler/rustc_ast/src/token.rs

@@ -153,6 +153,7 @@ pub fn ident_can_begin_expr(name: Symbol, span: Span, is_raw: bool) -> bool {
             kw::Do,
             kw::Box,
             kw::Break,
+            kw::Const,
             kw::Continue,
             kw::False,
             kw::For,

compiler/rustc_ast/src/util/parser.rs

@@ -282,6 +282,7 @@ pub enum ExprPrecedence {
     ForLoop,
     Loop,
     Match,
+    ConstBlock,
     Block,
     TryBlock,
     Struct,
@@ -346,6 +347,7 @@ impl ExprPrecedence {
             ExprPrecedence::ForLoop |
             ExprPrecedence::Loop |
             ExprPrecedence::Match |
+            ExprPrecedence::ConstBlock |
             ExprPrecedence::Block |
             ExprPrecedence::TryBlock |
             ExprPrecedence::Async |

compiler/rustc_ast/src/visit.rs

@@ -200,11 +200,7 @@ pub trait Visitor<'ast>: Sized {
         walk_generic_args(self, path_span, generic_args)
     }
     fn visit_generic_arg(&mut self, generic_arg: &'ast GenericArg) {
-        match generic_arg {
-            GenericArg::Lifetime(lt) => self.visit_lifetime(lt),
-            GenericArg::Type(ty) => self.visit_ty(ty),
-            GenericArg::Const(ct) => self.visit_anon_const(ct),
-        }
+        walk_generic_arg(self, generic_arg)
     }
     fn visit_assoc_ty_constraint(&mut self, constraint: &'ast AssocTyConstraint) {
         walk_assoc_ty_constraint(self, constraint)
@@ -486,6 +482,17 @@ where
     }
 }
 
+pub fn walk_generic_arg<'a, V>(visitor: &mut V, generic_arg: &'a GenericArg)
+where
+    V: Visitor<'a>,
+{
+    match generic_arg {
+        GenericArg::Lifetime(lt) => visitor.visit_lifetime(lt),
+        GenericArg::Type(ty) => visitor.visit_ty(ty),
+        GenericArg::Const(ct) => visitor.visit_anon_const(ct),
+    }
+}
+
 pub fn walk_assoc_ty_constraint<'a, V: Visitor<'a>>(
     visitor: &mut V,
     constraint: &'a AssocTyConstraint,
@@ -717,6 +724,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Array(ref subexpressions) => {
             walk_list!(visitor, visit_expr, subexpressions);
         }
+        ExprKind::ConstBlock(ref anon_const) => visitor.visit_anon_const(anon_const),
         ExprKind::Repeat(ref element, ref count) => {
             visitor.visit_expr(element);
             visitor.visit_anon_const(count)

compiler/rustc_ast_lowering/src/expr.rs

@@ -30,6 +30,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
             let kind = match e.kind {
                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
                 ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
+                ExprKind::ConstBlock(ref anon_const) => {
+                    let anon_const = self.lower_anon_const(anon_const);
+                    hir::ExprKind::ConstBlock(anon_const)
+                }
                 ExprKind::Repeat(ref expr, ref count) => {
                     let expr = self.lower_expr(expr);
                     let count = self.lower_anon_const(count);

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -287,7 +287,7 @@ impl<'a> AstValidator<'a> {
     // ```
     fn check_expr_within_pat(&self, expr: &Expr, allow_paths: bool) {
         match expr.kind {
-            ExprKind::Lit(..) | ExprKind::Err => {}
+            ExprKind::Lit(..) | ExprKind::ConstBlock(..) | ExprKind::Err => {}
             ExprKind::Path(..) if allow_paths => {}
             ExprKind::Unary(UnOp::Neg, ref inner) if matches!(inner.kind, ExprKind::Lit(_)) => {}
             _ => self.err_handler().span_err(

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -629,6 +629,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(const_trait_bound_opt_out, "`?const` on trait bounds is experimental");
     gate_all!(const_trait_impl, "const trait impls are experimental");
     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
+    gate_all!(inline_const, "inline-const is experimental");
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1714,6 +1714,14 @@ impl<'a> State<'a> {
         self.end();
     }
 
