Update no_std to use the attribute template
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These don't really belong where they are, and are just generally helpful things about the attribute.
The current text was a little bit of a mess and had some subtle inaccuracies. This reworks so that the intro follows the template, and just generally introduces the attribute. `allowed-positions` now just specifies the allowed positions. I reworked the behavior changes caused by no_std into proper separate rules. `std` is not injected into the crate root anymore starting with edition 2018.
I do not know how this was missing, I've looked at it a million times. I feel like I was maybe missing something, but I really can't find anything that directly refers to this, nor can I find any open issues. I believe this is now done with this relatively subtle bit: https://github.com/rust-lang/rust/blob/4d08223c054cf5a56d9761ca925fd46ffebe7115/compiler/rustc_builtin_macros/src/standard_library_imports.rs#L42 In the past it used to use a different approach of emulating `extern crate std as _;`: https://github.com/rust-lang/rust/blob/c8cf9f5a025bb475804b5a90f54aca310b952526/src/libsyntax_ext/standard_library_imports.rs#L42-L46
More closely align with the template, and some minor word tweaks.
We had said here that the `no_std` attribute is used to "prevent the automatic linking of the std crate", but then further down we say, "using `no_std` does not prevent the standard library from being linked in". The important distinction here is the word "automatically", but that gets a bit lost due to reusing the "prevent" phrasing. Probably "prevent" is just too strong a word to use when what we're really saying is that we're causing something to not happen automatically. Let's make that more clear and increase sentence parallelism.
Some sentences here could be improved with more sentence parallelism and a bit of reordering, so let's do that.
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This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed. Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers. |
traviscross
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Nov 11, 2025
We had text in the intro about "deferring to the core crate instead", but it may not have been sufficiently clear what we mean exactly by that. Let's talk specifically about how `no_std` causes us to use the `core` crate for the standard library prelude and for the `macro_use` prelude.
We want to highlight how using `no_std` doesn't prevent `std` from being linked. Let's revise and elaborate this text for clarity.
The text here could be read in such a way as to imply that `no_std` affected whether or not `core` was added to the extern prelude, but that isn't the case. Let's make it clear that this section is about what is added to the `macro_use` prelude and speak to that directly.
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New rules:
names.preludes.extern.no_std.syntaxnames.preludes.extern.no_std.duplicatesnames.preludes.extern.no_std.edition2018Renamed rules:
names.preludes.extern.no_std.externandnames.preludes.extern.no_std.coreis nownames.preludes.extern.no_std.inject