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Regression in async generator and fmt internals: loss of Send #93274
Description
If an
.awaitoccurs inside a formatting macro, the resulting generator is no longerSend.
Context
This has already tripped up some people which seem to get quite confused with this error message, so I'd rather we not regress here:
Code
I tried this code:
async fn foo() {
println!("{} {:?}", "", async {}.await);
}
const _ASSERT_SEND: fn() = || {
let _: &dyn Send = &foo();
};I expected to see this happen: Code compiles fine: the type of foo() is Send.
Instead, this happened: The type of foo() is not send, since the backing state-machine generator produced for it now captures part of the fmt internals involving a & reference to the extern { type Opaque; }, which is not Sync.
Version it worked on
It most recently worked on: beta
Version with regression
rustc --version --verbose:
nightly-2022-01-23 84322efad553c7a79c80
@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged
Activity
matthiaskrgr commentedon Jan 24, 2022
Might be related to #91032 / #93165 ?
danielhenrymantilla commentedon Jan 24, 2022
@matthiaskrgr I was also suspecting one of those, although I have to admit not having looked into it any further
lqd commentedon Jan 25, 2022
Does it work with #93284 btw ? That should completely disable drop range tracking. (I've launched a try build there so that we have artifacts ready for that soon).
There were also recent changes in the formatting macros/machinery last week IIRC. That could be related as well.
This should be easy enough to bisect, and make sure where the regression is coming from.
lqd commentedon Jan 25, 2022
Answering my own questions:
523be2e05da322daaecf1ecc8f2c0d625f5f46e3doesn't reproduce the issueformat_argschange I mentioned was Emit simpler code from format_args #91359 and its commit0bcacb391b28460f5a50fd627f01f670dfcfc7ccdoes reproduce the issue. This PR seems to have caused it indeed.danielhenrymantilla commentedon Jan 25, 2022
Nice investigation @lqd!
apiraino commentedon Jan 27, 2022
Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.
@rustbot label -I-prioritize +P-high
Rollup merge of rust-lang#93461 - dtolnay:fmtyield, r=davidtwco
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