implement `replace_with` and `replacen_with` for unstable feature `replace_with` - #147741

alpaylan · 2025-10-15T20:22:59Z

This is the PR for the tracking issue #147731 for implementation of replace_with.

rustbot · 2025-10-15T20:23:04Z

rustbot has assigned @joboet.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Kivooeo · 2025-10-16T07:14:14Z

+        if let Some(from_byte) = match from.as_utf8_pattern() {
+            Some(Utf8Pattern::StringPattern([from_byte])) => Some(*from_byte),
+            Some(Utf8Pattern::CharPattern(c)) => c.as_ascii().map(|ascii_char| ascii_char.to_u8()),
+            _ => None,
+        } {
+            if let Some(sref) = from_byte.as_ascii().map(|ascii_char| f(ascii_char.as_str())) {
+                if let [to_byte] = sref.as_ref().as_bytes() {
+                    return unsafe { replace_ascii(self.as_bytes(), from_byte, *to_byte) };
+                }
+            }
+        }


Would it be possible to use let chains here?

Updated with an if-let chain, did you have something else in mind or is this fine? (f70b5e1)

Kivooeo · 2025-10-16T12:58:11Z

Hm, that's weird because alloc is 2024 edition

rust/library/alloc/Cargo.toml

Line 11 in f524236

edition = "2024"

Kivooeo · 2025-10-16T13:03:13Z

It does compile with edition 2021 anyway tho

Command { cmd: ... "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/alloc-no-oom-handling/rmake_out" 


"--edition" "2021" 


"-Dwarnings" ... }, already_executed: true }

Well, I guess that's fine to revert this then :)

If this ever will get 2024 edition I will cleanup this accordingly

joboet · 2025-10-21T08:32:17Z

+        from: P,
+        mut f: impl FnMut(&str) -> S,
+    ) -> String {
+        // Fast path for replacing a single ASCII character with another.


f might not be side-effect-free, so this optimisation is incorrect.

Ah, that is correct. I'm assuming I should just remove it, or are there any other potential optimization?

SpriteOvO · 2026-05-03T16:43:45Z

Hi, ping from triage team. This PR has been inactive for a while. Are there any updates on this? I assume this PR should now be waiting for review?

@rustbot ready

joboet

Just a few nits I wanted to pick...

View changes since this review

joboet · 2026-05-07T09:47:07Z

        result
    }

+    /// Replaces first N matches of a pattern with another string.


Suggested change

/// Replaces first N matches of a pattern with another string.

/// Replaces the first N matches of a pattern with another string.

This is also an issue that's similarly present in replacen (https://doc.rust-lang.org/src/alloc/str.rs.html#296)
I assume I should fix both.

joboet · 2026-05-07T09:47:40Z


+    /// Replaces first N matches of a pattern with another string.
+    ///
+    /// `replacen` creates a new [`String`], and copies the data from this string slice into it.


This paragraph still needs updating.

joboet · 2026-05-07T09:49:50Z

+        // Hope to reduce the times of re-allocation
+        let mut result = String::with_capacity(32);


This is a bit inconsistent, replace_with doesn't do this preallocation. I'd do it in replace_with, too.

This is again replicating a pattern in replacen. I can do it of course but I had assumed there was a concrete reasoning behind it based on the usage pattern of replace vs replacen

rustbot · 2026-05-07T09:50:36Z

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

…place_with`

rustbot · 2026-06-01T23:27:12Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 15, 2025

rustbot assigned joboet Oct 15, 2025

rustbot added the T-libs Relevant to the library team, which will review and decide on the PR/issue. label Oct 15, 2025

alpaylan mentioned this pull request Oct 15, 2025

Tracking Issue for str::replace_with #147731

Open

4 tasks

Kivooeo reviewed Oct 16, 2025

View reviewed changes

This comment has been minimized.

Sign in to view

alpaylan force-pushed the master branch from f70b5e1 to 4ff1245 Compare October 16, 2025 13:10

joboet requested changes Oct 21, 2025

View reviewed changes

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 Oct 21, 2025

lygstate reviewed Dec 20, 2025

View reviewed changes

Comment thread library/alloc/src/str.rs

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 3, 2026

joboet requested changes May 7, 2026

View reviewed changes

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 May 7, 2026

This comment has been minimized.

Sign in to view

alpaylan added 3 commits June 1, 2026 19:22

implement replace_with and replacen_with for unstable feature `re…

08dd3a3

…place_with`

remove the incorrect fast path implementation

eb99813

fix grammar: change 'Replaces first' to 'Replaces the first'

585e45a

alpaylan force-pushed the master branch from 68b98a8 to 585e45a Compare June 1, 2026 23:27

	/// Replaces first N matches of a pattern with another string.
	/// Replaces the first N matches of a pattern with another string.

		// Hope to reduce the times of re-allocation
		let mut result = String::with_capacity(32);

Uh oh!

Uh oh!

Conversation

alpaylan commented Oct 15, 2025

Uh oh!

rustbot commented Oct 15, 2025

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

This comment has been minimized.

Kivooeo commented Oct 16, 2025

Uh oh!

Kivooeo commented Oct 16, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

SpriteOvO commented May 3, 2026

Uh oh!

joboet left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

rustbot commented May 7, 2026

Uh oh!

This comment has been minimized.

rustbot commented Jun 1, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

7 participants

Kivooeo commented Oct 16, 2025 •

edited

Loading

joboet left a comment •

edited by rustbot

Loading