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Auto merge of #104321 - Swatinem:async-gen, r=oli-obk

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Avoid `GenFuture` shim when compiling async constructs

Previously, async constructs would be lowered to "normal" generators, with an additional `from_generator` / `GenFuture` shim in between to convert from `Generator` to `Future`.

The compiler will now special-case these generators internally so that async constructs will *directly* implement `Future` without the need to go through the `from_generator` / `GenFuture` shim.

The primary motivation for this change was hiding this implementation detail in stack traces and debuginfo, but it can in theory also help the optimizer as there is less abstractions to see through.

---

Given this demo code:

```rust
pub async fn a(arg: u32) -> Backtrace {
    let bt = b().await;
    let _arg = arg;
    bt
}

pub async fn b() -> Backtrace {
    Backtrace::force_capture()
}
```

I would get the following with the latest stable compiler (on Windows):

```
   4: async_codegen:🅱️:async_fn$0
             at .\src\lib.rs:10
   5: core::future::from_generator::impl$1::poll<enum2$<async_codegen:🅱️:async_fn_env$0> >
             at /rustc/897e37553bba8b42751c67658967889d11ecd120\library\core\src\future\mod.rs:91
   6: async_codegen:🅰️:async_fn$0
             at .\src\lib.rs:4
   7: core::future::from_generator::impl$1::poll<enum2$<async_codegen:🅰️:async_fn_env$0> >
             at /rustc/897e37553bba8b42751c67658967889d11ecd120\library\core\src\future\mod.rs:91
```

whereas now I get a much cleaner stack trace:

```
   3: async_codegen:🅱️:async_fn$0
             at .\src\lib.rs:10
   4: async_codegen:🅰️:async_fn$0
             at .\src\lib.rs:4
```
This commit is contained in:
bors 2022-11-24 17:14:42 +00:00
commit 5dfb4b0afa
41 changed files with 459 additions and 211 deletions
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36
compiler/rustc_ast_lowering/src/expr.rs
View file

@ -149,7 +149,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
*capture_clause,
*closure_node_id,
None,
block.span,
e.span,
hir::AsyncGeneratorKind::Block,
|this| this.with_new_scopes(|this| this.lower_block_expr(block)),
),
@ -569,12 +569,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
}
}
/// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
/// Lower an `async` construct to a generator that implements `Future`.
///
/// This results in:
///
/// ```text
/// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
/// std::future::identity_future(static move? |_task_context| -> <ret_ty> {
/// <body>
/// })
/// ```
@ -589,12 +589,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
) -> hir::ExprKind<'hir> {
let output = ret_ty.unwrap_or_else(|| hir::FnRetTy::DefaultReturn(self.lower_span(span)));
// Resume argument type. We let the compiler infer this to simplify the lowering. It is
// fully constrained by `future::from_generator`.
// Resume argument type: `ResumeTy`
let unstable_span =
self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
let resume_ty = hir::QPath::LangItem(hir::LangItem::ResumeTy, unstable_span, None);
let input_ty = hir::Ty {
hir_id: self.next_id(),
kind: hir::TyKind::Infer,
span: self.lower_span(span),
kind: hir::TyKind::Path(resume_ty),
span: unstable_span,
};
// The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
@ -677,16 +679,24 @@ impl<'hir> LoweringContext<'_, 'hir> {
let generator = hir::Expr { hir_id, kind: generator_kind, span: self.lower_span(span) };
// `future::from_generator`:
let gen_future = self.expr_lang_item_path(
// FIXME(swatinem):
// For some reason, the async block needs to flow through *any*
// call (like the identity function), as otherwise type and lifetime
// inference have a hard time figuring things out.
// Without this, we would get:
// E0720 in src/test/ui/impl-trait/in-trait/default-body-with-rpit.rs
// E0700 in src/test/ui/self/self_lifetime-async.rs
// `future::identity_future`:
let identity_future = self.expr_lang_item_path(
unstable_span,
hir::LangItem::FromGenerator,
hir::LangItem::IdentityFuture,
AttrVec::new(),
None,
);
// `future::from_generator(generator)`:
hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
// `future::identity_future(generator)`:
hir::ExprKind::Call(self.arena.alloc(identity_future), arena_vec![self; generator])
}
/// Desugar `<expr>.await` into:
@ -990,7 +1000,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
}
// Transform `async |x: u8| -> X { ... }` into
// `|x: u8| future_from_generator(|| -> X { ... })`.
// `|x: u8| identity_future(|| -> X { ... })`.
let body_id = this.lower_fn_body(&outer_decl, |this| {
let async_ret_ty = if let FnRetTy::Ty(ty) = &decl.output {
let itctx = ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock);