Add generic conversion traits #23538

aturon · Mar 20, 2015

This commit:

Introduces std::convert, providing an implementation of
RFC 529.
Deprecates the AsPath, AsOsStr, and IntoBytes traits, all
in favor of the corresponding generic conversion traits.

Consequently, various IO APIs now take As<Path> rather than
AsPath, and so on. Since the types provided by std implement both
traits, this should cause relatively little breakage.
Deprecates many from_foo constructors in favor of from.
Changes PathBuf::new to take no argument (creating an empty buffer,
as per convention). The previous behavior is now available as
PathBuf::from.
De-stabilizes IntoCow. It's not clear whether we need this separate trait.

Closes #14433
Closes #22751

[breaking-change]

r? @alexcrichton
cc @Gankro @erickt @cmr

aturon · Mar 20, 2015

PRELIMINARY PR: Do not merge until the RFC is approved.

aturon · Mar 20, 2015

cc @sfackler @wycats

aturon · Mar 20, 2015

Per IRC: @alexcrichton is very uncomfortable with these living in libcollections rather than libcore. This placement was to facilitate impls like impl<T: Clone> To<Vec<T>> for [T] which would otherwise be disallowed due to coherence.

aturon · Mar 20, 2015

cc @reem

aturon · Mar 20, 2015

Per IRC: @reem points out that the same coherence problems make it much more difficult to define "implicit conversions" via generics targetting user-land types: you cannot implement To<MyType<T>> for StdType<T> due to coherence.

Introducing an ad hoc conversion trait, on the other hand, makes it possible to do this in userland.

aturon · Mar 20, 2015

Updated: I've now moved the traits into libcore, and have a (rather sad) impl of To<Vec<u8>> for [u8] which coherence permits.

bors · Mar 20, 2015

☔️ The latest upstream changes (presumably #23548) made this pull request unmergeable. Please resolve the merge conflicts.

aturon · Mar 20, 2015

I believe this is now ready to go, once the RFC itself has been merged.

alexcrichton · Mar 20, 2015

alexcrichton reviewed Mar 20, 2015

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alexcrichton · Mar 20, 2015

src/libcollections/borrow.rs

@@ -14,6 +14,7 @@

 use core::clone::Clone;
 use core::cmp::{Eq, Ord, Ordering, PartialEq, PartialOrd};
+use core::convert::As;


Could these traits also go into the libcore prelude? I think files like this can have use core::prelude::* at the top as well (slowly been migrating over time).

alexcrichton · Mar 20, 2015

alexcrichton reviewed Mar 20, 2015

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alexcrichton · Mar 20, 2015

src/libcore/convert.rs

+/// A cheap, reference-to-reference conversion.
+pub trait As<T: ?Sized> {
+    /// Perform the conversion.
+    fn convert_as(&self) -> &T;


I do quite like your suggestions on the RFC of as_ref, as_mut, to, and into. The only one that's "wrong" is as_ref (e.g. it should be called as) but it seems worth it!

Wouldn't as_ref and as_mut conflict with several adhoc methods (thinking of RawPtr and Option off the top of my head)?

They would overlap, but inherent impls trump trait ones. (Still, the overlap is one reason we might not want to choose these names, or might consider renaming the others...)

alexcrichton · Mar 20, 2015

alexcrichton reviewed Mar 20, 2015

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alexcrichton · Mar 20, 2015

src/libcore/convert.rs

+    fn convert_as(&self) -> &[T] {
+        self.as_slice()
+    }
+}


I thought we've been pretty hesitant about this in the past? Slices from Option and Result I thought would continue to be unstable functionality.

I'm willing to mark this unstable, but personally I have no objection to this functionality, especially in the context of generic conversions.

My fear in marking these unstable is that there's no warning on usage of an unstable impl, only on an unstable trait, and it's not clear that we'll get it fixed for 1.0.

alexcrichton · Mar 20, 2015

alexcrichton reviewed Mar 20, 2015

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alexcrichton · Mar 20, 2015

src/libcore/option.rs

+
+    /// Convert from `Option<T>` to `&[T]` (without copying)
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]


I would personally prefer this continue to remain unstable

(same for Result)

ping, perhaps this could remain unstable?

alexcrichton · Mar 20, 2015

alexcrichton reviewed Mar 20, 2015

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alexcrichton · Mar 20, 2015

src/librustc/session/search_paths.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.

+use std::convert::From;


This should be in the prelude, right?

(same for a few files below)

It's in the prelude, yes; I probably added a few redundant imports mechanically.

alexcrichton · Mar 20, 2015

alexcrichton reviewed Mar 20, 2015

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alexcrichton · Mar 20, 2015

src/libstd/ffi/os_str.rs

+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl From<String> for OsString {


I wonder if these would be better expressed as Into? For example functions may want to take T: Into<OsString>.

alexcrichton · Mar 20, 2015

alexcrichton reviewed Mar 20, 2015

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alexcrichton · Mar 20, 2015

src/libstd/path.rs

@@ -999,16 +1036,58 @@ impl PathBuf {
 }

 #[stable(feature = "rust1", since = "1.0.0")]
-impl<P: AsPath> iter::FromIterator<P> for PathBuf {
+impl<'a> From<&'a str> for PathBuf {


Similarly to the OsString case, these may be better expressed via Into.

