Reject async closures in AST lowering

Author: compiler-errors

compiler/rustc_ast_lowering/messages.ftl

@@ -48,7 +48,7 @@ ast_lowering_bad_return_type_notation_position = return type notation not allowe
 ast_lowering_clobber_abi_not_supported =
     `clobber_abi` is not supported on this target
 
-ast_lowering_closure_cannot_be_static = closures cannot be static
+ast_lowering_closure_cannot_be_static = {$modifier}closures cannot be static
 
 ast_lowering_coroutine_too_many_parameters =
     too many parameters for a coroutine (expected 0 or 1 parameters)

compiler/rustc_ast_lowering/src/errors.rs

@@ -126,9 +126,10 @@ pub(crate) struct CoroutineTooManyParameters {
 
 #[derive(Diagnostic)]
 #[diag(ast_lowering_closure_cannot_be_static, code = E0697)]
-pub(crate) struct ClosureCannotBeStatic {
+pub(crate) struct ClosureCannotBeStatic<'a> {
     #[primary_span]
     pub fn_decl_span: Span,
+    pub modifier: &'a str,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_ast_lowering/src/expr.rs

@@ -226,6 +226,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         e.id,
                         expr_hir_id,
                         *coroutine_kind,
+                        *movability,
                         fn_decl,
                         body,
                         *fn_decl_span,
@@ -1157,7 +1158,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
             None => {
                 if movability == Movability::Static {
-                    self.dcx().emit_err(ClosureCannotBeStatic { fn_decl_span });
+                    self.dcx().emit_err(ClosureCannotBeStatic { modifier: "", fn_decl_span });
                 }
                 hir::ClosureKind::Closure
             }
@@ -1186,6 +1187,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         closure_id: NodeId,
         closure_hir_id: HirId,
         coroutine_kind: CoroutineKind,
+        movability: Movability,
         decl: &FnDecl,
         body: &Expr,
         fn_decl_span: Span,
@@ -1202,6 +1204,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
         };
 
+        // Movability doesn't make sense on an async/gen closure, so reject outright.
+        match movability {
+            Movability::Static => {
+                self.dcx().emit_err(ClosureCannotBeStatic {
+                    modifier: &format!("{coroutine_desugaring:#}"),
+                    fn_decl_span,
+                });
+            }
+            Movability::Movable => {}
+        }
+
         let body = self.with_new_scopes(fn_decl_span, |this| {
             let inner_decl =
                 FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };

tests/ui/unpretty/exhaustive.expanded.stdout

@@ -185,8 +185,10 @@ mod expressions {
         async move || value;
         static || value;
         static move || value;
-        (static async || value);
-        (static async move || value);
+        (static async ||
+                value);
+        (static async move ||
+                value);
         || -> u8 { value };
         1 + || {};
     }

tests/ui/unpretty/exhaustive.hir.stderr

@@ -10,6 +10,18 @@ error[E0697]: closures cannot be static
 LL |         static move || value;
    |         ^^^^^^^^^^^^^^
 
+error[E0697]: `async` closures cannot be static
+  --> $DIR/exhaustive.rs:212:10
+   |
+LL |         (static async || value);
+   |          ^^^^^^^^^^^^^^^
+
+error[E0697]: `async` closures cannot be static
+  --> $DIR/exhaustive.rs:213:10
+   |
+LL |         (static async move || value);
+   |          ^^^^^^^^^^^^^^^^^^^^
+
 error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/exhaustive.rs:239:13
    |
@@ -166,7 +178,7 @@ LL |         let _: impl for<'a> Send;
    = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 20 previous errors
+error: aborting due to 22 previous errors
 
 Some errors have detailed explanations: E0214, E0562, E0697, E0703, E0728.
 For more information about an error, try `rustc --explain E0214`.

tests/ui/unpretty/exhaustive.hir.stdout

@@ -215,8 +215,16 @@ mod expressions {
         move || |mut _task_context: ResumeTy| { { let _t = value; _t } };
         || value;
         move || value;
-        || |mut _task_context: ResumeTy| { { let _t = value; _t } };
-        move || |mut _task_context: ResumeTy| { { let _t = value; _t } };
+        ||
+            |mut _task_context: ResumeTy|
+                {
+                    { let _t = value; _t }
+                };
+        move ||
+            |mut _task_context: ResumeTy|
+                {
+                    { let _t = value; _t }
+                };
         || -> u8 { value };
         1 + (|| { });
     }

tests/ui/unpretty/exhaustive.rs

@@ -208,8 +208,8 @@ mod expressions {
         async move || value;
         static || value;            //[hir]~ ERROR closures cannot be static
         static move || value;       //[hir]~ ERROR closures cannot be static
-        (static async || value);
-        (static async move || value);
+        (static async || value);    //[hir]~ ERROR `async` closures cannot be static
+        (static async move || value); //[hir]~ ERROR `async` closures cannot be static
         || -> u8 { value };
         1 + || {};
     }