Error on using yield without also using #[coroutine] on the closure

And suggest adding the `#[coroutine]` to the closure

Author: oli-obk

compiler/rustc_ast_lowering/messages.ftl

@@ -161,3 +161,6 @@ ast_lowering_underscore_expr_lhs_assign =
     .label = `_` not allowed here
 
 ast_lowering_use_angle_brackets = use angle brackets instead
+ast_lowering_yield_in_closure =
+    `yield` can only be used in `#[coroutine]` closures, or `gen` blocks
+    .suggestion = use `#[coroutine]` to make this closure a coroutine

compiler/rustc_ast_lowering/src/errors.rs

@@ -414,3 +414,12 @@ pub(crate) struct AsyncBoundOnlyForFnTraits {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_lowering_yield_in_closure)]
+pub(crate) struct YieldInClosure {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = "#[coroutine] ", applicability = "maybe-incorrect", style = "verbose")]
+    pub suggestion: Option<Span>,
+}

compiler/rustc_ast_lowering/src/expr.rs

@@ -8,6 +8,7 @@ use super::errors::{
 };
 use super::ResolverAstLoweringExt;
 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
+use crate::errors::YieldInClosure;
 use crate::{FnDeclKind, ImplTraitPosition};
 use rustc_ast::ptr::P as AstP;
 use rustc_ast::*;
@@ -979,6 +980,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 None
             };
             let body_id = this.lower_fn_body(decl, |this| {
+                this.coroutine_kind = coroutine_kind;
                 let e = this.lower_expr_mut(body);
                 coroutine_kind = this.coroutine_kind;
                 e
@@ -1577,7 +1579,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     )
                     .emit();
                 }
+                let suggestion = self.current_item.map(|s| s.shrink_to_lo());
+                self.dcx().emit_err(YieldInClosure { span, suggestion });
                 self.coroutine_kind = Some(hir::CoroutineKind::Coroutine(Movability::Movable));
+
                 false
             }
         };

compiler/rustc_ast_lowering/src/item.rs

@@ -203,7 +203,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 body,
                 ..
             }) => {
-                self.with_new_scopes(ident.span, |this| {
+                self.with_new_scopes(*fn_sig_span, |this| {
                     // Note: we don't need to change the return type from `T` to
                     // `impl Future<Output = T>` here because lower_body
                     // only cares about the input argument patterns in the function

compiler/rustc_codegen_cranelift/example/polymorphize_coroutine.rs

@@ -1,4 +1,4 @@
-#![feature(coroutines, coroutine_trait)]
+#![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
 
 use std::ops::Coroutine;
 use std::pin::Pin;
@@ -8,7 +8,8 @@ fn main() {
 }
 
 fn run_coroutine<T>() {
-    let mut coroutine = || {
+    let mut coroutine = #[coroutine]
+    || {
         yield;
         return;
     };

compiler/rustc_codegen_cranelift/example/std_example.rs

@@ -1,6 +1,7 @@
 #![feature(
     core_intrinsics,
     coroutines,
+    stmt_expr_attributes,
     coroutine_trait,
     is_sorted,
     repr_simd,
@@ -122,9 +123,12 @@ fn main() {
         test_simd();
     }
 
-    Box::pin(move |mut _task_context| {
-        yield ();
-    })
+    Box::pin(
+        #[coroutines]
+        move |mut _task_context| {
+            yield ();
+        },
+    )
     .as_mut()
     .resume(0);
 

compiler/rustc_codegen_gcc/example/std_example.rs

@@ -1,5 +1,5 @@
 #![allow(internal_features)]
-#![feature(core_intrinsics, coroutines, coroutine_trait, is_sorted)]
+#![feature(core_intrinsics, coroutines, coroutine_trait, is_sorted, stmt_expr_attributes)]
 
 #[cfg(feature="master")]
 #[cfg(target_arch="x86_64")]
@@ -103,7 +103,7 @@ fn main() {
         test_simd();
     }
 
