Merge #5971

5971: Implement async blocks r=flodiebold a=oxalica

Fix #4018

@flodiebold already gave a generic guide in the issue. Here's some concern about implementation detail:
- Chalk doesn't support generator type yet.
- Adding generator type as a brand new type (ctor) can be complex and need to *re-introduced* builtin impls. (Like how we implement closures before native closure support of chalk, which is already removed in #5401 )
- The output type of async block should be known after type inference of the whole body.
  - We cannot directly get the type from source like return-positon-impl-trait. But we still need to provide trait bounds when chalk asking for `opaque_ty_data`.
  - During the inference, the output type of async block can be temporary unknown and participate the later inference.
    `let a = async { None }; let _: i32 = a.await.unwrap();`

So in this PR, the type of async blocks is inferred as an opaque type parameterized by the `Future::Output` type it should be, like what we do with closure type.
And it really works now.

Well, I still have some questions:
- The bounds `AsyncBlockImplType<T>: Future<Output = T>` is currently generated in `opaque_ty_data`. I'm not sure if we should put this code here.
- Type of async block is now rendered as `impl Future<Output = OutputType>`. Do we need to special display to hint that it's a async block? Note that closure type has its special format, instead of `impl Fn(..) -> ..` or function type.



Co-authored-by: oxalica <[email protected]>

1 files changed, 9 insertions, 2 deletions

diff --git a/crates/hir_ty/src/infer/expr.rs b/crates/hir_ty/src/infer/expr.rs
index a2f849d02..0a141b9cb 100644
--- a/crates/hir_ty/src/infer/expr.rs
+++ b/crates/hir_ty/src/infer/expr.rs
@@ -17,8 +17,8 @@ use crate::{
     autoderef, method_resolution, op,
     traits::{FnTrait, InEnvironment},
     utils::{generics, variant_data, Generics},
-    ApplicationTy, Binders, CallableDefId, InferTy, IntTy, Mutability, Obligation, Rawness, Substs,
-    TraitRef, Ty, TypeCtor,
+    ApplicationTy, Binders, CallableDefId, InferTy, IntTy, Mutability, Obligation, OpaqueTyId,
+    Rawness, Substs, TraitRef, Ty, TypeCtor,
 };
 
 use super::{
@@ -146,6 +146,13 @@ impl<'a> InferenceContext<'a> {
                 // FIXME should be std::result::Result<{inner}, _>
                 Ty::Unknown
             }
+            Expr::Async { body } => {
+                // Use the first type parameter as the output type of future.
+                // existenail type AsyncBlockImplTrait<InnerType>: Future<Output = InnerType>
+                let inner_ty = self.infer_expr(*body, &Expectation::none());
+                let opaque_ty_id = OpaqueTyId::AsyncBlockTypeImplTrait(self.owner, *body);
+                Ty::apply_one(TypeCtor::OpaqueType(opaque_ty_id), inner_ty)
+            }
             Expr::Loop { body, label } => {
                 self.breakables.push(BreakableContext {
                     may_break: false,