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, 7 insertions, 0 deletions

diff --git a/crates/hir/src/code_model.rs b/crates/hir/src/code_model.rs
index c2fc819e7..7a9747fc7 100644
--- a/crates/hir/src/code_model.rs
+++ b/crates/hir/src/code_model.rs
@@ -1283,6 +1283,8 @@ impl Type {
     /// Checks that particular type `ty` implements `std::future::Future`.
     /// This function is used in `.await` syntax completion.
     pub fn impls_future(&self, db: &dyn HirDatabase) -> bool {
+        // No special case for the type of async block, since Chalk can figure it out.
+
         let krate = self.krate;
 
         let std_future_trait =
@@ -1600,6 +1602,11 @@ impl Type {
                                 cb(type_.derived(ty.clone()));
                             }
                         }
+                        TypeCtor::OpaqueType(..) => {
+                            if let Some(bounds) = ty.impl_trait_bounds(db) {
+                                walk_bounds(db, &type_.derived(ty.clone()), &bounds, cb);
+                            }
+                        }
                         _ => (),
                     }