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Stabilize debug_closure_helpers #146099

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@coolreader18 coolreader18 commented Sep 1, 2025 •
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Resolves #117729. The tracking issue still needs an FCP, but I'm hoping that creating a stabilization PR will prompt one.

r? libs-api

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. labels Sep 1, 2025
@coolreader18 coolreader18 changed the title Stabilize debug closure helpers Stabilize debug_closure_helpers Sep 1, 2025
@tgross35 tgross35 added the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Sep 2, 2025
@kornelski kornelski mentioned this pull request Sep 6, 2025
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@Amanieu Amanieu added I-libs-api-nominated Nominated for discussion during a libs-api team meeting. and removed I-libs-api-nominated Nominated for discussion during a libs-api team meeting. labels Sep 23, 2025
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bors commented Sep 27, 2025

☔ The latest upstream changes (presumably #146636) made this pull request unmergeable. Please resolve the merge conflicts.

@Amanieu Amanieu removed the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Sep 30, 2025
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coolreader18 commented Oct 20, 2025

Once the FCP is complete, I think it might make sense to merge #145915 first and then merge this on top of that.

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hanna-kruppe reviewed Oct 20, 2025
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library/core/src/fmt/builders.rs
Comment on lines 141 to 143
pub fn field_with<F>(&mut self, name: &str, value_fmt: F) -> &mut Self
where
F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
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@hanna-kruppe hanna-kruppe Oct 20, 2025 •
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Having the closure as a generic parameter (and not type-erasing it internally) leads to a lot of code bloat compared to the corresponding stable APIs that take &dyn Debug as value to format. According to cargo +nightly llvm-lines a simple program formatting a struct with two fields generates 694 lines of LLVM IR while the same program using field() generates 42 lines. So at least with the current implementation, these functions have an annoying downside compared to e.g. .field("foo", &fmt::from_fn(|f| ...)

I think this can be addressed after stabilization, without changing the signatures. But I wanted to flag it so the reviewer can think about it as well. It's not entirely obvious to me for the helpers that deal in FnOnce. I'm aware of one workaround (put the closure into an Option, then pass a &dyn FnMut that unwraps this option to call the original impl FnOnce) but it's not quite zero cost in several dimensions.

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@jmillikin jmillikin Oct 21, 2025 •
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Could this be solved by putting the main logic back into field(), and having field_with be an #[inline] wrapper around it?

I don't want to send in any PRs that might cause Git conflicts with the stabilization, but if the public API is good then I'd be happy to experiment with internal reorganization / optimization after both have landed.

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field takes a &dyn Debug. How do you invoke a FnOnce or FnMut from the &self argument of Debug::fmt?

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Cell<Option<F>> , then .replace(None).unwrap()? It doesn't need to survive more than one call.

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I think if field_with were changed to use dyn, it'd be important to ensure that it doesn't cause undue stack usage, since that was brought up as an issue with the existing functions in #117729 (comment)

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That report is in the context of a total stack size of 4 KiB. The DebugStruct::field function in my nightly toolchain's libcore.rlib allocates a very reasonable amount of stack space (add $0x48,%rsp). It then goes on to call other functions, but those seem to be functions shared with most of the formatting infrastructure. It's frankly a miracle that any variation of the code fits within a 4 KiB stack, and the cases that work probably depends on the code being duplicated and specialized a bit for the particular usage, which is fundamentally at odds with optimizing for code size (as core::fmt generally does). So I wouldn't give much weight to that report.

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jmillikin commented Nov 4, 2025

The stabilization PR for fmt::from_fn has merged, so this one is now ready to rebase.

gentle ping @the8472 for review

@coolreader18 coolreader18 force-pushed the stabilize-debug_closure_helpers branch from a51fcbe to b1bde94 Compare November 4, 2025 18:52
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bors commented Nov 6, 2025

☔ The latest upstream changes (presumably #148544) made this pull request unmergeable. Please resolve the merge conflicts.

@coolreader18 coolreader18 force-pushed the stabilize-debug_closure_helpers branch from b1bde94 to 1e39a61 Compare November 6, 2025 02:49
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rustbot commented Nov 6, 2025

This PR was rebased onto a different master 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.

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rust-log-analyzer commented Nov 6, 2025

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot) 
[WARNING] `tests/rustdoc-gui/search-result-display.goml` line 39: Delta is 0 for "x", maybe try to use `compare-elements-position` instead?

======== tests/rustdoc-gui/source-code-wrapping.goml ========

[ERROR] `tests/rustdoc-gui/source-code-wrapping.goml` line 30: Error: Navigating frame was detached: for command `go-to: "file://" + |DOC_PATH| + "/test_docs/trait_bounds/index.html"`
`tests/rustdoc-gui/source-code-wrapping.goml` source-code-wrapping output:
Protocol error: Connection closed. Most likely the page has been closed.
stack: Error: Protocol error: Connection closed. Most likely the page has been closed.
    at assert (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/util/assert.js:18:15)
    at CdpPage.close (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/cdp/Page.js:814:36)
    at async runAllCommands (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:433:13)
    at async innerRunTestCode (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:696:21)
    at async innerRunTests (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:633:17)
    at async runTests (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:824:27)


======== tests/rustdoc-gui/source-code-wrapping.goml ========

[ERROR] `tests/rustdoc-gui/source-code-wrapping.goml` line 30: Error: Navigating frame was detached: for command `go-to: "file://" + |DOC_PATH| + "/test_docs/trait_bounds/index.html"`
`tests/rustdoc-gui/source-code-wrapping.goml` source-code-wrapping output:
Protocol error: Connection closed. Most likely the page has been closed.
stack: Error: Protocol error: Connection closed. Most likely the page has been closed.
    at assert (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/util/assert.js:18:15)
    at CdpPage.close (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/puppeteer-core/lib/cjs/puppeteer/cdp/Page.js:814:36)
    at async runAllCommands (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:433:13)
    at async innerRunTestCode (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:696:21)
    at async innerRunTests (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:633:17)
    at async runTests (/checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/node_modules/browser-ui-test/src/index.js:824:27)



<= doc-ui tests done: 142 succeeded, 1 failed, 0 filtered out

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jmillikin commented Nov 6, 2025

@bors retry

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bors commented Nov 6, 2025

@jmillikin: 🔑 Insufficient privileges: not in try users

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Tracking Issue for debug_closure_helpers

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