Implement `TryFrom<OsString>` for `String` by Kixunil · Pull Request #151822 · rust-lang/rust

Kixunil · 2026-01-29T13:46:35Z

This is draft because I was unable to compile locally, it will be probably broken for a few iterations.

Being able to generically convert strings can be beneficial for argument parsing code and similar situations especially in case of conditional conversions. The standard library already provided this converion, just not via a trait. This commit fills the gap by adding the impl.

This addition was approved in ACP 732. It was requested that FromUtf8Error should be made generic over the input and this commit obeys the request. However some challenges were encountered:

The fields were private and the type had no constructor - solved by making the fields public but unstable and #[doc(hidden)].
There is a method to perform lossy conversion and it looks like it should be provided for OsString as well - this is not yet resolved.
into_os_string method was requested but the types are in different crates - solved with #[rustc_allow_incoherent_impl].

Being able to generically convert strings can be beneficial for argument parsing code and similar situations especially in case of conditional conversions. The standard library already provided this converion, just not via a trait. This commit fills the gap by adding the impl. This addition was approved in [ACP 732]. It was requested that `FromUtf8Error` should be made generic over the input and this commit obeys the request. However some challenges were encountered: * The fields were private and the type had no constructor - solved by making the fields public but unstable and `#[doc(hidden)]`. * There is a method to perform lossy conversion and it looks like it should be provided for `OsString` as well - this is not yet resolved. * `into_os_string` method was requested but the types are in different crates - solved with `#[rustc_allow_incoherent_impl]`. [ACP 732]: rust-lang/libs-team#732

rustbot · 2026-01-29T18:37:05Z

r? @tgross35

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tgross35 · 2026-01-30T20:48:01Z

Kicking off a build for crater.

@bors try

Implement `TryFrom<OsString>` for `String`

tgross35 · 2026-01-30T20:50:28Z

Since you have already been thinking about the design in the ACP,

r? programmerjake

tgross35 · 2026-01-30T20:51:13Z

library/std/src/ffi/os_str.rs

+    /// Returns an [`OsStr`] slice that was attempted to convert to a `String`.
+    #[stable(feature = "tryfrom_os_string_for_string", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_allow_incoherent_impl]
+    pub fn as_os_str(&self) -> &OsStr {
+        &self.input[..]
+    }
+
+    /// Returns the [`OsString`] that was attempted to convert to a `String`.
+    ///
+    /// This method is carefully constructed to avoid allocation. It will
+    /// consume the error, moving out the string, so that a copy of the string
+    /// does not need to be made.
+    #[stable(feature = "tryfrom_os_string_for_string", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_allow_incoherent_impl]
+    pub fn into_os_string(self) -> OsString {
+        self.input
+    }


These can be added unstably (unlike traits). I think it will probably be fine to put up a stabilization PR shortly after this merges.

technically trait impls can be unstable too (afaik they actually work now), but idk if it's standard practice to use that yet.

🎉 that's news to me! I think we may as well try it

see https://rustc-dev-guide.rust-lang.org/stability.html#unstable_feature_bound for docs.
You'll need to create a tracking issue, and replace the below NNN with that issue number.

#[unstable(feature = "tryfrom_os_string_for_string", issue = "NNN")] #[unstable_feature_bound(tryfrom_os_string_for_string)] impl TryFrom<OsString> for String { ... }

also, you'll want to add #[must_use = ...] to those methods to match the methods on FromUtf8Error<Vec<u8>>

programmerjake · 2026-01-30T22:04:31Z

library/std/src/ffi/os_str.rs

+
+    /// Attempts to convert an [`OsString`] into a [`String`].
+    ///
+    /// This conversion does not allocate or copy memory.


It does technically copy memory, just not the contents of the OsString.

The docs should probably be based on the docs of OsString::into_string:

rust/library/std/src/ffi/os_str.rs

Lines 227 to 239 in 36e2b8a

/// Converts the `OsString` into a [`String`] if it contains valid Unicode data.

