Update error message for const_gen_fn.rs

Author: eholk

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -503,7 +503,7 @@ impl<'tcx> NonConstOp<'tcx> for Coroutine {
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        let msg = format!("{:#}s are not allowed in {}s", self.0, ccx.const_kind());
+        let msg = format!("{} are not allowed in {}s", self.0.to_plural_string(), ccx.const_kind());
         if let Status::Unstable { gate, .. } = self.status_in_item(ccx) {
             ccx.tcx.sess.create_feature_err(errors::UnallowedOpInConstContext { span, msg }, gate)
         } else {

compiler/rustc_hir/src/hir.rs

@@ -2065,6 +2065,15 @@ impl CoroutineKind {
     pub fn is_fn_like(self) -> bool {
         matches!(self, CoroutineKind::Desugared(_, CoroutineSource::Fn))
     }
+
+    pub fn to_plural_string(&self) -> String {
+        match self {
+            CoroutineKind::Desugared(d, CoroutineSource::Fn) => format!("{d:#}fn bodies"),
+            CoroutineKind::Desugared(d, CoroutineSource::Block) => format!("{d:#}blocks"),
+            CoroutineKind::Desugared(d, CoroutineSource::Closure) => format!("{d:#}closure bodies"),
+            CoroutineKind::Coroutine(_) => "coroutines".to_string(),
+        }
+    }
 }
 
 impl fmt::Display for CoroutineKind {

tests/ui/coroutine/const_gen_fn.rs

@@ -4,6 +4,7 @@
 
 const gen fn a() {}
 //~^ ERROR functions cannot be both `const` and `gen`
+//~^^ ERROR `gen` fn bodies are not allowed in constant functions
 
 const async gen fn b() {}
 //~^ ERROR functions cannot be both `const` and `async gen`

tests/ui/coroutine/const_gen_fn.stderr

@@ -8,13 +8,19 @@ LL | const gen fn a() {}
    | `const` because of this
 
 error: functions cannot be both `const` and `async gen`
-  --> $DIR/const_gen_fn.rs:8:1
+  --> $DIR/const_gen_fn.rs:9:1
    |
 LL | const async gen fn b() {}
    | ^^^^^-^^^^^^^^^----------
    | |     |
    | |     `async gen` because of this
    | `const` because of this
 
-error: aborting due to 2 previous errors
+error: `gen` fn bodies are not allowed in constant functions
+  --> $DIR/const_gen_fn.rs:5:18
+   |
+LL | const gen fn a() {}
+   |                  ^^
+
+error: aborting due to 3 previous errors