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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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