From e31e627238d75b3f24532ccb86fac7b4fd5e735d Mon Sep 17 00:00:00 2001 From: Alexis Bourget Date: Mon, 23 Nov 2020 09:18:13 +0100 Subject: [PATCH] Add doc for 'as _' about '_' and its possibilities and problems --- library/std/src/keyword_docs.rs | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-) diff --git a/library/std/src/keyword_docs.rs b/library/std/src/keyword_docs.rs index b990b785703..9fd322adcfc 100644 --- a/library/std/src/keyword_docs.rs +++ b/library/std/src/keyword_docs.rs @@ -20,19 +20,30 @@ /// explicitly using `as` allows a few more coercions that aren't allowed implicitly, such as /// changing the type of a raw pointer or turning closures into raw pointers. /// -/// `as` is also used to rename imports in [`use`] and [`extern crate`] statements: +/// `as` can be seen as the primitive for `From` and `Into`: `as` only works with primitives +/// (`u8`, `bool`, `str`, pointers, ...) whereas `From` and `Into` also works with types like +/// `String` or `Vec`. +/// +/// `as` can also be used with the `_` placeholder when the destination type can be inferred. Note +/// that this can cause inference breakage and usually such code should use an explicit type for +/// both clarity and stability. This is most useful when converting pointers using `as *const _` or +/// `as *mut _` though the [`cast`][const-cast] method is recommended over `as *const _` and it is +/// [the same][mut-cast] for `as *mut _`: those methods make the intent clearer. +/// +/// `as` is also used to rename imports in [`use`] and [`extern crate`][`crate`] statements: /// /// ``` /// # #[allow(unused_imports)] /// use std::{mem as memory, net as network}; /// // Now you can use the names `memory` and `network` to refer to `std::mem` and `std::net`. /// ``` -/// /// For more information on what `as` is capable of, see the [Reference]. /// /// [Reference]: ../reference/expressions/operator-expr.html#type-cast-expressions +/// [`crate`]: keyword.crate.html /// [`use`]: keyword.use.html -/// [`extern crate`]: keyword.crate.html +/// [const-cast]: primitive.pointer.html#method.cast +/// [mut-cast]: primitive.pointer.html#method.cast-1 mod as_keyword {} #[doc(keyword = "break")]