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Rollup merge of rust-lang#62528 - SimonSapin:concat, r=alexcrichton
Add joining slices of slices with a slice separator, not just a single item
rust-lang#27747 (comment)
> It's kinda annoying to be able to join strings with a str (which can have multiple chars), but joining a slice of slices, you can only join with a single element.
This turns out to be fixable, with some possible inference regressions.
# TL;DR
Related trait(s) are unstable and tracked at rust-lang#27747, but the `[T]::join` method that is being extended here is already stable.
Example use of the new insta-stable functionality:
```rust
let nested: Vec<Vec<Foo>> = /* … */;
let separator: &[Foo] = /* … */; // Previously: could only be a single &Foo
nested.join(separator)
```
Complete API affected by this PR, after changes:
```rust
impl<T> [T] {
pub fn concat<Item: ?Sized>(&self) -> <Self as Concat<Item>>::Output
where Self: Concat<Item>
{
Concat::concat(self)
}
pub fn join<Separator>(&self, sep: Separator) -> <Self as Join<Separator>>::Output
where Self: Join<Separator>
{
Join::join(self, sep)
}
}
// The `Item` parameter is only useful for the the slice-of-slices impl.
pub trait Concat<Item: ?Sized> {
type Output;
fn concat(slice: &Self) -> Self::Output;
}
pub trait Join<Separator> {
type Output;
fn join(slice: &Self, sep: Separator) -> Self::Output;
}
impl<T: Clone, V: Borrow<[T]>> Concat<T> for [V] {
type Output = Vec<T>;
}
impl<T: Clone, V: Borrow<[T]>> Join<&'_ T> for [V] {
type Output = Vec<T>;
}
// New functionality here!
impl<T: Clone, V: Borrow<[T]>> Join<&'_ [T]> for [V] {
type Output = Vec<T>;
}
impl<S: Borrow<str>> Concat<str> for [S] {
type Output = String;
}
impl<S: Borrow<str>> Join<&'_ str> for [S] {
type Output = String;
}
```
# Details
After rust-lang#62403 but before this PR, the API is:
```rust
impl<T> [T] {
pub fn concat<Separator: ?Sized>(&self) -> T::Output
where T: SliceConcat<Separator>
{
SliceConcat::concat(self)
}
pub fn join<Separator: ?Sized>(&self, sep: &Separator) -> T::Output
where T: SliceConcat<Separator>
{
SliceConcat::join(self, sep)
}
}
pub trait SliceConcat<Separator: ?Sized>: Sized {
type Output;
fn concat(slice: &[Self]) -> Self::Output;
fn join(slice: &[Self], sep: &Separator) -> Self::Output;
}
impl<T: Clone, V: Borrow<[T]>> SliceConcat<T> for V {
type Output = Vec<T>;
}
impl<S: Borrow<str>> SliceConcat<str> for S {
type Output = String;
}
```
By adding a trait impl we should be able to accept a slice of `T` as the separator, as an alternative to a single `T` value.
In a `some_slice.join(some_separator)` call, trait resolution will pick an impl or the other based on the type of `some_separator`. In `some_slice.concat()` however there is no separator, so this call would become ambiguous. Some regression in type inference or trait resolution may be acceptable on principle, but requiring a turbofish for every single call to `concat` isn’t great.
The solution to that is splitting the `SliceConcat` trait into two `Concat` and `Join` traits, one for each eponymous method. Only `Join` would gain a new impl, so that `some_slice.concat()` would not become ambiguous.
Now, at the trait level the `Concat` trait does not need a `Separator` parameter anymore. However, simply removing it causes one of the impls not to be accepted anymore:
```rust
error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
--> src/liballoc/slice.rs:608:6
|
608 | impl<T: Clone, V: Borrow<[T]>> Concat for [V] {
| ^ unconstrained type parameter
```
This makes sense: if `[V]::concat` is a method that is itself not generic, then its return type (which is the `Concat::Output` associated type) needs to be determined based on solely `V`. And although there is no such type in the standard library, there is nothing stopping another crate from defining a `V` type that implements both `Borrow<[Foo]>` and `Borrow<[Bar]>`. It might not be a good idea, but it’s possible. Both would apply here, and there would be no way to determine `T`.
This could be a warning sign that this API is too generic. Perhaps we’d be better off having one less type variable, and only implement `Concat for [&'_ [T]]` and `Concat for [Vec<T>]` etc. However this aspect of `[V]::concat` is already stable, so we’re stuck with it.
The solution is to keep a dummy type parameter on the `Concat` trait. That way, if a type has multiple `Borrow<[_]>` impls, it’ll end up with multiple corresponding `Concat<_>` impls.
In `impl<S: Borrow<str>> Concat<str> for [S]`, the second occurrence of `str` is not meaningful. It could be any type. As long as there is only once such type with an applicable impl, trait resolution will be appeased without demanding turbofishes.
# Joining strings with `char`
For symmetry I also tried adding this impl (because why not):
```rust
impl<S: Borrow<str>> Join<char> for [S] {
type Output = String;
}
```
This immediately caused an inference regression in a dependency of rustc:
```rust
error[E0277]: the trait bound `std::string::String: std::borrow::Borrow<[std::string::String]>` is not satisfied
--> /home/simon/.cargo/registry/src/github.com-1ecc6299db9ec823/getopts-0.2.19/src/lib.rs:595:37
|
595 | row.push_str(&desc_rows.join(&desc_sep));
| ^^^^ the trait `std::borrow::Borrow<[std::string::String]>` is not implemented for `std::string::String`
|
= help: the following implementations were found:
<std::string::String as std::borrow::Borrow<str>>
= note: required because of the requirements on the impl of `std::slice::Join<&std::string::String>` for `[std::string::String]`
```
In the context of this code, two facts are known:
* `desc_rows` is a `Vec<String>`
* `desc_sep` is a `String`
Previously the first fact alone reduces the resolution of `join` to only one solution, where its argument it expected to be `&str`. Then, `&String` is coerced to `&str`.
With the new `Join` impl, the first fact leavs two applicable impls where the separator can be either `&str` or `char`. But `&String` is neither of these things. It appears that possible coercions are not accounted for, in the search for a solution in trait resolution.
I have not included this new impl in this PR. It’s still possible to add later, but the `getopts` breakage does not need to block the rest of the PR. And the functionality easy for end-user to duplicate: `slice_of_strings.join(&*char_separator.encode_utf8(&mut [0_u8, 4]))`
The `&*` part of that last code snippet is another case of the same issue: `encode_utf8` returns `&mut str` which can be coerced to `&str`, but isn’t when trait resolution is ambiguous.
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