delay_span_bug if const-checking an async function

This errors during AST lowering. Any errors we emit here are just noise.

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -1,8 +1,8 @@
 //! The `Visitor` responsible for actually checking a `mir::Body` for invalid operations.
 
 use rustc_errors::struct_span_err;
-use rustc_hir::{self as hir, LangItem};
+use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::{def_id::DefId, HirId};
+use rustc_hir::{self as hir, HirId, LangItem};
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
@@ -204,6 +204,13 @@ impl Validator<'mir, 'tcx> {
     pub fn check_body(&mut self) {
         let ConstCx { tcx, body, def_id, .. } = *self.ccx;
 
+        // `async` functions cannot be `const fn`. This is checked during AST lowering, so there's
+        // no need to emit duplicate errors here.
+        if is_async_fn(tcx, def_id) || body.generator_kind.is_some() {
+            tcx.sess.delay_span_bug(body.span, "`async` functions cannot be `const fn`");
+            return;
+        }
+
         // The local type and predicate checks are not free and only relevant for `const fn`s.
         if self.const_kind() == hir::ConstContext::ConstFn {
             // Prevent const trait methods from being annotated as `stable`.
@@ -877,3 +884,11 @@ fn place_as_reborrow(
 fn is_int_bool_or_char(ty: Ty<'_>) -> bool {
     ty.is_bool() || ty.is_integral() || ty.is_char()
 }
+
+fn is_async_fn(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
+    let hir_map = tcx.hir();
+    let hir_id = hir_map.local_def_id_to_hir_id(def_id);
+    hir_map
+        .fn_sig_by_hir_id(hir_id)
+        .map_or(false, |sig| sig.header.asyncness == hir::IsAsync::Async)
+}

src/test/ui/async-await/no-const-async.rs

@@ -3,4 +3,3 @@
 
 pub const async fn x() {}
 //~^ ERROR functions cannot be both `const` and `async`
-//~| ERROR `impl Trait` in const fn is unstable

src/test/ui/async-await/no-const-async.stderr

@@ -7,15 +7,5 @@ LL | pub const async fn x() {}
    |     |     `async` because of this
    |     `const` because of this
 
-error[E0723]: `impl Trait` in const fn is unstable
+error: aborting due to previous error
-  --> $DIR/no-const-async.rs:4:24
-   |
-LL | pub const async fn x() {}
-   |                        ^
-   |
-   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
-   = help: add `#![feature(const_fn)]` to the crate attributes to enable
 
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0723`.

src/test/ui/parser/fn-header-semantic-fail.rs

@@ -12,7 +12,6 @@ fn main() {
     extern "C" fn ff4() {} // OK.
     const async unsafe extern "C" fn ff5() {} // OK.
     //~^ ERROR functions cannot be both `const` and `async`
-    //~| ERROR `from_generator` is not yet stable as a const fn
 
     trait X {
         async fn ft1(); //~ ERROR functions in traits cannot be declared `async`
@@ -35,7 +34,6 @@ fn main() {
         const async unsafe extern "C" fn ft5() {}
         //~^ ERROR functions in traits cannot be declared `async`
         //~| ERROR functions in traits cannot be declared const
-        //~| ERROR `from_generator` is not yet stable as a const fn
         //~| ERROR method `ft5` has an incompatible type for trait
         //~| ERROR functions cannot be both `const` and `async`
     }
@@ -47,7 +45,6 @@ fn main() {
         extern "C" fn fi4() {} // OK.
         const async unsafe extern "C" fn fi5() {}
         //~^ ERROR functions cannot be both `const` and `async`
-        //~| ERROR `from_generator` is not yet stable as a const fn
     }
 
     extern {

src/test/ui/parser/fn-header-semantic-fail.stderr

@@ -8,7 +8,7 @@ LL |     const async unsafe extern "C" fn ff5() {} // OK.
    |     `const` because of this
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:18:9
+  --> $DIR/fn-header-semantic-fail.rs:17:9
    |
 LL |         async fn ft1();
    |         -----^^^^^^^^^^
@@ -19,19 +19,19 @@ LL |         async fn ft1();
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:20:9
+  --> $DIR/fn-header-semantic-fail.rs:19:9
    |
 LL |         const fn ft3();
    |         ^^^^^ functions in traits cannot be const
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:22:9
+  --> $DIR/fn-header-semantic-fail.rs:21:9
    |
 LL |         const async unsafe extern "C" fn ft5();
    |         ^^^^^ functions in traits cannot be const
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:22:9
+  --> $DIR/fn-header-semantic-fail.rs:21:9
    |
 LL |         const async unsafe extern "C" fn ft5();
    |         ^^^^^^-----^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -42,7 +42,7 @@ LL |         const async unsafe extern "C" fn ft5();
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:22:9
+  --> $DIR/fn-header-semantic-fail.rs:21:9
    |
 LL |         const async unsafe extern "C" fn ft5();
    |         ^^^^^-^^^^^----------------------------
@@ -51,7 +51,7 @@ LL |         const async unsafe extern "C" fn ft5();
    |         `const` because of this
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:30:9
+  --> $DIR/fn-header-semantic-fail.rs:29:9
    |
 LL |         async fn ft1() {}
    |         -----^^^^^^^^^^^^
@@ -62,19 +62,19 @@ LL |         async fn ft1() {}
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:33:9
+  --> $DIR/fn-header-semantic-fail.rs:32:9
    |
 LL |         const fn ft3() {}
    |         ^^^^^ functions in traits cannot be const
 
