Add very basic "comptime" fn implementation

compiler/rustc_ast_lowering/src/errors.rs

@@ -3,6 +3,17 @@ use rustc_errors::{DiagArgFromDisplay, DiagSymbolList};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{Ident, Span, Symbol};
 
+#[derive(Diagnostic)]
+#[diag("a function cannot be both `comptime` and `const`")]
+pub(crate) struct ConstComptimeFn {
+    #[primary_span]
+    #[suggestion("remove the `const`", applicability = "machine-applicable", code = "")]
+    #[note("`const` implies the function can be called at runtime, too")]
+    pub span: Span,
+    #[label("`comptime` because of this")]
+    pub attr_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag("parenthesized type parameters may only be used with a `Fn` trait", code = E0214)]
 pub(crate) struct GenericTypeWithParentheses {

compiler/rustc_ast_lowering/src/item.rs

@@ -25,6 +25,7 @@ use super::{
     AstOwner, FnDeclKind, GenericArgsMode, ImplTraitContext, ImplTraitPosition, LoweringContext,
     ParamMode, RelaxedBoundForbiddenReason, RelaxedBoundPolicy, ResolverAstLoweringExt,
 };
+use crate::errors::ConstComptimeFn;
 
 /// Wraps either IndexVec (during `hir_crate`), which acts like a primary
 /// storage for most of the MaybeOwners, or FxIndexMap during delayed AST -> HIR
@@ -1736,12 +1737,23 @@ impl<'hir> LoweringContext<'_, 'hir> {
             safety.into()
         };
 
-        hir::FnHeader {
-            safety,
-            asyncness,
-            constness: self.lower_constness(h.constness),
-            abi: self.lower_extern(h.ext),
+        let mut constness = self.lower_constness(h.constness);
+        if let Some(&attr_span) = find_attr!(attrs, Comptime(span) => span) {
+            match std::mem::replace(&mut constness, rustc_hir::Constness::Const { always: true }) {
+                rustc_hir::Constness::Const { always: true } => {
+                    unreachable!("lower_constness cannot produce comptime")
+                }
+                // A function can't be `const` and `comptime` at the same time
+                rustc_hir::Constness::Const { always: false } => {
+                    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, constness, abi: self.lower_extern(h.ext) }
     }
 
     pub(super) fn lower_abi(&mut self, abi_str: StrLit) -> ExternAbi {
@@ -1803,7 +1815,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
     pub(super) fn lower_constness(&mut self, c: Const) -> hir::Constness {
         match c {
-            Const::Yes(_) => hir::Constness::Const,
+            Const::Yes(_) => hir::Constness::Const { always: false },
             Const::No => hir::Constness::NotConst,
         }
     }

compiler/rustc_attr_parsing/src/attributes/semantics.rs

@@ -6,3 +6,14 @@ impl NoArgsAttributeParser 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 NoArgsAttributeParser for ComptimeParser {
+    const PATH: &[Symbol] = &[sym::rustc_comptime];
+    const ON_DUPLICATE: OnDuplicate = 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

@@ -53,7 +53,7 @@ use crate::attributes::repr::*;
 use crate::attributes::rustc_allocator::*;
 use crate::attributes::rustc_dump::*;
 use crate::attributes::rustc_internal::*;
-use crate::attributes::semantics::*;
+use crate::attributes::semantics::{ComptimeParser, *};
 use crate::attributes::stability::*;
 use crate::attributes::test_attrs::*;
 use crate::attributes::traits::*;
@@ -229,6 +229,7 @@ attribute_parsers!(
         Single<WithoutArgs<AutomaticallyDerivedParser>>,
         Single<WithoutArgs<ColdParser>>,
         Single<WithoutArgs<CompilerBuiltinsParser>>,
+        Single<WithoutArgs<ComptimeParser>>,
         Single<WithoutArgs<ConstContinueParser>>,
         Single<WithoutArgs<CoroutineParser>>,
         Single<WithoutArgs<DefaultLibAllocatorParser>>,

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -779,7 +779,8 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     // to do different checks than usual.
 
                     trace!("attempting to call a trait method");
-                    let is_const = tcx.constness(callee) == hir::Constness::Const;
+                    let is_const =
+                        matches!(tcx.constness(callee), hir::Constness::Const { always: false });
 
                     // Only consider a trait to be const if the const conditions hold.
                     // Otherwise, it's really misleading to call something "conditionally"

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -423,7 +423,8 @@ fn build_error_for_const_call<'tcx>(
                         err.help("const traits are not yet supported on stable Rust");
                     }
                 }
-            } else if ccx.tcx.constness(callee) != hir::Constness::Const {
+            } else if !matches!(ccx.tcx.constness(callee), hir::Constness::Const { always: false })
+            {
                 let name = ccx.tcx.item_name(callee);
                 err.span_note(
                     ccx.tcx.def_span(callee),

