Check closure's constness validity in the constness query

instead of during ast lowering, where it's not easily possible to obtain all the right information in time

Author: oli-obk

compiler/rustc_ast_lowering/src/expr.rs

@@ -1052,7 +1052,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         binder: &ClosureBinder,
         capture_clause: CaptureBy,
         closure_id: NodeId,
-        mut constness: Const,
+        constness: Const,
         movability: Movability,
         decl: &FnDecl,
         body: &Expr,
@@ -1062,18 +1062,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let closure_def_id = self.local_def_id(closure_id);
         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 
-        if let Const::Yes(span) = constness {
-            if !self.is_in_const_context {
-                self.dcx().span_err(span, "cannot use `const` closures outside of const contexts");
-                constness = Const::No;
-            }
-        }
-
         let (body_id, closure_kind) = self.with_new_scopes(fn_decl_span, move |this| {
             let mut coroutine_kind = find_attr!(attrs, Coroutine(_) => hir::CoroutineKind::Coroutine(Movability::Movable));
 
             // FIXME(contracts): Support contracts on closures?
-            let body_id = this.lower_fn_body(decl, None, constness, |this| {
+            let body_id = this.lower_fn_body(decl, None, |this| {
                 this.coroutine_kind = coroutine_kind;
                 let e = this.lower_expr_mut(body);
                 coroutine_kind = this.coroutine_kind;

compiler/rustc_ast_lowering/src/item.rs

@@ -1,4 +1,3 @@
-use std::mem;
 
 use rustc_abi::ExternAbi;
 use rustc_ast::visit::AssocCtxt;
@@ -378,7 +377,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         body.as_deref(),
                         attrs,
                         contract.as_deref(),
-                        header.constness,
                     );
 
                     let itctx = ImplTraitContext::Universal;
@@ -1068,7 +1066,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     Some(body),
                     attrs,
                     contract.as_deref(),
-                    sig.header.constness,
                 );
                 let (generics, sig) = self.lower_method_sig(
                     generics,
@@ -1262,7 +1259,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     body.as_deref(),
                     attrs,
                     contract.as_deref(),
-                    sig.header.constness,
                 );
                 let (generics, sig) = self.lower_method_sig(
                     generics,
@@ -1391,13 +1387,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
     ) -> hir::BodyId {
         let prev_coroutine_kind = self.coroutine_kind.take();
-        let prev_is_in_const_context = mem::take(&mut self.is_in_const_context);
         let task_context = self.task_context.take();
         let (parameters, result) = f(self);
         let body_id = self.record_body(parameters, result);
         self.task_context = task_context;
         self.coroutine_kind = prev_coroutine_kind;
-        self.is_in_const_context = prev_is_in_const_context;
         body_id
     }
 
@@ -1416,13 +1410,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &mut self,
         decl: &FnDecl,
         contract: Option<&FnContract>,
-        constness: Const,
         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
     ) -> hir::BodyId {
         self.lower_body(|this| {
-            if let Const::Yes(_) = constness {
-                this.is_in_const_context = true;
-            }
             let params =
                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x)));
 
@@ -1440,20 +1430,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
         decl: &FnDecl,
         body: &Block,
         contract: Option<&FnContract>,
-        constness: Const,
     ) -> hir::BodyId {
-        self.lower_fn_body(decl, contract, constness, |this| this.lower_block_expr(body))
+        self.lower_fn_body(decl, contract, |this| this.lower_block_expr(body))
     }
 
     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
         self.lower_body(|this| {
             (
                 &[],
                 match expr {
-                    Some(expr) => {
-                        this.is_in_const_context = true;
-                        this.lower_expr_mut(expr)
-                    }
+                    Some(expr) => this.lower_expr_mut(expr),
                     None => this.expr_err(span, this.dcx().span_delayed_bug(span, "no block")),
                 },
             )
@@ -1472,13 +1458,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
         body: Option<&Block>,
         attrs: &'hir [hir::Attribute],
         contract: Option<&FnContract>,
-        constness: Const,
     ) -> hir::BodyId {
         let Some(body) = body else {
             // Functions without a body are an error, except if this is an intrinsic. For those we
             // create a fake body so that the entire rest of the compiler doesn't have to deal with
             // this as a special case.
-            return self.lower_fn_body(decl, contract, constness, |this| {
+            return self.lower_fn_body(decl, contract, |this| {
                 if find_attr!(attrs, RustcIntrinsic) || this.tcx.is_sdylib_interface_build() {
                     let span = this.lower_span(span);
                     let empty_block = hir::Block {
@@ -1503,7 +1488,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         };
         let Some(coroutine_kind) = coroutine_kind else {
             // Typical case: not a coroutine.
-            return self.lower_fn_body_block(decl, body, contract, constness);
+            return self.lower_fn_body_block(decl, body, contract);
         };
         // FIXME(contracts): Support contracts on async fn.
         self.lower_body(|this| {

