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Stabilize debug_closure_helpers #146099

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2 changes: 1 addition & 1 deletion compiler/rustc_middle/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,7 @@
#![allow(rustc::diagnostic_outside_of_impl)]
#![allow(rustc::direct_use_of_rustc_type_ir)]
#![allow(rustc::untranslatable_diagnostic)]
#![cfg_attr(bootstrap, feature(debug_closure_helpers))]
#![feature(allocator_api)]
#![feature(array_windows)]
#![feature(assert_matches)]
Expand All @@ -37,7 +38,6 @@
#![feature(box_patterns)]
#![feature(closure_track_caller)]
#![feature(core_intrinsics)]
#![feature(debug_closure_helpers)]
#![feature(decl_macro)]
#![feature(discriminant_kind)]
#![feature(extern_types)]
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2 changes: 1 addition & 1 deletion compiler/rustc_parse/src/lib.rs
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Expand Up @@ -3,9 +3,9 @@
// tidy-alphabetical-start
#![allow(rustc::diagnostic_outside_of_impl)]
#![allow(rustc::untranslatable_diagnostic)]
#![cfg_attr(bootstrap, feature(debug_closure_helpers))]
#![feature(assert_matches)]
#![feature(box_patterns)]
#![feature(debug_closure_helpers)]
#![feature(default_field_values)]
#![feature(if_let_guard)]
#![feature(iter_intersperse)]
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12 changes: 6 additions & 6 deletions library/core/src/fmt/builders.rs
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Expand Up @@ -137,7 +137,7 @@ impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
///
/// This method is equivalent to [`DebugStruct::field`], but formats the
/// value using a provided closure rather than by calling [`Debug::fmt`].
#[unstable(feature = "debug_closure_helpers", issue = "117729")]
#[stable(feature = "debug_closure_helpers", since = "CURRENT_RUSTC_VERSION")]
pub fn field_with<F>(&mut self, name: &str, value_fmt: F) -> &mut Self
where
F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
Comment on lines 141 to 143
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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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@hanna-kruppe hanna-kruppe Oct 21, 2025

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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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@jmillikin jmillikin Oct 21, 2025

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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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@coolreader18 coolreader18 Oct 22, 2025

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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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@hanna-kruppe hanna-kruppe Oct 22, 2025 •
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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.

Expand Down Expand Up @@ -334,7 +334,7 @@ impl<'a, 'b: 'a> DebugTuple<'a, 'b> {
///
/// This method is equivalent to [`DebugTuple::field`], but formats the
/// value using a provided closure rather than by calling [`Debug::fmt`].
#[unstable(feature = "debug_closure_helpers", issue = "117729")]
#[stable(feature = "debug_closure_helpers", since = "CURRENT_RUSTC_VERSION")]
pub fn field_with<F>(&mut self, value_fmt: F) -> &mut Self
where
F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
Expand Down Expand Up @@ -554,7 +554,7 @@ impl<'a, 'b: 'a> DebugSet<'a, 'b> {
///
/// This method is equivalent to [`DebugSet::entry`], but formats the
/// entry using a provided closure rather than by calling [`Debug::fmt`].
#[unstable(feature = "debug_closure_helpers", issue = "117729")]
#[stable(feature = "debug_closure_helpers", since = "CURRENT_RUSTC_VERSION")]
pub fn entry_with<F>(&mut self, entry_fmt: F) -> &mut Self
where
F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
Expand Down Expand Up @@ -746,7 +746,7 @@ impl<'a, 'b: 'a> DebugList<'a, 'b> {
///
/// This method is equivalent to [`DebugList::entry`], but formats the
/// entry using a provided closure rather than by calling [`Debug::fmt`].
#[unstable(feature = "debug_closure_helpers", issue = "117729")]
#[stable(feature = "debug_closure_helpers", since = "CURRENT_RUSTC_VERSION")]
pub fn entry_with<F>(&mut self, entry_fmt: F) -> &mut Self
where
F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
Expand Down Expand Up @@ -976,7 +976,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
///
/// This method is equivalent to [`DebugMap::key`], but formats the
/// key using a provided closure rather than by calling [`Debug::fmt`].
#[unstable(feature = "debug_closure_helpers", issue = "117729")]
#[stable(feature = "debug_closure_helpers", since = "CURRENT_RUSTC_VERSION")]
pub fn key_with<F>(&mut self, key_fmt: F) -> &mut Self
where
F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
Expand Down Expand Up @@ -1052,7 +1052,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
///
/// This method is equivalent to [`DebugMap::value`], but formats the
/// value using a provided closure rather than by calling [`Debug::fmt`].
#[unstable(feature = "debug_closure_helpers", issue = "117729")]
#[stable(feature = "debug_closure_helpers", since = "CURRENT_RUSTC_VERSION")]
pub fn value_with<F>(&mut self, value_fmt: F) -> &mut Self
where
F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result,
Expand Down
1 change: 0 additions & 1 deletion library/coretests/tests/lib.rs
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Expand Up @@ -37,7 +37,6 @@
#![feature(core_private_bignum)]
#![feature(core_private_diy_float)]
#![feature(cstr_display)]
#![feature(debug_closure_helpers)]
#![feature(dec2flt)]
#![feature(drop_guard)]
#![feature(duration_constants)]
Expand Down
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