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -11,13 +11,13 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Constness {
     let node = tcx.hir_node_by_def_id(def_id);
 
     match node {
-        Node::Ctor(VariantData::Tuple(..)) => Constness::Const,
+        Node::Ctor(VariantData::Tuple(..)) => Constness::Const { always: false },
         Node::ForeignItem(item) if let ForeignItemKind::Fn(..) = item.kind => {
             // Foreign functions cannot be evaluated at compile-time.
             Constness::NotConst
         }
         Node::Expr(e) if let ExprKind::Closure(c) = e.kind => {
-            if let Constness::Const = c.constness && tcx.hir_body_const_context(tcx.local_parent(def_id)).is_none() {
+            if let Constness::Const { .. } = c.constness && tcx.hir_body_const_context(tcx.local_parent(def_id)).is_none() {
                 tcx.dcx().span_err(tcx.def_span(def_id), "cannot use `const` closures outside of const contexts");
                 return Constness::NotConst;
             }
@@ -37,7 +37,7 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Constness {
             ..
         }) => {
             match sig.header.constness {
-                Constness::Const => Constness::Const,
+                Constness::Const{always} => Constness::Const {always},
                 // inherent impl could be const
                 Constness::NotConst => tcx.constness(tcx.local_parent(def_id)),
             }

compiler/rustc_feature/src/builtin_attrs.rs

@@ -736,6 +736,10 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_no_writable,
         "`#[rustc_no_writable]` stops the compiler from considering mutable reference arguments of this function as implicitly writable"
     ),
+    rustc_attr!(
+        rustc_comptime,
+        "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

@@ -967,6 +967,9 @@ pub enum AttributeKind {
     /// Represents `#[compiler_builtins]`.
     CompilerBuiltins,
 
+    /// Represents `#[rustc_comptime]`
+    Comptime(Span),
+
     /// Represents `#[const_continue]`.
     ConstContinue(Span),
 

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -27,6 +27,7 @@ impl AttributeKind {
             Cold => No,
             CollapseDebugInfo(..) => Yes,
             CompilerBuiltins => No,
+            Comptime(..) => No,
             ConstContinue(..) => No,
             Coroutine => No,
             Coverage(..) => No,

compiler/rustc_hir/src/hir.rs

@@ -2377,10 +2377,12 @@ pub enum ConstContext {
     /// - Array length expressions
     /// - Enum discriminants
     /// - Const generics
-    ///
-    /// For the most part, other contexts are treated just like a regular `const`, so they are
-    /// lumped into the same category.
-    Const { inline: bool },
+    Const {
+        /// For backwards compatibility `const` items allow
+        /// calls to `const fn` to get promoted.
+        /// We forbid that in comptime fns and inline consts.
+        allow_const_fn_promotion: bool,
+    },
 }
 
 impl ConstContext {
@@ -4544,17 +4546,22 @@ impl fmt::Display for Safety {
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Encodable, Decodable, StableHash)]
-#[derive(Default)]
 pub enum Constness {
-    #[default]
-    Const,
+    Const { always: bool },
     NotConst,
 }
 
+impl Default for Constness {
+    fn default() -> Self {
+        Self::Const { always: false }
+    }
+}
+
 impl fmt::Display for Constness {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str(match *self {
-            Self::Const => "const",
+            Self::Const { always: true } => "comptime",
+            Self::Const { always: false } => "const",
             Self::NotConst => "non-const",
         })
     }
@@ -4608,10 +4615,6 @@ impl FnHeader {
         matches!(self.asyncness, IsAsync::Async(_))
     }
 
-    pub fn is_const(&self) -> bool {
-        matches!(self.constness, Constness::Const)
-    }
-
     pub fn is_unsafe(&self) -> bool {
         self.safety().is_unsafe()
     }

compiler/rustc_hir_pretty/src/lib.rs

@@ -716,7 +716,7 @@ impl<'a> State<'a> {
 
                 match of_trait {
                     None => {
-                        if let hir::Constness::Const = constness {
+                        if let hir::Constness::Const { always: false } = constness {
                             self.word_nbsp("const");
                         }
                         impl_generics(self)
@@ -733,7 +733,7 @@ impl<'a> State<'a> {
 
                         impl_generics(self);
 
-                        if let hir::Constness::Const = constness {
+                        if let hir::Constness::Const { always: false } = constness {
                             self.word_nbsp("const");
                         }
 
