Make diagnostics know about more coro closures than async closures

Author: oli-obk

compiler/rustc_hir/src/hir.rs

@@ -1893,9 +1893,7 @@ impl CoroutineKind {
             CoroutineKind::Coroutine(mov) => mov,
         }
     }
-}
 
-impl CoroutineKind {
     pub fn is_fn_like(self) -> bool {
         matches!(self, CoroutineKind::Desugared(_, CoroutineSource::Fn))
     }

compiler/rustc_trait_selection/messages.ftl

@@ -72,8 +72,6 @@ trait_selection_adjust_signature_remove_borrow = consider adjusting the signatur
 
 trait_selection_ascribe_user_type_prove_predicate = ...so that the where clause holds
 
-trait_selection_async_closure_not_fn = async closure does not implement `{$kind}` because it captures state from its environment
-
 trait_selection_await_both_futures = consider `await`ing on both `Future`s
 trait_selection_await_future = consider `await`ing on the `Future`
 trait_selection_await_note = calling an async function returns a future
@@ -123,6 +121,8 @@ trait_selection_closure_kind_requirement = the requirement to implement `{$trait
 
 trait_selection_compare_impl_item_obligation = ...so that the definition in impl matches the definition from the trait
 trait_selection_consider_specifying_length = consider specifying the actual array length
+trait_selection_coro_closure_not_fn = {$coro_kind}closure does not implement `{$kind}` because it captures state from its environment
+
 trait_selection_data_flows = ...but data{$label_var1_exists ->
     [true] {" "}from `{$label_var1}`
     *[false] {""}

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -37,9 +37,7 @@ use super::{
 use crate::error_reporting::TypeErrCtxt;
 use crate::error_reporting::infer::TyCategory;
 use crate::error_reporting::traits::report_dyn_incompatibility;
-use crate::errors::{
-    AsyncClosureNotFn, ClosureFnMutLabel, ClosureFnOnceLabel, ClosureKindMismatch,
-};
+use crate::errors::{ClosureFnMutLabel, ClosureFnOnceLabel, ClosureKindMismatch, CoroClosureNotFn};
 use crate::infer::{self, InferCtxt, InferCtxtExt as _};
 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
 use crate::traits::{
@@ -887,15 +885,24 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             }
 
             // If the closure has captures, then perhaps the reason that the trait
-            // is unimplemented is because async closures don't implement `Fn`/`FnMut`
+            // is unimplemented is because coro closures don't implement `Fn`/`FnMut`
             // if they have captures.
             if let Some(by_ref_captures) = by_ref_captures
                 && let ty::FnPtr(sig_tys, _) = by_ref_captures.kind()
                 && !sig_tys.skip_binder().output().is_unit()
             {
-                let mut err = self.dcx().create_err(AsyncClosureNotFn {
+                let coro_kind = match self
+                    .tcx
+                    .coroutine_kind(self.tcx.coroutine_for_closure(closure_def_id))
+                    .unwrap()
+                {
+                    rustc_hir::CoroutineKind::Desugared(desugaring, _) => desugaring.to_string(),
+                    coro => coro.to_string(),
+                };
+                let mut err = self.dcx().create_err(CoroClosureNotFn {
                     span: self.tcx.def_span(closure_def_id),
                     kind: expected_kind.as_str(),
+                    coro_kind,
                 });
                 self.note_obligation_cause(&mut err, &obligation);
                 return Some(err.emit());

compiler/rustc_trait_selection/src/errors.rs

@@ -170,11 +170,12 @@ pub struct ClosureFnMutLabel {
 }
 
 #[derive(Diagnostic)]
-#[diag(trait_selection_async_closure_not_fn)]
-pub(crate) struct AsyncClosureNotFn {
+#[diag(trait_selection_coro_closure_not_fn)]
+pub(crate) struct CoroClosureNotFn {
     #[primary_span]
     pub span: Span,
     pub kind: &'static str,
+    pub coro_kind: String,
 }
 
 #[derive(Diagnostic)]

tests/ui/iterators/generator_returned_from_fn.stderr

@@ -10,7 +10,7 @@ LL | |         }
 LL | |     } }
    | |_____^ returns a reference to data owned by the current function
 
-error: async closure does not implement `Fn` because it captures state from its environment
+error: `gen` closure does not implement `Fn` because it captures state from its environment
   --> $DIR/generator_returned_from_fn.rs:35:13
    |
 LL |       iter! { move || {