2a177c2047
Skip over args when determining if async-closure's inner coroutine consumes its upvars #125306 implements a strategy for when we have an `async move ||` async-closure that is inferred to be `async FnOnce`, it will force the inner coroutine to also be `move`, since we cannot borrow any upvars from the parent async-closure (since `FnOnce` is not lending):8e86c95671/compiler/rustc_hir_typeck/src/upvar.rs (L211-L229)However, when this strategy was implemented, it reused the `ExprUseVisitor` data from visiting the whole coroutine, which includes additional statements due to `async`-specific argument desugaring:8e86c95671/compiler/rustc_ast_lowering/src/item.rs (L1197-L1228)Well, it turns out that we don't care about these argument desugaring parameters, because arguments to the async-closure are not the *async-closure*'s captures -- they exist for only one invocation of the closure, and they're always consumed by construction (see the argument desugaring above), so they will force the coroutine's inferred kind to `FnOnce`. (Unless they're `Copy`, because we never consider `Copy` types to be consumed):8e86c95671/compiler/rustc_hir_typeck/src/expr_use_visitor.rs (L60-L66)However, since we *were* visiting these arg exprs, this resulted in us too-aggressively applying `move` to the inner coroutine, resulting in regressions. For example, this PR fixes #128516. Fascinatingly, the note above about how we never consume `Copy` types is why this only regressed when the argument types weren't all `Copy`. I tried to leave some comments inline to make this more clear :)
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