Require rustc_comptime fns to also be const

compiler/rustc_ast_lowering/src/diagnostics.rs

@@ -123,11 +123,13 @@ pub(crate) struct AwaitOnlyInAsyncFnAndBlocks {
 }
 
 #[derive(Diagnostic)]
-#[diag("a function cannot be both `comptime` and `const`")]
-pub(crate) struct ConstComptimeFn {
+#[diag("a function cannot just be `comptime`")]
+pub(crate) struct NonConstComptimeFn {
     #[primary_span]
-    #[suggestion("remove the `const`", applicability = "machine-applicable", code = "")]
-    #[note("`const` implies the function can be called at runtime, too")]
+    #[suggestion("add `const`", applicability = "machine-applicable", code = "const ")]
+    #[note(
+        "`const` is less restrictive than `comptime`, but already adds many restrictions `comptime` needs"
+    )]
     pub span: Span,
     #[label("`comptime` because of this")]
     pub attr_span: Span,

compiler/rustc_ast_lowering/src/item.rs

@@ -27,7 +27,7 @@ use super::{
     AstOwner, FnDeclKind, GenericArgsMode, ImplTraitContext, ImplTraitPosition, LoweringContext,
     ParamMode, RelaxedBoundForbiddenReason, RelaxedBoundPolicy, ResolverAstLoweringExt,
 };
-use crate::diagnostics::ConstComptimeFn;
+use crate::diagnostics::NonConstComptimeFn;
 
 /// Wraps either IndexVec (during `hir_crate`), which acts like a primary
 /// storage for most of the MaybeOwners, or FxIndexMap during delayed AST -> HIR
@@ -353,15 +353,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     self.record_body(&[], body)
                 }),
             ),
-            ItemKind::Fn(Fn {
-                sig: FnSig { decl, header, span: fn_sig_span },
-                ident,
-                generics,
-                body,
-                contract,
-                define_opaque,
-                ..
-            }) => {
+            ItemKind::Fn(Fn { sig, ident, generics, body, contract, define_opaque, .. }) => {
+                let FnSig { decl, header, span: fn_sig_span } = sig;
                 self.with_new_scopes(*fn_sig_span, |this| {
                     // Note: we don't need to change the return type from `T` to
                     // `impl Future<Output = T>` here because lower_body
@@ -385,7 +378,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     });
                     let sig = hir::FnSig {
                         decl,
-                        header: this.lower_fn_header(*header, hir::Safety::Safe, attrs),
+                        header: this.lower_fn_header(
+                            *header,
+                            sig.header_span(),
+                            hir::Safety::Safe,
+                            attrs,
+                        ),
                         span: this.lower_span(*fn_sig_span),
                     };
                     this.lower_define_opaque(hir_id, define_opaque);
@@ -808,7 +806,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     });
 
                 // Unmarked safety in unsafe block defaults to unsafe.
-                let header = self.lower_fn_header(sig.header, hir::Safety::Unsafe, attrs);
+                let header =
+                    self.lower_fn_header(sig.header, sig.header_span(), hir::Safety::Unsafe, attrs);
 
                 if define_opaque.is_some() {
                     self.dcx().span_err(i.span, "foreign functions cannot define opaque types");
@@ -1742,7 +1741,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         coroutine_kind: Option<CoroutineKind>,
         attrs: &[hir::Attribute],
     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
-        let header = self.lower_fn_header(sig.header, hir::Safety::Safe, attrs);
+        let header = self.lower_fn_header(sig.header, sig.header_span(), hir::Safety::Safe, attrs);
         let itctx = ImplTraitContext::Universal;
         let (generics, decl) = self.lower_generics(generics, id, itctx, |this| {
             this.lower_fn_decl(&sig.decl, id, sig.span, kind, coroutine_kind)
@@ -1753,6 +1752,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     pub(super) fn lower_fn_header(
         &mut self,
         h: FnHeader,
+        header_span: Span,
         default_safety: hir::Safety,
         attrs: &[hir::Attribute],
     ) -> hir::FnHeader {
@@ -1780,13 +1780,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 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 => {}
+                rustc_hir::Constness::Const { always: false } => {}
+                // A function can't just be `comptime`, it must also be `const`.
+                rustc_hir::Constness::NotConst => {
+                    self.dcx().emit_err(NonConstComptimeFn { span: header_span, attr_span });
+                }
             }
         }
 

tests/ui/comptime/const_comptime.rs

@@ -1,7 +1,7 @@
 #![feature(rustc_attrs)]
 
 #[rustc_comptime]
-const fn foo() {}
-//~^ ERROR a function cannot be both `comptime` and `const`
+fn foo() {}
+//~^ ERROR a function cannot just be `comptime`
 
 fn main() {}

tests/ui/comptime/const_comptime.stderr

@@ -1,16 +1,16 @@
-error: a function cannot be both `comptime` and `const`
+error: a function cannot just be `comptime`
   --> $DIR/const_comptime.rs:4:1
    |
 LL | #[rustc_comptime]
    | ----------------- `comptime` because of this
-LL | const fn foo() {}
-   | ^^^^^ help: remove the `const`
+LL | fn foo() {}
+   | ^ help: add `const`: `const`
    |
-note: `const` implies the function can be called at runtime, too
+note: `const` is less restrictive than `comptime`, but already adds many restrictions `comptime` needs
   --> $DIR/const_comptime.rs:4:1
    |
-LL | const fn foo() {}
-   | ^^^^^
+LL | fn foo() {}
+   | ^
 
 error: aborting due to 1 previous error
 

tests/ui/comptime/feature-gate-test.rs

@@ -1,5 +1,5 @@
 #[rustc_comptime]
 //~^ ERROR use of an internal attribute
-fn foo() {}
+const fn foo() {}
 
 fn main() {}

tests/ui/comptime/not_callable.rs

@@ -1,7 +1,7 @@
 #![feature(rustc_attrs, const_trait_impl)]
 
 #[rustc_comptime]
-fn foo() {}
+const fn foo() {}
 
 fn main() {
     // Ok
@@ -16,7 +16,7 @@ const fn bar() {
 }
 
 #[rustc_comptime]
-fn baz() {
+const fn baz() {
     // Ok
     foo();
 }

tests/ui/comptime/trait_bounds.rs

@@ -5,19 +5,18 @@ const trait Trait {
 }
 
 #[rustc_comptime]
-fn always_const<T: const Trait>() {
+const fn always_const<T: const Trait>() {
     T::method()
 }
 
 #[rustc_comptime]
-fn conditionally_const<T: [const] Trait>() {
-    //~^ ERROR: `[const]` is not allowed here
+const fn conditionally_const<T: [const] Trait>() {
     T::method()
     //~^ ERROR: `T: const Trait` is not satisfied
 }
 
 #[rustc_comptime]
-fn non_const<T: Trait>() {
+const fn non_const<T: Trait>() {
     T::method()
     //~^ ERROR: `T: const Trait` is not satisfied
 }

tests/ui/comptime/trait_bounds.stderr

@@ -1,27 +1,15 @@
-error: `[const]` is not allowed here
-  --> $DIR/trait_bounds.rs:13:27
-   |
-LL | fn conditionally_const<T: [const] Trait>() {
-   |                           ^^^^^^^
-   |
-note: this function is not `const`, so it cannot have `[const]` trait bounds
-  --> $DIR/trait_bounds.rs:13:4
-   |
-LL | fn conditionally_const<T: [const] Trait>() {
-   |    ^^^^^^^^^^^^^^^^^^^
-
 error[E0277]: the trait bound `T: const Trait` is not satisfied
-  --> $DIR/trait_bounds.rs:15:5
+  --> $DIR/trait_bounds.rs:14:5
    |
 LL |     T::method()
    |     ^
 
 error[E0277]: the trait bound `T: const Trait` is not satisfied
-  --> $DIR/trait_bounds.rs:21:5
+  --> $DIR/trait_bounds.rs:20:5
    |
 LL |     T::method()
    |     ^
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/comptime/trait_comptime.rs

@@ -3,24 +3,27 @@
 trait Foo {
     #[rustc_comptime]
     //~^ ERROR: cannot be used on required trait methods
-    fn foo();
+    const fn foo();
+    //~^ ERROR: functions in traits cannot be declared const
 
     #[rustc_comptime]
     //~^ ERROR: cannot be used on provided trait methods
-    fn bar() {}
+    const fn bar() {}
+    //~^ ERROR: functions in traits cannot be declared const
 }
 
 struct Bar;
 
 impl Bar {
     #[rustc_comptime]
-    fn foo() {}
+    const fn foo() {}
 }
 
 impl Foo for Bar {
     #[rustc_comptime]
     //~^ ERROR: cannot be used on trait methods
-    fn foo() {}
+    const fn foo() {}
+    //~^ ERROR: functions in trait impls cannot be declared const
 }
 
 fn main() {}

tests/ui/comptime/trait_comptime.stderr

@@ -1,3 +1,30 @@
+error[E0379]: functions in traits cannot be declared const
+  --> $DIR/trait_comptime.rs:6:5
+   |
+LL |     const fn foo();
+   |     ^^^^^-
+   |     |
+   |     functions in traits cannot be const
+   |     help: remove the `const`
+
+error[E0379]: functions in traits cannot be declared const
+  --> $DIR/trait_comptime.rs:11:5
+   |
+LL |     const fn bar() {}
+   |     ^^^^^-
+   |     |
+   |     functions in traits cannot be const
+   |     help: remove the `const`
+
+error[E0379]: functions in trait impls cannot be declared const
+  --> $DIR/trait_comptime.rs:25:5
+   |
+LL |     const fn foo() {}
+   |     ^^^^^-
+   |     |
+   |     functions in trait impls cannot be const
+   |     help: remove the `const`
+
 error: `#[rustc_comptime]` attribute cannot be used on required trait methods
   --> $DIR/trait_comptime.rs:4:5
    |
@@ -7,20 +34,21 @@ LL |     #[rustc_comptime]
    = help: `#[rustc_comptime]` can only be applied to functions with a body
 
 error: `#[rustc_comptime]` attribute cannot be used on provided trait methods
-  --> $DIR/trait_comptime.rs:8:5
+  --> $DIR/trait_comptime.rs:9:5
    |
 LL |     #[rustc_comptime]
    |     ^^^^^^^^^^^^^^^^^
    |
    = help: `#[rustc_comptime]` can be applied to functions and inherent methods
 
 error: `#[rustc_comptime]` attribute cannot be used on trait methods in impl blocks
-  --> $DIR/trait_comptime.rs:21:5
+  --> $DIR/trait_comptime.rs:23:5
    |
 LL |     #[rustc_comptime]
    |     ^^^^^^^^^^^^^^^^^
    |
    = help: `#[rustc_comptime]` can be applied to functions and inherent methods
 
-error: aborting due to 3 previous errors
+error: aborting due to 6 previous errors
 
+For more information about this error, try `rustc --explain E0379`.