compiler/rustc_ast_lowering/src/lib.rs

@@ -124,7 +124,6 @@ struct LoweringContext<'a, 'hir> {
     loop_scope: Option<HirId>,
     is_in_loop_condition: bool,
     is_in_dyn_type: bool,
-    is_in_const_context: bool,
 
     current_hir_id_owner: hir::OwnerId,
     item_local_id_counter: hir::ItemLocalId,
@@ -193,7 +192,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             loop_scope: None,
             is_in_loop_condition: false,
             is_in_dyn_type: false,
-            is_in_const_context: false,
             coroutine_kind: None,
             task_context: None,
             current_item: None,

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -16,7 +16,13 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Constness {
             // Foreign functions cannot be evaluated at compile-time.
             Constness::NotConst
         }
-        Node::Expr(e) if let ExprKind::Closure(c) = e.kind => c.constness,
+        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() {
+                tcx.dcx().span_err(tcx.def_span(def_id), "cannot use `const` closures outside of const contexts");
+                return Constness::NotConst;
+            }
+            c.constness
+        },
         // FIXME(fee1-dead): extract this one out and rename this query to `fn_constness` so we don't need `is_const_fn` anymore.
         Node::Item(i) if let ItemKind::Impl(impl_) = i.kind => impl_.constness,
         Node::Item(Item { kind: ItemKind::Fn { sig, .. }, .. }) => sig.header.constness,

compiler/rustc_middle/src/hir/map.rs

@@ -319,17 +319,7 @@ impl<'tcx> TyCtxt<'tcx> {
             BodyOwnerKind::Fn if self.is_constructor(def_id) => return None,
             // Const closures use their parent's const context
             BodyOwnerKind::Closure if self.is_const_fn(def_id) => {
-                return Some(
-                    self.hir_body_const_context(self.local_parent(local_def_id)).unwrap_or_else(
-                        || {
-                            assert!(
-                                self.dcx().has_errors().is_some(),
-                                "`const` closure with no enclosing const context",
-                            );
-                            ConstContext::ConstFn
-                        },
-                    ),
-                );
+                return self.hir_body_const_context(self.local_parent(local_def_id));
             }
             BodyOwnerKind::Fn if self.is_const_fn(def_id) => ConstContext::ConstFn,
             BodyOwnerKind::Fn | BodyOwnerKind::Closure | BodyOwnerKind::GlobalAsm => return None,

tests/ui/consts/const-closure-in-trait-impl.rs

@@ -1,3 +1,5 @@
+//@ check-pass
+
 #![feature(const_closures, const_destruct, const_trait_impl)]
 
 use std::marker::Destruct;
@@ -17,8 +19,6 @@ impl const T for S {
 
     fn b(&mut self) {
         self.a(const || {});
-        //~^ ERROR: cannot use `const` closures outside of const contexts
-        //~| ERROR: [const] Fn()` is not satisfied
     }
 }
 

tests/ui/consts/const-closure-in-trait-impl.stderr

@@ -11,6 +11,7 @@ impl<T: Foo> Foo for &mut T {
     const fn test() -> impl [const] Fn() {
         //~^ ERROR functions in trait impls cannot be declared const
         const move || {}
+        //~^ ERROR: cannot use `const` closures outside of const contexts
     }
 }
 

tests/ui/traits/const-traits/const-closure-in-non-const-trait-impl-method.rs

@@ -14,6 +14,12 @@ help: ... and declare the impl to be const instead
 LL | impl<T: Foo> const Foo for &mut T {
    |              +++++
 
-error: aborting due to 1 previous error
+error: cannot use `const` closures outside of const contexts
+  --> $DIR/const-closure-in-non-const-trait-impl-method.rs:13:9
+   |
+LL |         const move || {}
+   |         ^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0379`.

tests/ui/traits/const-traits/const-closure-in-non-const-trait-impl-method.stderr

@@ -6,6 +6,7 @@ trait Tr {
     const fn test() {
         //~^ ERROR functions in traits cannot be declared const
         (const || {})()
+        //~^ ERROR cannot use `const` closures outside of const contexts
     }
 }