@@ -2631,7 +2631,8 @@ impl<'a> State<'a> {
     fn print_constness(&mut self, s: hir::Constness) {
         match s {
             hir::Constness::NotConst => {}
-            hir::Constness::Const => self.word_nbsp("const"),
+            hir::Constness::Const { always: false } => self.word_nbsp("const"),
+            hir::Constness::Const { always: true } => { /* printed as an attribute */ }
         }
     }
 

compiler/rustc_hir_typeck/src/callee.rs

@@ -923,6 +923,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         callee_did: DefId,
         callee_args: GenericArgsRef<'tcx>,
     ) {
+        let const_context = self.tcx.hir_body_const_context(self.body_id);
+
+        if let hir::Constness::Const { always: true } = self.tcx.constness(callee_did) {
+            match const_context {
+                Some(hir::ConstContext::Const { .. } | hir::ConstContext::Static(_)) => {}
+                Some(hir::ConstContext::ConstFn) | None => {
+                    self.dcx().span_err(span, "comptime fns can only be called at compile time");
+                }
+            }
+        }
+
         // FIXME(const_trait_impl): We should be enforcing these effects unconditionally.
         // This can be done as soon as we convert the standard library back to
         // using const traits, since if we were to enforce these conditions now,
@@ -936,7 +947,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return;
         }
 
-        let host = match self.tcx.hir_body_const_context(self.body_id) {
+        let host = match const_context {
             Some(hir::ConstContext::Const { .. } | hir::ConstContext::Static(_)) => {
                 ty::BoundConstness::Const
             }

compiler/rustc_metadata/src/rmeta/table.rs

@@ -225,8 +225,9 @@ defaulted_enum! {
 
 defaulted_enum! {
     hir::Constness {
-        ( Const    )
+        ( Const { always: false } )
         ( NotConst )
+        ( Const { always: true } )
     }
 }
 

compiler/rustc_middle/src/hir/map.rs

@@ -313,12 +313,18 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn hir_body_const_context(self, local_def_id: LocalDefId) -> Option<ConstContext> {
         let def_id = local_def_id.into();
         let ccx = match self.hir_body_owner_kind(def_id) {
-            BodyOwnerKind::Const { inline } => ConstContext::Const { inline },
+            BodyOwnerKind::Const { inline } => {
+                ConstContext::Const { allow_const_fn_promotion: !inline }
+            }
             BodyOwnerKind::Static(mutability) => ConstContext::Static(mutability),
 
             BodyOwnerKind::Fn if self.is_constructor(def_id) => return None,
             BodyOwnerKind::Fn | BodyOwnerKind::Closure if self.is_const_fn(def_id) => {
-                ConstContext::ConstFn
+                if matches!(self.constness(def_id), rustc_hir::Constness::Const { always: true }) {
+                    ConstContext::Const { allow_const_fn_promotion: false }
+                } else {
+                    ConstContext::ConstFn
+                }
             }
             BodyOwnerKind::Fn | BodyOwnerKind::Closure | BodyOwnerKind::GlobalAsm => return None,
         };

compiler/rustc_middle/src/ty/adt.rs

@@ -310,7 +310,8 @@ impl<'tcx> rustc_type_ir::inherent::AdtDef<TyCtxt<'tcx>> for AdtDef<'tcx> {
 
     fn destructor(self, tcx: TyCtxt<'tcx>) -> Option<AdtDestructorKind> {
         Some(match tcx.constness(self.destructor(tcx)?.did) {
-            hir::Constness::Const => AdtDestructorKind::Const,
+            hir::Constness::Const { always: true } => todo!("FIXME(comptime)"),
+            hir::Constness::Const { always: false } => AdtDestructorKind::Const,
             hir::Constness::NotConst => AdtDestructorKind::NotConst,
         })
     }

compiler/rustc_middle/src/ty/context.rs

@@ -2620,7 +2620,10 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Whether the trait impl is marked const. This does not consider stability or feature gates.
     pub fn is_const_trait_impl(self, def_id: DefId) -> bool {
         self.def_kind(def_id) == DefKind::Impl { of_trait: true }
-            && self.impl_trait_header(def_id).constness == hir::Constness::Const
+            && matches!(
+                self.impl_trait_header(def_id).constness,
+                hir::Constness::Const { always: false }
+            )
     }
 
     pub fn is_sdylib_interface_build(self) -> bool {

compiler/rustc_middle/src/ty/context/impl_interner.rs

@@ -435,7 +435,7 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
 
     fn closure_is_const(self, def_id: DefId) -> bool {
         debug_assert_matches!(self.def_kind(def_id), DefKind::Closure);
-        self.constness(def_id) == hir::Constness::Const
+        matches!(self.constness(def_id), hir::Constness::Const { always: false })
     }
 
     fn alias_has_const_conditions(self, def_id: DefId) -> bool {