It's also a little sad that the same set of impls as From<T> for OsString needs to be listed here, but not necessarily the end of the world.

Yes, the inability to do blanket "transportation" of one conversion to another is a loss here.

Note that specialization may someday allow us to regain this via automatic reflexive and transitive blankets. It's not clear.

Yeah I'm willing to just understand that this is a fact of life :)

Gankro · Mar 20, 2015

Gankro reviewed Mar 20, 2015

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Gankro · Mar 20, 2015

src/compiletest/header.rs

      None => {
        if parse_name_directive(line, "pp-exact") {
-            testfile.file_name().map(|s| PathBuf::new(s))
+            testfile.file_name().map(|s| PathBuf::from(s))


This can just be map(PathBuf::from)

Gankro · Mar 20, 2015

Gankro reviewed Mar 20, 2015

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Gankro · Mar 20, 2015

src/libcollections/slice.rs

        let mut result = Vec::with_capacity(size + self.len());
        let mut first = true;
        for v in self {
            if first { first = false } else { result.push(sep.clone()) }
-            result.push_all(v.as_slice())
+            result.push_all(v.convert_as())


This is kind of an unfortunate readability loss.

Yeah, it's probably more clear to perform the conversion up-front and rebind the variable (perhaps with type annotation, even).

Note, though, that v.as_ref::<[_]>() would probably still be readable.

Alternatively if these traits were hooked into coercions you could just result.push_all(&v), which is less informative but somehow less cryptic than covert_as::<???>().

alexcrichton · Mar 20, 2015

cc #23567

aturon · Mar 23, 2015

Updated:

Removed To for the time being.
Renamed As to AsRef.
Renamed all methods to match their trait names.
All conversion functionality now in libcore; added to core::prelude
Added From => Into implementation, which makes it possible to add conversions in both directions without running afoul of coherence. For example, we now have From<[T]> for Vec<T> where T: Clone, which yields the corresponding Into going in the other direction -- despite the fact that the two types live in different crates.

I also believe this addresses a few concerns about things implementing From instead of Into
Removed AsRef impls for slices from Option/Result, which was causing a bad interaction with their as_ref methods. This functionality is available by calling as_slice directly.

All in all, I feel much better with the form this PR is taking now. It feels cleaner, leaner, more consistent and more extensible. Thanks for the feedback making this possible!

@alexcrichton: re-r?

alexcrichton · Mar 23, 2015

alexcrichton reviewed Mar 23, 2015

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alexcrichton · Mar 23, 2015

src/libstd/env.rs

@@ -67,8 +67,8 @@ pub fn current_dir() -> io::Result<PathBuf> {
 /// println!("Successfully changed working directory to {}!", root.display());
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
-pub fn set_current_dir<P: AsPath + ?Sized>(p: &P) -> io::Result<()> {
-    os_imp::chdir(p.as_path())
+pub fn set_current_dir<P: AsRef<path::Path> + ?Sized>(p: &P) -> io::Result<()> {


Could this module import Path directly and use AsRef<Path>?

Ahha! It can now. I think when I first wrote it Path was still in the prelude :)

alexcrichton · Mar 23, 2015

I, too, am quite happy with how this is turning out! r=me with the as_slice methods on Option and Result remaining #[unstable] (as they were before)

aturon · Mar 23, 2015

pings addressed :)

alexcrichton · Mar 23, 2015

@bors: r+ 8389253

aturon · Mar 23, 2015

@bors: r=alexcrichton 97c8e43

alexcrichton · Mar 24, 2015

Rolled into #23654

bors · Mar 24, 2015

☔️ The latest upstream changes (presumably #23654) made this pull request unmergeable. Please resolve the merge conflicts.

rust-highfive assigned alexcrichton Mar 20, 2015

aturon force-pushed the aturon:conversion branch from 9fcd79f to 844c115 Mar 20, 2015

aturon referenced this pull request Mar 20, 2015
Merged
RFC: Generic conversion traits #529

aturon force-pushed the aturon:conversion branch 6 times, most recently from c8671be to 3c62067 Mar 20, 2015

aturon force-pushed the aturon:conversion branch from 3c62067 to dbd9480 Mar 23, 2015

aturon changed the title ~~[WIP] Add generic conversion traits~~ Add generic conversion traits Mar 23, 2015

aturon force-pushed the aturon:conversion branch from dbd9480 to a686d27 Mar 23, 2015

aturon force-pushed the aturon:conversion branch from a686d27 to 8389253 Mar 23, 2015

aturon force-pushed the aturon:conversion branch from 8389253 to 97c8e43 Mar 23, 2015

alexcrichton added a commit to alexcrichton/rust that referenced this pull request Mar 23, 2015

rollup merge of rust-lang#23538: aturon/conversion
Conflicts: src/librustc_back/rpath.rs

fd13400

aturon force-pushed the aturon:conversion branch from 97c8e43 to 08ee682 Mar 23, 2015

alexcrichton closed this Mar 24, 2015

SimonSapin referenced this pull request Mar 25, 2015
Closed
Implement generic conversion traits #23567

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alexcrichton commented Mar 20, 2015

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