part of Support gen syntax (feature gen_blocks) #16156 by asukaminato0721 · Pull Request #21987 · rust-lang/rust-analyzer

asukaminato0721 · 2026-04-07T12:26:05Z

add basic supports for gen { } async gen { } gen fn async gen fn

fix some type signature show.

add some tests.

Copilot

Pull request overview

Adds initial support for the gen / async gen syntax (blocks and functions) across HIR lowering, type inference, and type display, including wiring up an AsyncIterator lang item and adding regression tests.

Changes:

Introduce new HIR expression variants for gen {} / async gen {} and lower gen fn / async gen fn bodies into them.
Extend type inference / solver interop and type display to represent gen as impl Iterator<Item = ...> and async gen as impl AsyncIterator<Item = ...>.
Add AsyncIterator minicore + lang item plumbing and new inference/coercion tests.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
crates/test-utils/src/minicore.rs	Adds minicore support for `core::async_iter::AsyncIterator` plus `#[lang]` wiring for `Option`/`Poll`.
crates/intern/src/symbol/symbols.rs	Registers new symbols used by the async iterator/lang item plumbing.
crates/hir/src/display.rs	Displays `gen fn`/`async gen fn` and strips wrapped return types similarly to `async fn`.
crates/hir-ty/src/tests/simple.rs	Adds inference tests for `gen`/`async gen` blocks and functions.
crates/hir-ty/src/tests/coercion.rs	Adds coercion tests for `yield` inside `gen`/`async gen` blocks.
crates/hir-ty/src/next_solver/interner.rs	Updates solver interner to recognize `Gen`/`AsyncGen` and map `AsyncIterator` lang item.
crates/hir-ty/src/mir/lower.rs	Explicitly marks `gen`/`async gen` blocks as not supported in MIR lowering.
crates/hir-ty/src/infer/mutability.rs	Includes `Gen`/`AsyncGen` in mutability inference traversal.
crates/hir-ty/src/infer/expr.rs	Implements inference + coroutine type lowering for `Gen`/`AsyncGen` blocks.
crates/hir-ty/src/infer/closure/analysis.rs	Includes `Gen`/`AsyncGen` in closure capture analysis traversal.
crates/hir-ty/src/display.rs	Displays coroutine types originating from `gen`/`async gen` as `impl Iterator/AsyncIterator`.
crates/hir-ty/src/diagnostics/unsafe_check.rs	Traverses `Gen`/`AsyncGen` blocks in unsafe diagnostics.
crates/hir-ty/src/diagnostics/expr.rs	Validates `Gen`/`AsyncGen` blocks like other block expressions.
crates/hir-expand/src/mod_path.rs	Adds known path for `core::async_iter::AsyncIterator`.
crates/hir-def/src/signatures.rs	Adds `GEN` flag and `is_gen()` on function signatures.
crates/hir-def/src/lang_item.rs	Adds `AsyncIterator` to the lang item table.
crates/hir-def/src/hir.rs	Adds `Expr::Gen` and `Expr::AsyncGen` HIR variants.
crates/hir-def/src/expr_store/scope.rs	Ensures scopes are computed for `Gen`/`AsyncGen` blocks.
crates/hir-def/src/expr_store/pretty.rs	Pretty-prints `gen` and `async gen` blocks.
crates/hir-def/src/expr_store/lower.rs	Lowers `gen`/`async gen` blocks and wraps `gen fn`/`async gen fn` bodies; wraps function return type in Iterator/AsyncIterator impl-trait.
crates/hir-def/src/expr_store/body.rs	Threads `is_gen_fn` into body lowering.
crates/hir-def/src/expr_store.rs	Traversal updates to treat `Gen`/`AsyncGen` as block-like expressions.

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View changes since this review

ChayimFriedman2 · 2026-04-15T17:11:45Z

I prefer to wait with this until my closure rewrite will be merged, which will mean you will have a rebase to do, but gen blocks/closures will be supported much better. I can also direct you (after I finish my branch of course), we need to follow rustc closely.

rustbot · 2026-04-20T12:20:39Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

ChayimFriedman2 · 2026-04-20T14:28:46Z

So: my PR was merged, what you need to do now is complete the things:

Some things are called async blocks/closures, you need to generalize the name to coroutines/coroutine closures respectively. Some will also need to know the kind. E.g. ExprCollector::lower_async_block_with_moved_arguments(), ExprCollector::async_block(). Lowering gen blocks is just like lowering async blocks, and gen closures are just like async closures. Similarly for async gen.
hir_def::ClosureKind is tricky. I think it will be the best as the following:

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum CoroutineKind {
    Async,
    Gen,
    AsyncGen,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ClosureKind {
    Closure,
    OldCoroutine(Movability), // `#[coroutine]`, previously this variant was called `Coroutine`. I don't have a better name :(
    Coroutine { kind: CoroutineKind, source: CoroutineSource }, // Previously `AsyncBlock`
    CoroutineClosure(CoroutineKind), // Previously `AsyncClosure`
}

In hir-ty/src/infer/closure.rs, inside infer_closure(), it was copied (and adjusted of course) from https://github.com/rust-lang/rust/blob/main/compiler/rustc_hir_typeck/src/closure.rs, but I removed the parts of gen and async gen closure. The structure is preserved, though: look at how we handle async blocks and async closures, and look at that rustc file, and copy the rest.
Add the lang items and fill the minicore as needed.
In hir-ty/src/display.rs, almost nothing needs to change, but when printing coroutines/coroutine closures, you will need to print gen or async gen instead of async when needed.

