Stabilize {std, core}::prelude::rust_*.
This stabilizes the `{core, std}::prelude::{rust_2015, rust_2018, rust_2021}` modules.
The usage of these modules as the prelude in those editions was already stabilized. This just stabilizes the modules themselves, making it possible for a user to explicitly refer to them.
Tracking issue: https://github.com/rust-lang/rust/issues/85684
FCP on the RFC that included this finished here: https://github.com/rust-lang/rfcs/pull/3114#issuecomment-840577395
Remove must_use from ALLOWED_ATTRIBUTES
This is a fairly common attribute on methods, but is not something you need to know when reading the method docs - the purpose of the attribute is for the compiler to tell you about it if you forget to use a value.
Removing reclaims some valuable space in the summary of methods, particularly when the attribute has a long string value.
As discussed in #84309. Partially addresses #81482.
r? ```@Manishearth```
Add functions `Duration::try_from_secs_{f32, f64}`
These functions allow constructing a Duration from a floating point value that could be out of range without panicking.
Tracking issue: #83400
Explain non-dropped sender recv in docs
Original senders that are still hanging around could cause
Receiver::recv to not block since this is a potential footgun
for beginners, clarify more on this in the docs for readers to
be aware about it.
Maybe it would be better to show an example of the pattern where `drop(tx)` is used when it is being cloned multiple times? Although I have seen it in quite a few articles but I am surprised that this part is not very clear with the current words without careful reading.
> If the corresponding Sender has disconnected, or it disconnects while this call is blocking, this call will wake up and return Err to indicate that no more messages can ever be received on this channel. However, since channels are buffered, messages sent before the disconnect will still be properly received.
Some words there may seemed similar if I carefully read and relate it but if I am new, I probably does not know "drop" makes it "disconnected". So I mention the words "drop" and "alive" to make it more relatable to lifetime.
rustdoc: revert deref recur to resume inclusion of impl ExtTrait<Local> for ExtType
As discussed here: https://github.com/rust-lang/rust/issues/82465#issuecomment-829290384, Revert PR #80653 to resolve issue #82465.
Issue #82465 was we had stopped including certain trait implementations, namely implementations on an imported type of an imported trait *instantiated on a local type*. That bug was injected by PR #80653.
Reverting #80653 means we don't list all the methods that you have accessible via recursively applying `Deref`.
[Discussion in last week's rustc triage meeting](https://zulip-archive.rust-lang.org/238009tcompilermeetings/19557weekly2021042954818.html#236680594) led us to conclude that the bug was worse than the enhancement, and there was not an obvious fix for the bug itself. So for the short term we remove the enhancement, while in the long term we will work on figuring out a way to have our imported trait implementation cake and eat it too.
Improve CTFE UB validation error messages
As mentioned in https://github.com/rust-lang/rust/pull/86245#discussion_r650494012 this PR slightly improves the formatting of validation errors, to move the path to the error prefix.
From:
`type validation failed: encountered invalid vtable: size is bigger than largest supported object at .0`
To:
`type validation failed at .0: encountered invalid vtable: size is bigger than largest supported object`.
Previously, coverage tests were writing profiler data to files based on
their pid. As rustdoc spawns each doctest as its own process, it might
be possible in rare cases that a pid is being reused which would cause
a file to be overwritten, leading to incorrect coverage results.