 error[E0379]: functions in traits cannot be declared const
-  --> $DIR/fn-header-semantic-fail.rs:35:9
+  --> $DIR/fn-header-semantic-fail.rs:34:9
    |
 LL |         const async unsafe extern "C" fn ft5() {}
    |         ^^^^^ functions in traits cannot be const
 
 error[E0706]: functions in traits cannot be declared `async`
-  --> $DIR/fn-header-semantic-fail.rs:35:9
+  --> $DIR/fn-header-semantic-fail.rs:34:9
    |
 LL |         const async unsafe extern "C" fn ft5() {}
    |         ^^^^^^-----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -85,7 +85,7 @@ LL |         const async unsafe extern "C" fn ft5() {}
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:35:9
+  --> $DIR/fn-header-semantic-fail.rs:34:9
    |
 LL |         const async unsafe extern "C" fn ft5() {}
    |         ^^^^^-^^^^^------------------------------
@@ -94,7 +94,7 @@ LL |         const async unsafe extern "C" fn ft5() {}
    |         `const` because of this
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:48:9
+  --> $DIR/fn-header-semantic-fail.rs:46:9
    |
 LL |         const async unsafe extern "C" fn fi5() {}
    |         ^^^^^-^^^^^------------------------------
@@ -103,7 +103,7 @@ LL |         const async unsafe extern "C" fn fi5() {}
    |         `const` because of this
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:54:18
+  --> $DIR/fn-header-semantic-fail.rs:51:18
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -113,7 +113,7 @@ LL |         async fn fe1();
    |         help: remove the qualifiers: `fn`
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:55:19
+  --> $DIR/fn-header-semantic-fail.rs:52:19
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -124,7 +124,7 @@ LL |         unsafe fn fe2();
    |         help: remove the qualifiers: `fn`
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:56:18
+  --> $DIR/fn-header-semantic-fail.rs:53:18
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -135,7 +135,7 @@ LL |         const fn fe3();
    |         help: remove the qualifiers: `fn`
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:57:23
+  --> $DIR/fn-header-semantic-fail.rs:54:23
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -146,7 +146,7 @@ LL |         extern "C" fn fe4();
    |         help: remove the qualifiers: `fn`
 
 error: functions in `extern` blocks cannot have qualifiers
-  --> $DIR/fn-header-semantic-fail.rs:58:42
+  --> $DIR/fn-header-semantic-fail.rs:55:42
    |
 LL |     extern {
    |     ------ in this `extern` block
@@ -157,7 +157,7 @@ LL |         const async unsafe extern "C" fn fe5();
    |         help: remove the qualifiers: `fn`
 
 error: functions cannot be both `const` and `async`
-  --> $DIR/fn-header-semantic-fail.rs:58:9
+  --> $DIR/fn-header-semantic-fail.rs:55:9
    |
 LL |         const async unsafe extern "C" fn fe5();
    |         ^^^^^-^^^^^----------------------------
@@ -165,16 +165,8 @@ LL |         const async unsafe extern "C" fn fe5();
    |         |     `async` because of this
    |         `const` because of this
 
-error: `from_generator` is not yet stable as a const fn
-  --> $DIR/fn-header-semantic-fail.rs:13:44
-   |
-LL |     const async unsafe extern "C" fn ff5() {} // OK.
-   |                                            ^^
-   |
-   = help: add `#![feature(gen_future)]` to the crate attributes to enable
-
 error[E0053]: method `ft1` has an incompatible type for trait
-  --> $DIR/fn-header-semantic-fail.rs:30:24
+  --> $DIR/fn-header-semantic-fail.rs:29:24
    |
 LL |         async fn ft1();
    |                       - type in trait
@@ -189,7 +181,7 @@ LL |         async fn ft1() {}
               found fn pointer `fn() -> impl Future`
 
 error[E0053]: method `ft5` has an incompatible type for trait
-  --> $DIR/fn-header-semantic-fail.rs:35:48
+  --> $DIR/fn-header-semantic-fail.rs:34:48
    |
 LL |         const async unsafe extern "C" fn ft5();
    |                                               - type in trait
@@ -203,23 +195,7 @@ LL |         const async unsafe extern "C" fn ft5() {}
    = note: expected fn pointer `unsafe extern "C" fn()`
               found fn pointer `unsafe extern "C" fn() -> impl Future`
 
-error: `from_generator` is not yet stable as a const fn
+error: aborting due to 20 previous errors
-  --> $DIR/fn-header-semantic-fail.rs:35:48
-   |
-LL |         const async unsafe extern "C" fn ft5() {}
-   |                                                ^^
-   |
-   = help: add `#![feature(gen_future)]` to the crate attributes to enable
-
-error: `from_generator` is not yet stable as a const fn
-  --> $DIR/fn-header-semantic-fail.rs:48:48
-   |
-LL |         const async unsafe extern "C" fn fi5() {}
-   |                                                ^^
-   |
-   = help: add `#![feature(gen_future)]` to the crate attributes to enable
-
-error: aborting due to 23 previous errors
 
 Some errors have detailed explanations: E0053, E0379, E0706.
 For more information about an error, try `rustc --explain E0053`.