+    fn print_expr_anon_const(&mut self, expr: &ast::AnonConst, attrs: &[ast::Attribute]) {
+        self.ibox(INDENT_UNIT);
+        self.s.word("const");
+        self.print_inner_attributes_inline(attrs);
+        self.print_expr(&expr.value);
+        self.end();
+    }
+
     fn print_expr_repeat(
         &mut self,
         element: &ast::Expr,
@@ -1890,6 +1898,9 @@ impl<'a> State<'a> {
             ast::ExprKind::Array(ref exprs) => {
                 self.print_expr_vec(&exprs[..], attrs);
             }
+            ast::ExprKind::ConstBlock(ref anon_const) => {
+                self.print_expr_anon_const(anon_const, attrs);
+            }
             ast::ExprKind::Repeat(ref element, ref count) => {
                 self.print_expr_repeat(element, count, attrs);
             }

compiler/rustc_feature/src/active.rs

@@ -598,6 +598,9 @@ declare_features! (
     /// Allows `#[instruction_set(_)]` attribute
     (active, isa_attribute, "1.48.0", Some(74727), None),
 
+    /// Allow anonymous constants from an inline `const` block
+    (active, inline_const, "1.49.0", Some(76001), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -618,6 +621,7 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::const_trait_bound_opt_out,
     sym::lazy_normalization_consts,
     sym::specialization,
+    sym::inline_const,
 ];
 
 /// Some features are not allowed to be used together at the same time, if

compiler/rustc_hir/src/hir.rs

@@ -1361,6 +1361,7 @@ impl Expr<'_> {
     pub fn precedence(&self) -> ExprPrecedence {
         match self.kind {
             ExprKind::Box(_) => ExprPrecedence::Box,
+            ExprKind::ConstBlock(_) => ExprPrecedence::ConstBlock,
             ExprKind::Array(_) => ExprPrecedence::Array,
             ExprKind::Call(..) => ExprPrecedence::Call,
             ExprKind::MethodCall(..) => ExprPrecedence::MethodCall,
@@ -1446,6 +1447,7 @@ impl Expr<'_> {
             | ExprKind::LlvmInlineAsm(..)
             | ExprKind::AssignOp(..)
             | ExprKind::Lit(_)
+            | ExprKind::ConstBlock(..)
             | ExprKind::Unary(..)
             | ExprKind::Box(..)
             | ExprKind::AddrOf(..)
@@ -1501,6 +1503,8 @@ pub fn is_range_literal(expr: &Expr<'_>) -> bool {
 pub enum ExprKind<'hir> {
     /// A `box x` expression.
     Box(&'hir Expr<'hir>),
+    /// Allow anonymous constants from an inline `const` block
+    ConstBlock(AnonConst),
     /// An array (e.g., `[a, b, c, d]`).
     Array(&'hir [Expr<'hir>]),
     /// A function call.

compiler/rustc_hir/src/intravisit.rs

@@ -1068,6 +1068,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
         ExprKind::Array(subexpressions) => {
             walk_list!(visitor, visit_expr, subexpressions);
         }
+        ExprKind::ConstBlock(ref anon_const) => visitor.visit_anon_const(anon_const),
         ExprKind::Repeat(ref element, ref count) => {
             visitor.visit_expr(element);
             visitor.visit_anon_const(count)

compiler/rustc_hir_pretty/src/lib.rs

@@ -1135,6 +1135,15 @@ impl<'a> State<'a> {
         self.end()
     }
 
+    fn print_expr_anon_const(&mut self, anon_const: &hir::AnonConst) {
+        self.ibox(INDENT_UNIT);
+        self.s.word_space("const");
+        self.s.word("{");
+        self.print_anon_const(anon_const);
+        self.s.word("}");
+        self.end()
+    }
+
     fn print_expr_repeat(&mut self, element: &hir::Expr<'_>, count: &hir::AnonConst) {
         self.ibox(INDENT_UNIT);
         self.s.word("[");
@@ -1287,6 +1296,9 @@ impl<'a> State<'a> {
             hir::ExprKind::Array(ref exprs) => {
                 self.print_expr_vec(exprs);
             }
+            hir::ExprKind::ConstBlock(ref anon_const) => {
+                self.print_expr_anon_const(anon_const);
+            }
             hir::ExprKind::Repeat(ref element, ref count) => {
                 self.print_expr_repeat(&element, count);
             }

