Implement std::convert traits for char by SimonSapin · Pull Request #35755 · rust-lang/rust

SimonSapin · 2016-08-17T17:13:57Z

This is motivated by avoiding the as operator, which sometimes silently truncates, and instead use conversions that are explicitly lossless and infallible.

I’m less certain that From<u8> for char should be implemented: while it matches an existing behavior of as, it’s not necessarily the right thing to use for non-ASCII bytes. It effectively decodes bytes as ISO/IEC 8859-1 (since Unicode designed its first 256 code points to be compatible with that encoding), but that is not apparent in the API name.

rust-highfive · 2016-08-17T17:14:05Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

sfackler · 2016-08-17T19:40:31Z

We're never going to need to add any extra cases to this enum, right? Should we stick a __ForExtensibility variant just in case?

sfackler · 2016-08-17T19:42:12Z

I personally feel okay about the u8 -> char impl as it seems like what everyone would expect to happen, but am interested to see what other people think.

cc @rust-lang/libs

alexcrichton · 2016-08-18T02:38:28Z

Seems reasonable to me, but I'd prefer to use an opaque struct with optional method accessors rather than an enum for the error type in TryFrom

SimonSapin · 2016-08-18T06:06:05Z

@sfackler Re extensibility, I don’t expect this to ever be needed. The range of Unicode Scalar Values changed exactly once in the history of Unicode. At first it was “16 bits ought to be enough for everybody” 0x0000...0xFFFF. When that turned out not to be enough and a lot of systems were already using u16 as a code unit, UTF-16 was introduced. The current range, 0x0000...0xD7FF | 0xE000...0x10FFFF, is exactly what can be encoded by UTF-16. (UTF-8’s original design can support up to 0x7FFF_FFFF with 6-bytes sequences. It is now artificially restricted to match.)

In Unicode 9.0, 76% of the million and some code points are unassigned, so they’re not expected to run out. And breaking compatibility with UTF-16 is such a breaking change that I imagine it’s not even considered.

And this concern disappears with…

@alexcrichton Yeah, I also considered an opaque struct. Even without accessor method since I can’t think of a use case for it. (Code like a WTF-8 implementation that wants to deal with surrogate code points will likely do its own code point arithmetic anyway.) And a method cal always be added later. I’ve updated the PR.

alexcrichton · 2016-08-23T05:48:10Z

Discussed during @rust-lang/libs triage today, conclusion was to merge. Thanks for the update @SimonSapin!

@bors: r+

bors · 2016-08-23T05:48:11Z

📌 Commit 82678c5 has been approved by alexcrichton

…hton Implement std::convert traits for char This is motivated by avoiding the `as` operator, which sometimes silently truncates, and instead use conversions that are explicitly lossless and infallible. I’m less certain that `From<u8> for char` should be implemented: while it matches an existing behavior of `as`, it’s not necessarily the right thing to use for non-ASCII bytes. It effectively decodes bytes as ISO/IEC 8859-1 (since Unicode designed its first 256 code points to be compatible with that encoding), but that is not apparent in the API name.

bors · 2016-08-23T18:18:42Z

☔ The latest upstream changes (presumably #35656) made this pull request unmergeable. Please resolve the merge conflicts.

ollie27 · 2016-08-27T15:40:26Z

 transmute(i)
 }

+#[stable(feature = "char_convert", since = "1.12.0")]


Should these not be "1.13.0"?

Not at the time I first opened this PR, but now yes. Fixed.

bors · 2016-08-29T08:56:44Z

🔒 Merge conflict

These fit with other From implementations between integer types. This helps the coding style of avoiding the 'as' operator that sometimes silently truncates, and signals that these specific conversions are lossless and infaillible.

For symmetry with From<char> for u32.

alexcrichton · 2016-08-29T17:18:35Z

@bors: r+ f040208

bors · 2016-09-01T09:53:29Z

⌛ Testing commit f040208 with merge b2799a5...

Implement std::convert traits for char This is motivated by avoiding the `as` operator, which sometimes silently truncates, and instead use conversions that are explicitly lossless and infallible. I’m less certain that `From<u8> for char` should be implemented: while it matches an existing behavior of `as`, it’s not necessarily the right thing to use for non-ASCII bytes. It effectively decodes bytes as ISO/IEC 8859-1 (since Unicode designed its first 256 code points to be compatible with that encoding), but that is not apparent in the API name.

bors · 2016-09-01T13:05:02Z

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: alexcrichton
Pushing b2799a5 to master...

bluss · 2016-09-08T13:13:31Z

Mini-reminder: let's tag more user-visible stuff with relnotes

rust-highfive assigned aturon Aug 17, 2016

sfackler added the T-libs-api [DEPRECATED; DO NOT USE] label Aug 17, 2016

sfackler reviewed Aug 17, 2016
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SimonSapin force-pushed the char_convert branch from 249b789 to 82678c5Compare August 18, 2016 06:05

eddyb mentioned this pull request Aug 23, 2016
Rollup of 15 pull requests #35944
Closed

ollie27 reviewed Aug 27, 2016
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SimonSapin added 2 commits August 29, 2016 17:34

Implement From<char> for u32, and From<u8> for char
41d0a89
These fit with other From implementations between integer types. This helps the coding style of avoiding the 'as' operator that sometimes silently truncates, and signals that these specific conversions are lossless and infaillible.

Implement TryFrom<u32> for char
f040208
For symmetry with From<char> for u32.

SimonSapin force-pushed the char_convert branch from 82678c5 to f040208Compare August 29, 2016 15:34

bors merged commit f040208 into rust-lang:masterSep 1, 2016

bluss added the relnotes Marks issues that should be documented in the release notes of the next release. label Sep 8, 2016

SimonSapin deleted the char_convert branch September 15, 2016 07:06

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