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Rollup merge of #73421 - janikrabe:master, r=joshtriplett

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Clarify effect of orphan rule changes on From/Into

Updated documentation for `std::convert` and `std::convert::From` to reflect changes to orphan rule in Rust 1.41. It should no longer be necessary to implement `Into` directly, unless targeting an older version.

r? @steveklabnik
This commit is contained in:
Manish Goregaokar 2020-07-16 00:01:02 -07:00 committed by GitHub
commit 31121cbec3
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1 changed files with 7 additions and 4 deletions
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src/libcore/convert/mod.rs
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@ -18,8 +18,9 @@
//! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],
//! as [`From`] and [`TryFrom`] provide greater flexibility and offer
//! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
//! blanket implementation in the standard library. Only implement [`Into`] or [`TryInto`]
//! when a conversion to a type outside the current crate is required.
//! blanket implementation in the standard library. When targeting a version prior to Rust 1.41, it
//! may be necessary to implement [`Into`] or [`TryInto`] directly when converting to a type
//! outside the current crate.
//!
//! # Generic Implementations
//!
@ -298,8 +299,10 @@ pub trait Into<T>: Sized {
/// because implementing `From` automatically provides one with an implementation of [`Into`]
/// thanks to the blanket implementation in the standard library.
///
/// Only implement [`Into`] if a conversion to a type outside the current crate is required.
/// `From` cannot do these type of conversions because of Rust's orphaning rules.
/// Only implement [`Into`] when targeting a version prior to Rust 1.41 and converting to a type
/// outside the current crate.
/// `From` was not able to do these types of conversions in earlier versions because of Rust's
/// orphaning rules.
/// See [`Into`] for more details.
///
/// Prefer using [`Into`] over using `From` when specifying trait bounds on a generic function.