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| -rw-r--r-- | library/core/src/convert/mod.rs | 40 |
1 files changed, 40 insertions, 0 deletions
diff --git a/library/core/src/convert/mod.rs b/library/core/src/convert/mod.rs index 9e4fdcda06e..fc8d19d1a58 100644 --- a/library/core/src/convert/mod.rs +++ b/library/core/src/convert/mod.rs @@ -479,6 +479,46 @@ pub trait Into<T>: Sized { /// - `From<T> for U` implies [`Into`]`<U> for T` /// - `From` is reflexive, which means that `From<T> for T` is implemented /// +/// # When to implement `From` +/// +/// While there's no technical restrictions on which conversions can be done using +/// a `From` implementation, the general expectation is that the conversions +/// should typically be restricted as follows: +/// +/// * The conversion is *infallible*: if the conversion can fail, use [`TryFrom`] +/// instead; don't provide a `From` impl that panics. +/// +/// * The conversion is *lossless*: semantically, it should not lose or discard +/// information. For example, `i32: From<u16>` exists, where the original +/// value can be recovered using `u16: TryFrom<i32>`. And `String: From<&str>` +/// exists, where you can get something equivalent to the original value via +/// `Deref`. But `From` cannot be used to convert from `u32` to `u16`, since +/// that cannot succeed in a lossless way. (There's some wiggle room here for +/// information not considered semantically relevant. For example, +/// `Box<[T]>: From<Vec<T>>` exists even though it might not preserve capacity, +/// like how two vectors can be equal despite differing capacities.) +/// +/// * The conversion is *value-preserving*: the conceptual kind and meaning of +/// the resulting value is the same, even though the Rust type and technical +/// representation might be different. For example `-1_i8 as u8` is *lossless*, +/// since `as` casting back can recover the original value, but that conversion +/// is *not* available via `From` because `-1` and `255` are different conceptual +/// values (despite being identical bit patterns technically). But +/// `f32: From<i16>` *is* available because `1_i16` and `1.0_f32` are conceptually +/// the same real number (despite having very different bit patterns technically). +/// `String: From<char>` is available because they're both *text*, but +/// `String: From<u32>` is *not* available, since `1` (a number) and `"1"` +/// (text) are too different. (Converting values to text is instead covered +/// by the [`Display`](crate::fmt::Display) trait.) +/// +/// * The conversion is *obvious*: it's the only reasonable conversion between +/// the two types. Otherwise it's better to have it be a named method or +/// constructor, like how [`str::as_bytes`] is a method and how integers have +/// methods like [`u32::from_ne_bytes`], [`u32::from_le_bytes`], and +/// [`u32::from_be_bytes`], none of which are `From` implementations. Whereas +/// there's only one reasonable way to wrap an [`Ipv6Addr`](crate::net::Ipv6Addr) +/// into an [`IpAddr`](crate::net::IpAddr), thus `IpAddr: From<Ipv6Addr>` exists. +/// /// # Examples /// /// [`String`] implements `From<&str>`: |