That's it I think! Feel free to ask me questions.

update based on comment

ChayimFriedman2

I have a suspect that you're using AI tools for code generation without disclosing that.

Please note that we require disclosure for AI usage, and properly understanding and reviewing their output.

View changes since this review

ChayimFriedman2 · 2026-04-20T19:14:54Z

-            // all async closures would default to `FnOnce` as their calling mode.
-            CaptureBy::Ref,
+        let capture_by = match kind {
+            // Async closures use by-ref by default so they don't all collapse to `FnOnce`.


Nope, gen and async gen closures also need CaptureBy::Ref.

ChayimFriedman2 · 2026-04-20T19:17:15Z

        const EXPLICIT_SAFE = 1 << 11;
        const HAS_LEGACY_CONST_GENERICS = 1 << 12;
        const RUSTC_INTRINSIC = 1 << 13;
+        const GEN = 1 << 14;


Put it near ASYNC?

ChayimFriedman2 · 2026-04-20T19:20:58Z

            let body = this.collect_expr_opt(expr);
-            if awaitable == Awaitable::Yes {
-                this.lower_async_block_with_moved_arguments(params, body, CoroutineSource::Fn)
+            if is_gen_fn {


This should also just call lower_coroutine_with_moved_arguments().

ChayimFriedman2 · 2026-04-20T19:21:54Z

+    fn desugared_coroutine_expr(
        &mut self,
-        source: CoroutineSource,
+        closure_kind: ClosureKind,


Take instead CoroutineKind and CoroutineSource?

ChayimFriedman2 · 2026-04-20T19:29:32Z

-                        };
-                        ClosureKind::Coroutine(movability)
+                    let closure_kind = if let Some(kind) =
+                        if e.async_token().is_some() && e.gen_token().is_some()


I don't like the nested if here. Split it into a local.

ChayimFriedman2 · 2026-04-20T19:32:50Z

        // would naturally unify these two trait hierarchies in the most
        // general way.
-        let call_trait_choices = if self.shallow_resolve(adjusted_ty).is_coroutine_closure() {
+        let prefer_async_fn_traits = match self.shallow_resolve(adjusted_ty).kind() {


rustc doesn't do this and keeps the old self.shallow_resolve(adjusted_ty).is_coroutine_closure() check, so it's good enough for us.

ChayimFriedman2 · 2026-04-20T19:34:35Z

-                    ClosureKind::Coroutine(_) => self.table.next_ty_var(),
-                    ClosureKind::AsyncBlock { .. } => self.types.types.unit,
+                    ClosureKind::OldCoroutine(_)
+                    | ClosureKind::Coroutine { kind: CoroutineKind::Gen | CoroutineKind::AsyncGen, .. } => {


Did you look at the rustc code I linked? That's not what it does.

ChayimFriedman2 · 2026-04-20T19:37:04Z

                        .async_fn_trait_kind_from_def_id(trait_def_id)
                        .or_else(|| self.fn_trait_kind_from_def_id(trait_def_id)),
+                    ClosureKind::CoroutineClosure(CoroutineKind::Gen)
+                    | ClosureKind::CoroutineClosure(CoroutineKind::AsyncGen) => {


This is again not what rustc does. Look carefully.

ChayimFriedman2 · 2026-04-20T19:38:17Z

+            {
                self.extract_sig_from_projection_and_future_bound(projection)
            }
+            ClosureKind::CoroutineClosure(CoroutineKind::Gen | CoroutineKind::AsyncGen)


And here too.

Copilot AI review requested due to automatic review settings April 7, 2026 12:26

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 7, 2026

Copilot started reviewing on behalf of asukaminato0721 April 7, 2026 12:26 View session

asukaminato0721 force-pushed the 16156 branch 2 times, most recently from b4d4360 to e737273 Compare April 7, 2026 12:28

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asukaminato0721 force-pushed the 16156 branch from e737273 to 55c4bca Compare April 15, 2026 14:43

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asukaminato0721 force-pushed the 16156 branch from 55c4bca to 401c4e1 Compare April 15, 2026 14:52

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asukaminato0721 force-pushed the 16156 branch from 401c4e1 to 813c613 Compare April 20, 2026 12:20

fix

84b2f21

update based on comment

asukaminato0721 force-pushed the 16156 branch from 813c613 to 84b2f21 Compare April 20, 2026 18:59

ChayimFriedman2 requested changes Apr 20, 2026

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asukaminato0721 commented Apr 7, 2026 •

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