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Reflect explicit macro import in reference #2077

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Reflect explicit macro import in reference #2077

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Reflect explicit macro import in reference
Voultapher Nov 8, 2025
1e9df55
Restore line break after rule ID and unwrap
traviscross Nov 9, 2025
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9 changes: 2 additions & 7 deletions src/names/preludes.md
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Expand Up @@ -80,11 +80,7 @@ r[names.preludes.extern.no_std]
### The `no_std` attribute

r[names.preludes.extern.no_std.intro]
By default, the standard library is automatically included in the crate root
module. The [`std`] crate is added to the root, along with an implicit
[`macro_use` attribute] pulling in all macros exported from `std` into the
[`macro_use` prelude]. Both [`core`] and [`std`] are added to the [extern
prelude].
By default, the standard library is automatically included in the crate root module. The [`std`] crate is added to the root. Both [`core`] and [`std`] are added to the [extern prelude]. Standard library macros are imported via [use declarations] and are *not* part of the [`macro_use` prelude].
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@traviscross traviscross Nov 9, 2025

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The [std] crate is added to the root.

Is this sentence needed? I.e., is there any sense in which "the std crate is added to the root" but the core crate is not, or that this is not covered by "Both [core] and [std] are added to the [extern prelude]"?

My reading of the old text is that the sentence was there to support the restrictive clause (that erroneously was preceded by a comma, making the clause seem non-restrictive, i.e. that the part before the comma could stand on its own without the part after it):

The [std] crate is added to the root, along with an implicit [macro_use attribute] pulling in all macros exported from std into the [macro_use prelude].

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@traviscross traviscross Nov 9, 2025 •
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Or, maybe this language and the line below

It does three things... Injects the [core] crate into the crate root instead of [std].

are leftovers from Rust 2015.

In Rust 2018 and later, this is a valid library:;

use ::core as c1;
use ::std as c2;

But it's not in Rust 2015. It gives the error

error[E0432]: unresolved import `core`
 --> src/lib.rs:1:5
  |
1 | use ::core as c1;
  |     ^^^^^^^^^^^^ no `core` in the root

unless #![no_std] is used, in which case it accepts that line but not use ::std. Interestingly, I don't see this documented specifically in the 2018 edition guide:

https://doc.rust-lang.org/nightly/edition-guide/rust-2018/path-changes.html

Perhaps this is simply an artifact of the change to make crates in the extern prelude accessible to use declarations. Did that then make it fully unobservable whether core is separately added to the crate root or not, or is there still some way to tell?

cc @ehuss

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@traviscross traviscross Nov 9, 2025

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Looking, this is all covered in:

I'm going to go get that merged.

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@Voultapher Voultapher Nov 10, 2025

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Ok, I'll wait for #1913 to be merged and will update this PR on that.


r[names.preludes.extern.no_std.allowed-positions]
The *`no_std` [attribute]* may be applied at the crate level to prevent the
Expand All @@ -97,8 +93,7 @@ r[names.preludes.extern.no_std.extern]
r[names.preludes.extern.no_std.module]
* Affects which module is used to make up the [standard library prelude] (as described above).
r[names.preludes.extern.no_std.core]
* Injects the [`core`] crate into the crate root instead of [`std`], and pulls
in all macros exported from `core` in the [`macro_use` prelude].
* Injects the [`core`] crate into the crate root instead of [`std`].

> [!NOTE]
> Using the core prelude over the standard prelude is useful when either the crate is targeting a platform that does not support the standard library or is purposefully not using the capabilities of the standard library. Those capabilities are mainly dynamic memory allocation (e.g. `Box` and `Vec`) and file and network capabilities (e.g. `std::fs` and `std::io`).
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