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Document std limitations before/after main #115247

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Sep 16, 2023
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Document std limitations before/after main #115247

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26 changes: 25 additions & 1 deletion library/std/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -152,6 +152,31 @@
//! contains further primitive shared memory types, including [`atomic`] and
//! [`mpsc`], which contains the channel types for message passing.
//!
//! # Use before and after `main()`
//!
//! Many parts of the standard library are expected to work before and after `main()`;
//! but this is not guaranteed or ensured by tests. It is recommended that you write your own tests
//! and run them on each platform you wish to support.
//! This means that use of `std` before/after main, especially of features that interact with the
//! OS or global state, is exempted from stability and portability guarantees and instead only
//! provided on a best-effort basis. Nevertheless bug reports are appreciated.
//!
//! On the other hand `core` and `alloc` are most likely to work in such environments with
//! the caveat that any hookable behavior such as panics, oom handling or allocators will also
//! depend on the compatibility of the hooks.
//!
//! Some features may also behave differently outside main, e.g. stdio could become unbuffered,
//! some panics might turn into aborts, backtraces might not get symbolicated or similar.
//!
//! Non-exhaustive list of known limitations:
//!
//! - after-main use of thread-locals, which also affects additional features:
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It's not just after main that these can cause issues, they may panic during TLS destruction when any thread is exiting.

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Sure, but this section (and the filed issue it is trying to address) is about before/after main, not about non-main-thread lifecycles.

We could generalize it to "outside std-managed lifecycle scopes" or some similar mouthful?

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I just think this one sentence could be generalized to apply to TLS destruction in general, not just when exiting the main thread.

//! - [`thread::current()`]
//! - [`thread::scope()`]
//! - [`sync::mpsc`]
//! - before-main stdio file descriptors are not guaranteed to be open on unix platforms
//!
//!
//! [I/O]: io
//! [`MIN`]: i32::MIN
//! [`MAX`]: i32::MAX
Expand Down Expand Up @@ -187,7 +212,6 @@
//! [rust-discord]: https://discord.gg/rust-lang
//! [array]: prim@array
//! [slice]: prim@slice

// To run std tests without x.py without ending up with two copies of std, Miri needs to be
// able to "empty" this crate. See <https://github.com/rust-lang/miri-test-libstd/issues/4>.
// rustc itself never sets the feature, so this line has no effect there.
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