compiler/rustc_lint/src/early.rs

@@ -195,6 +195,11 @@ impl<'a, T: EarlyLintPass> ast_visit::Visitor<'a> for EarlyContextAndPass<'a, T>
         run_early_pass!(self, check_expr_post, e);
     }
 
+    fn visit_generic_arg(&mut self, arg: &'a ast::GenericArg) {
+        run_early_pass!(self, check_generic_arg, arg);
+        ast_visit::walk_generic_arg(self, arg);
+    }
+
     fn visit_generic_param(&mut self, param: &'a ast::GenericParam) {
         run_early_pass!(self, check_generic_param, param);
         ast_visit::walk_generic_param(self, param);

compiler/rustc_lint/src/passes.rs

@@ -33,6 +33,7 @@ macro_rules! late_lint_methods {
             fn check_expr(a: &$hir hir::Expr<$hir>);
             fn check_expr_post(a: &$hir hir::Expr<$hir>);
             fn check_ty(a: &$hir hir::Ty<$hir>);
+            fn check_generic_arg(a: &$hir hir::GenericArg<$hir>);
             fn check_generic_param(a: &$hir hir::GenericParam<$hir>);
             fn check_generics(a: &$hir hir::Generics<$hir>);
             fn check_where_predicate(a: &$hir hir::WherePredicate<$hir>);
@@ -176,6 +177,7 @@ macro_rules! early_lint_methods {
             fn check_expr(a: &ast::Expr);
             fn check_expr_post(a: &ast::Expr);
             fn check_ty(a: &ast::Ty);
+            fn check_generic_arg(a: &ast::GenericArg);
             fn check_generic_param(a: &ast::GenericParam);
             fn check_generics(a: &ast::Generics);
             fn check_where_predicate(a: &ast::WherePredicate);

compiler/rustc_lint/src/unused.rs

@@ -839,10 +839,6 @@ impl EarlyLintPass for UnusedParens {
         }
     }
 
-    fn check_anon_const(&mut self, cx: &EarlyContext<'_>, c: &ast::AnonConst) {
-        self.check_unused_delims_expr(cx, &c.value, UnusedDelimsCtx::AnonConst, false, None, None);
-    }
-
     fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
         if let StmtKind::Local(ref local) = s.kind {
             self.check_unused_parens_pat(cx, &local.pat, false, false);
@@ -965,13 +961,6 @@ impl UnusedDelimLint for UnusedBraces {
                         if !Self::is_expr_delims_necessary(expr, followed_by_block)
                             && (ctx != UnusedDelimsCtx::AnonConst
                                 || matches!(expr.kind, ast::ExprKind::Lit(_)))
-                            // array length expressions are checked during `check_anon_const` and `check_ty`,
-                            // once as `ArrayLenExpr` and once as `AnonConst`.
-                            //
-                            // As we do not want to lint this twice, we do not emit an error for
-                            // `ArrayLenExpr` if `AnonConst` would do the same.
-                            && (ctx != UnusedDelimsCtx::ArrayLenExpr
-                                || !matches!(expr.kind, ast::ExprKind::Lit(_)))
                             && !cx.sess().source_map().is_multiline(value.span)
                             && value.attrs.is_empty()
                             && !value.span.from_expansion()
@@ -999,21 +988,54 @@ impl UnusedDelimLint for UnusedBraces {
 }
 
 impl EarlyLintPass for UnusedBraces {
+    fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
+        <Self as UnusedDelimLint>::check_stmt(self, cx, s)
+    }
+
     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
-        <Self as UnusedDelimLint>::check_expr(self, cx, e)
+        <Self as UnusedDelimLint>::check_expr(self, cx, e);
+
+        if let ExprKind::Repeat(_, ref anon_const) = e.kind {
+            self.check_unused_delims_expr(
+                cx,
+                &anon_const.value,
+                UnusedDelimsCtx::AnonConst,
+                false,
+                None,
+                None,
+            );
+        }
     }
 
