impl AsRef<Path> for Cow<'_, str> #73390
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There is already `impl AsRef<OsStr> for str` and `impl<'_> AsRef<Path> for Cow<'_, OsStr>` but one still need to do `s.as_ref()` for functions that takes in `AsRef<Path>` for `Cow<'_ str>` such as `Path::join()`.
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dtolnay
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Cow<str> is a quite widely used type with currently one unique AsRef impl, so I think it would be too disruptive to do this. It breaks the following code:
use std::borrow::Cow;
fn main() {
let s = Cow::Borrowed("");
let _ = s.as_ref();
}error[E0283]: type annotations needed
--> src/main.rs:5:15
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5 | let _ = s.as_ref();
| ^^^^^^ cannot infer type for enum `std::borrow::Cow<'_, str>`
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= note: cannot satisfy `std::borrow::Cow<'_, str>: std::convert::AsRef<_>`
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Thanks anyway! |
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But isn't that the case only when they use I believe the use of |
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See the CI failures reported by rust-highfive. This breaks 2 places just in rustc_session so I believe it would break a significant fraction of crates.io where similar code is found. error[E0283]: type annotations needed
--> src/librustc_session/filesearch.rs:93:42
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93 | p.push(find_libdir(self.sysroot).as_ref());
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= note: cannot satisfy `std::borrow::Cow<'_, str>: std::convert::AsRef<_>`
error[E0283]: type annotations needed
--> src/librustc_session/filesearch.rs:102:52
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102 | let mut p = PathBuf::from(find_libdir(sysroot).as_ref());
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= note: cannot satisfy `std::borrow::Cow<'_, str>: std::convert::AsRef<_>` |
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The underlying reason for this problem is rooted deeper (in how |
matthiaskrgr
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Oct 3, 2022
…riplett docs: Improve AsRef / AsMut docs on blanket impls There are several issues with the current state of `AsRef` and `AsMut` as [discussed here on IRLO](https://internals.rust-lang.org/t/semantics-of-asref/17016). See also rust-lang#39397, rust-lang#45742, rust-lang#73390, rust-lang#98905, and the FIXMEs [here](https://github.com/rust-lang/rust/blob/1.62.0/library/core/src/convert/mod.rs#L509-L515) and [here](https://github.com/rust-lang/rust/blob/1.62.0/library/core/src/convert/mod.rs#L530-L536). These issues are difficult to fix. This PR aims to update the documentation to better reflect the status-quo and to give advice on how `AsRef` and `AsMut` should be used. In particular: - Explicitly mention that `AsRef` and `AsMut` do not auto-dereference generally for all dereferencable types (but only if inner type is a shared and/or mutable reference) - Give advice to not use `AsRef` or `AsMut` for the sole purpose of dereferencing - Suggest providing a transitive `AsRef` or `AsMut` implementation for types which implement `Deref` - Add new section "Reflexivity" in documentation comments for `AsRef` and `AsMut` - Provide better example for `AsMut` - Added heading "Relation to `Borrow`" in `AsRef`'s docs to improve structure
matthiaskrgr
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Oct 3, 2022
…riplett docs: Improve AsRef / AsMut docs on blanket impls There are several issues with the current state of `AsRef` and `AsMut` as [discussed here on IRLO](https://internals.rust-lang.org/t/semantics-of-asref/17016). See also rust-lang#39397, rust-lang#45742, rust-lang#73390, rust-lang#98905, and the FIXMEs [here](https://github.com/rust-lang/rust/blob/1.62.0/library/core/src/convert/mod.rs#L509-L515) and [here](https://github.com/rust-lang/rust/blob/1.62.0/library/core/src/convert/mod.rs#L530-L536). These issues are difficult to fix. This PR aims to update the documentation to better reflect the status-quo and to give advice on how `AsRef` and `AsMut` should be used. In particular: - Explicitly mention that `AsRef` and `AsMut` do not auto-dereference generally for all dereferencable types (but only if inner type is a shared and/or mutable reference) - Give advice to not use `AsRef` or `AsMut` for the sole purpose of dereferencing - Suggest providing a transitive `AsRef` or `AsMut` implementation for types which implement `Deref` - Add new section "Reflexivity" in documentation comments for `AsRef` and `AsMut` - Provide better example for `AsMut` - Added heading "Relation to `Borrow`" in `AsRef`'s docs to improve structure
matthiaskrgr
added a commit
to matthiaskrgr/rust
that referenced
this pull request
Oct 3, 2022
…riplett docs: Improve AsRef / AsMut docs on blanket impls There are several issues with the current state of `AsRef` and `AsMut` as [discussed here on IRLO](https://internals.rust-lang.org/t/semantics-of-asref/17016). See also rust-lang#39397, rust-lang#45742, rust-lang#73390, rust-lang#98905, and the FIXMEs [here](https://github.com/rust-lang/rust/blob/1.62.0/library/core/src/convert/mod.rs#L509-L515) and [here](https://github.com/rust-lang/rust/blob/1.62.0/library/core/src/convert/mod.rs#L530-L536). These issues are difficult to fix. This PR aims to update the documentation to better reflect the status-quo and to give advice on how `AsRef` and `AsMut` should be used. In particular: - Explicitly mention that `AsRef` and `AsMut` do not auto-dereference generally for all dereferencable types (but only if inner type is a shared and/or mutable reference) - Give advice to not use `AsRef` or `AsMut` for the sole purpose of dereferencing - Suggest providing a transitive `AsRef` or `AsMut` implementation for types which implement `Deref` - Add new section "Reflexivity" in documentation comments for `AsRef` and `AsMut` - Provide better example for `AsMut` - Added heading "Relation to `Borrow`" in `AsRef`'s docs to improve structure
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Maybe this should be reconsidered in some way, re "breakage of time crate" due to change in 1.80 recently: https://internals.rust-lang.org/t/type-inference-breakage-in-1-80-has-not-been-handled-well/21374/6 |
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There is already
impl AsRef<OsStr> for strandimpl<'_> AsRef<Path> for Cow<'_, OsStr>but one still need to dos.as_ref()for functions that takes inAsRef<Path>forCow<'_ str>such as
Path::join().