Check comptime fns

Author: oli-obk

compiler/rustc_ast_lowering/messages.ftl

@@ -50,6 +50,11 @@ ast_lowering_clobber_abi_not_supported =
 
 ast_lowering_closure_cannot_be_static = closures cannot be static
 
+ast_lowering_const_comptime_fn = a function cannot be both `comptime` and `const`
+    .suggestion = remove the `const`
+    .label = `comptime` because of this
+    .note = `const` implies the function can be called at runtime, too
+
 ast_lowering_coroutine_too_many_parameters =
     too many parameters for a coroutine (expected 0 or 1 parameters)
 

compiler/rustc_ast_lowering/src/errors.rs

@@ -3,6 +3,16 @@ use rustc_errors::codes::*;
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{Ident, Span, Symbol};
 
+#[derive(Diagnostic)]
+#[diag(ast_lowering_const_comptime_fn)]
+pub(crate) struct ConstComptimeFn {
+    #[primary_span]
+    #[suggestion(applicability = "machine-applicable", code = "")]
+    pub span: Span,
+    #[label]
+    pub attr_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_lowering_generic_type_with_parentheses, code = E0214)]
 pub(crate) struct GenericTypeWithParentheses {

compiler/rustc_ast_lowering/src/item.rs

@@ -23,6 +23,7 @@ use super::{
     AstOwner, FnDeclKind, ImplTraitContext, ImplTraitPosition, LoweringContext, ParamMode,
     RelaxedBoundForbiddenReason, RelaxedBoundPolicy, ResolverAstLoweringExt,
 };
+use crate::errors::ConstComptimeFn;
 
 pub(super) struct ItemLowerer<'a, 'hir> {
     pub(super) tcx: TyCtxt<'hir>,
@@ -1537,6 +1538,22 @@ impl<'hir> LoweringContext<'_, 'hir> {
             safety.into()
         };
 
+        let mut constness = self.lower_constness(h.constness);
+        if let Some(&attr_span) = find_attr!(attrs, AttributeKind::Comptime(span) => span) {
+            match std::mem::replace(&mut constness, rustc_hir::Constness::Comptime) {
+                rustc_hir::Constness::Comptime => {
+                    unreachable!("lower_constness cannot produce comptime")
+                }
+                // A function can't be `const` and `comptime` at the same time
+                rustc_hir::Constness::Const => {
+                    let Const::Yes(span) = h.constness else { unreachable!() };
+                    self.dcx().emit_err(ConstComptimeFn { span, attr_span });
+                }
+                // Good
+                rustc_hir::Constness::NotConst => {}
+            }
+        }
+
         hir::FnHeader {
             safety,
             asyncness,

compiler/rustc_attr_parsing/src/attributes/semantics.rs

@@ -7,3 +7,14 @@ impl<S: Stage> NoArgsAttributeParser<S> for MayDangleParser {
     const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(ALL_TARGETS); //FIXME Still checked fully in `check_attr.rs`
     const CREATE: fn(span: Span) -> AttributeKind = AttributeKind::MayDangle;
 }
+
+pub(crate) struct ComptimeParser;
+impl<S: Stage> NoArgsAttributeParser<S> for ComptimeParser {
+    const PATH: &[Symbol] = &[sym::rustc_comptime];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
+        Allow(Target::Method(MethodKind::Inherent)),
+        Allow(Target::Fn),
+    ]);
+    const CREATE: fn(Span) -> AttributeKind = AttributeKind::Comptime;
+}

compiler/rustc_attr_parsing/src/context.rs

@@ -58,7 +58,7 @@ use crate::attributes::rustc_internal::{
     RustcLayoutScalarValidRangeEndParser, RustcLayoutScalarValidRangeStartParser, RustcMainParser,
     RustcObjectLifetimeDefaultParser, RustcSimdMonomorphizeLaneLimitParser,
 };
-use crate::attributes::semantics::MayDangleParser;
+use crate::attributes::semantics::{ComptimeParser, MayDangleParser};
 use crate::attributes::stability::{
     BodyStabilityParser, ConstStabilityIndirectParser, ConstStabilityParser, StabilityParser,
 };
@@ -216,6 +216,7 @@ attribute_parsers!(
         Single<WithoutArgs<AutomaticallyDerivedParser>>,
         Single<WithoutArgs<CoinductiveParser>>,
         Single<WithoutArgs<ColdParser>>,
+        Single<WithoutArgs<ComptimeParser>>,
         Single<WithoutArgs<ConstContinueParser>>,
         Single<WithoutArgs<ConstStabilityIndirectParser>>,
         Single<WithoutArgs<CoroutineParser>>,

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -34,8 +34,8 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
         hir::Node::Item(i) if let hir::ItemKind::Impl(impl_) = i.kind => impl_.constness,
         _ => {
             if let Some(fn_kind) = node.fn_kind() {
-                if fn_kind.constness() == hir::Constness::Const {
-                    return hir::Constness::Const;
+                if fn_kind.constness() != hir::Constness::NotConst {
+                    return fn_kind.constness();
                 }
 
                 // If the function itself is not annotated with `const`, it may still be a `const fn`

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1407,6 +1407,11 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_force_inline, Normal, template!(Word, NameValueStr: "reason"), WarnFollowing, EncodeCrossCrate::Yes,
         "`#[rustc_force_inline]` forces a free function to be inlined"
     ),
+    rustc_attr!(
+        rustc_comptime, Normal, template!(Word), ErrorFollowing,
+        EncodeCrossCrate::No,
+        "the `#[rustc_comptime]` attribute is just used to avoid adding syntax for `comptime fn`"
+    ),
 
     // ==========================================================================
     // Internal attributes, Testing:

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -491,6 +491,9 @@ pub enum AttributeKind {
     /// Represents `#[cold]`.
     Cold(Span),
 
+    /// Represents `#[rustc_comptime]`
+    Comptime(Span),
+
     /// Represents `#[rustc_confusables]`.
     Confusables {
         symbols: ThinVec<Symbol>,

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -28,6 +28,7 @@ impl AttributeKind {
             BodyStability { .. } => No,
             Coinductive(..) => No,
             Cold(..) => No,
+            Comptime(..) => No,
             Confusables { .. } => Yes,
             ConstContinue(..) => No,
             ConstStability { .. } => Yes,

compiler/rustc_middle/src/ty/mod.rs

@@ -2137,7 +2137,10 @@ impl<'tcx> TyCtxt<'tcx> {
         matches!(
             self.def_kind(def_id),
             DefKind::Fn | DefKind::AssocFn | DefKind::Ctor(_, CtorKind::Fn) | DefKind::Closure
-        ) && self.constness(def_id) == hir::Constness::Const
+        ) && match self.constness(def_id) {
+            hir::Constness::Comptime | hir::Constness::Const => true,
+            hir::Constness::NotConst => false,
+        }
     }
 
     /// Whether this item is conditionally constant for the purposes of the