///

/// On failure, ownership of the original `OsString` is returned.

///

/// # Examples

///

/// ```

/// use std::ffi::OsString;

///

/// let os_string = OsString::from("foo");

/// let string = os_string.into_string();

/// assert_eq!(string, Ok(String::from("foo")));

/// ```

Right, I copied it from some other method that says the same thing in such scenario. I will look into it.

programmerjake · 2026-01-30T22:11:03Z

library/std/src/ffi/os_str.rs

+    /// Returns an [`OsStr`] slice that was attempted to convert to a `String`.
+    #[stable(feature = "tryfrom_os_string_for_string", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_allow_incoherent_impl]
+    pub fn as_os_str(&self) -> &OsStr {
+        &self.input[..]
+    }
+
+    /// Returns the [`OsString`] that was attempted to convert to a `String`.
+    ///
+    /// This method is carefully constructed to avoid allocation. It will
+    /// consume the error, moving out the string, so that a copy of the string
+    /// does not need to be made.
+    #[stable(feature = "tryfrom_os_string_for_string", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_allow_incoherent_impl]
+    pub fn into_os_string(self) -> OsString {
+        self.input
+    }


also, you'll want to add #[must_use = ...] to those methods to match the methods on FromUtf8Error<Vec<u8>>

programmerjake · 2026-01-30T22:15:16Z

So, as mentioned, please change the API to #[unstable(...)] and create a tracking issue.

@rustbot author

rustbot · 2026-01-30T22:15:20Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rust-bors · 2026-01-30T22:58:26Z

☀️ Try build successful (CI)
Build commit: e5b0618 (e5b0618d996a6a3941e423fd3e170737b8dbdc92, parent: 36e2b8a3a7aad93f8a92db6d254b746aa94ed6da)

tgross35 · 2026-01-30T23:18:16Z

@craterbot check

craterbot · 2026-01-30T23:18:45Z

👌 Experiment pr-151822 created and queued.
🤖 Automatically detected try build e5b0618
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-02-04T18:44:36Z

🚧 Experiment pr-151822 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-02-06T13:23:42Z

🎉 Experiment pr-151822 is completed!
📊 2 regressed and 3 fixed (800034 total)
📊 2203 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-151822/retry-regressed-list.txt ↩

programmerjake · 2026-02-06T16:20:39Z

looks like nothing significant changed in the crater run.

so I'm changing this back to waiting on the author: #151822 (comment)

@rustbot author

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jan 29, 2026

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Kixunil force-pushed the tryfrom-osstring-for-string branch from 2ff5e55 to 3a3227c Compare January 29, 2026 16:10

Kixunil marked this pull request as ready for review January 29, 2026 18:37

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 29, 2026

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rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jan 29, 2026

eliminmax mentioned this pull request Jan 30, 2026

ACP: add str::from_boxed_utf8 for API consistency rust-lang/libs-team#737

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rust-bors bot pushed a commit that referenced this pull request Jan 30, 2026

Auto merge of #151822 - Kixunil:tryfrom-osstring-for-string, r=<try>

e5b0618

Implement `TryFrom<OsString>` for `String`

rustbot assigned programmerjake and unassigned tgross35 Jan 30, 2026

tgross35 added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. labels Jan 30, 2026

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craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 30, 2026

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Feb 6, 2026

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 6, 2026

joshtriplett mentioned this pull request Feb 10, 2026

ACP: impl TryFrom<{OsString, &OsStr}> for String rust-lang/libs-team#732

Closed

	/// Converts the `OsString` into a [`String`] if it contains valid Unicode data.
	///
	/// On failure, ownership of the original `OsString` is returned.
	///
	/// # Examples
	///
	/// ```
	/// use std::ffi::OsString;
	///
	/// let os_string = OsString::from("foo");
	/// let string = os_string.into_string();
	/// assert_eq!(string, Ok(String::from("foo")));
	/// ```

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