-    Box::pin(move |mut _task_context| {
+    Box::pin(#[coroutine] move |mut _task_context| {
         yield ();
     }).as_mut().resume(0);
 

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1260,30 +1260,34 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         id: DefIndex,
         sess: &'a Session,
     ) -> impl Iterator<Item = ModChild> + 'a {
-        iter::from_coroutine(move || {
-            if let Some(data) = &self.root.proc_macro_data {
-                // If we are loading as a proc macro, we want to return
-                // the view of this crate as a proc macro crate.
-                if id == CRATE_DEF_INDEX {
-                    for child_index in data.macros.decode(self) {
+        iter::from_coroutine(
+            #[cfg_attr(not(bootstrap), coroutine)]
+            move || {
+                if let Some(data) = &self.root.proc_macro_data {
+                    // If we are loading as a proc macro, we want to return
+                    // the view of this crate as a proc macro crate.
+                    if id == CRATE_DEF_INDEX {
+                        for child_index in data.macros.decode(self) {
+                            yield self.get_mod_child(child_index, sess);
+                        }
+                    }
+                } else {
+                    // Iterate over all children.
+                    let non_reexports =
+                        self.root.tables.module_children_non_reexports.get(self, id);
+                    for child_index in non_reexports.unwrap().decode(self) {
                         yield self.get_mod_child(child_index, sess);
                     }
-                }
-            } else {
-                // Iterate over all children.
-                let non_reexports = self.root.tables.module_children_non_reexports.get(self, id);
-                for child_index in non_reexports.unwrap().decode(self) {
-                    yield self.get_mod_child(child_index, sess);
-                }
 
-                let reexports = self.root.tables.module_children_reexports.get(self, id);
-                if !reexports.is_default() {
-                    for reexport in reexports.decode((self, sess)) {
-                        yield reexport;
+                    let reexports = self.root.tables.module_children_reexports.get(self, id);
+                    if !reexports.is_default() {
+                        for reexport in reexports.decode((self, sess)) {
+                            yield reexport;
+                        }
                     }
                 }
-            }
-        })
+            },
+        )
     }
 
     fn is_ctfe_mir_available(self, id: DefIndex) -> bool {

compiler/rustc_middle/src/lib.rs

@@ -35,6 +35,7 @@
 #![feature(const_type_name)]
 #![feature(discriminant_kind)]
 #![feature(coroutines)]
+#![feature(stmt_expr_attributes)]
 #![feature(generic_nonzero)]
 #![feature(if_let_guard)]
 #![feature(inline_const)]

compiler/rustc_middle/src/ty/context.rs

@@ -1262,20 +1262,23 @@ impl<'tcx> TyCtxt<'tcx> {
         self.dep_graph.read_index(DepNodeIndex::FOREVER_RED_NODE);
 
         let definitions = &self.untracked.definitions;
-        std::iter::from_coroutine(|| {
-            let mut i = 0;
-
-            // Recompute the number of definitions each time, because our caller may be creating
-            // new ones.
-            while i < { definitions.read().num_definitions() } {
-                let local_def_index = rustc_span::def_id::DefIndex::from_usize(i);
-                yield LocalDefId { local_def_index };
-                i += 1;
-            }
+        std::iter::from_coroutine(
+            #[cfg_attr(not(bootstrap), coroutine)]
+            || {
+                let mut i = 0;
+
+                // Recompute the number of definitions each time, because our caller may be creating
+                // new ones.
+                while i < { definitions.read().num_definitions() } {
+                    let local_def_index = rustc_span::def_id::DefIndex::from_usize(i);
+                    yield LocalDefId { local_def_index };
+                    i += 1;
+                }
 
-            // Freeze definitions once we finish iterating on them, to prevent adding new ones.
-            definitions.freeze();
-        })
+                // Freeze definitions once we finish iterating on them, to prevent adding new ones.
+                definitions.freeze();
+            },
+        )
     }
 
     pub fn def_path_table(self) -> &'tcx rustc_hir::definitions::DefPathTable {