-    fn check_anon_const(&mut self, cx: &EarlyContext<'_>, c: &ast::AnonConst) {
-        self.check_unused_delims_expr(cx, &c.value, UnusedDelimsCtx::AnonConst, false, None, None);
+    fn check_generic_arg(&mut self, cx: &EarlyContext<'_>, arg: &ast::GenericArg) {
+        if let ast::GenericArg::Const(ct) = arg {
+            self.check_unused_delims_expr(
+                cx,
+                &ct.value,
+                UnusedDelimsCtx::AnonConst,
+                false,
+                None,
+                None,
+            );
+        }
     }
 
-    fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
-        <Self as UnusedDelimLint>::check_stmt(self, cx, s)
+    fn check_variant(&mut self, cx: &EarlyContext<'_>, v: &ast::Variant) {
+        if let Some(anon_const) = &v.disr_expr {
+            self.check_unused_delims_expr(
+                cx,
+                &anon_const.value,
+                UnusedDelimsCtx::AnonConst,
+                false,
+                None,
+                None,
+            );
+        }
     }
 
     fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
-        if let &ast::TyKind::Paren(ref r) = &ty.kind {
-            if let ast::TyKind::Array(_, ref len) = r.kind {
+        match ty.kind {
+            ast::TyKind::Array(_, ref len) => {
                 self.check_unused_delims_expr(
                     cx,
                     &len.value,
@@ -1023,6 +1045,19 @@ impl EarlyLintPass for UnusedBraces {
                     None,
                 );
             }
+
+            ast::TyKind::Typeof(ref anon_const) => {
+                self.check_unused_delims_expr(
+                    cx,
+                    &anon_const.value,
+                    UnusedDelimsCtx::AnonConst,
+                    false,
+                    None,
+                    None,
+                );
+            }
+
+            _ => {}
         }
     }
 

compiler/rustc_mir_build/src/build/expr/as_constant.rs

@@ -33,6 +33,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 Constant { span, user_ty, literal }
             }
             ExprKind::StaticRef { literal, .. } => Constant { span, user_ty: None, literal },
+            ExprKind::ConstBlock { value } => Constant { span, user_ty: None, literal: value },
             _ => span_bug!(span, "expression is not a valid constant {:?}", kind),
         }
     }

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -254,6 +254,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Continue { .. }
             | ExprKind::Return { .. }
             | ExprKind::Literal { .. }
+            | ExprKind::ConstBlock { .. }
             | ExprKind::StaticRef { .. }
             | ExprKind::InlineAsm { .. }
             | ExprKind::LlvmInlineAsm { .. }

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -234,6 +234,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
             ExprKind::Yield { .. }
             | ExprKind::Literal { .. }
+            | ExprKind::ConstBlock { .. }
             | ExprKind::StaticRef { .. }
             | ExprKind::Block { .. }
             | ExprKind::Match { .. }

compiler/rustc_mir_build/src/build/expr/category.rs

@@ -68,7 +68,9 @@ impl Category {
             | ExprKind::ThreadLocalRef(_)
             | ExprKind::LlvmInlineAsm { .. } => Some(Category::Rvalue(RvalueFunc::AsRvalue)),
 
-            ExprKind::Literal { .. } | ExprKind::StaticRef { .. } => Some(Category::Constant),
+            ExprKind::ConstBlock { .. } | ExprKind::Literal { .. } | ExprKind::StaticRef { .. } => {
+                Some(Category::Constant)
+            }
 
             ExprKind::Loop { .. }
             | ExprKind::Block { .. }

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -454,6 +454,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Array { .. }
             | ExprKind::Tuple { .. }
             | ExprKind::Closure { .. }
+            | ExprKind::ConstBlock { .. }
             | ExprKind::Literal { .. }
             | ExprKind::ThreadLocalRef(_)
             | ExprKind::StaticRef { .. } => {