This is an experimental new implementation offmt::Arguments. In this implementation,fmt::Arguments is only two pointers in size. (Instead of six, currently.) This makes it the same size as a&str and makes it fit in a register pair.

Additionally, thisfmt::Arguments can store a&'static strwithout any indirection or additional storage. This means that simple cases likeprint_fmt(format_args!("hello")) are now just as efficient for the caller asprint_str("hello"), as shown by this example:

code:

fnmain(){println!("Hello, world!");}

before:

main:subrsp,56learax,[rip+ .Lanon_hello_world]mov     qword ptr[rsp+8],raxmov     qword ptr[rsp+16],1mov     qword ptr[rsp+24],8xorpsxmm0,xmm0movups  xmmword ptr[rsp+32],xmm0leardi,[rsp+8]call    qword ptr[rip+std::io::stdio::_print]addrsp,56ret

after:

main:learsi,[rip+ .Lanon_hello_world]movedi,29jmp     qword ptr[rip+std::io::stdio::_print]

Similarly,panic!("Hello, world!"); shows the same change.

To do:

Performance testing
Figure out why things are slower to compile
Fix compilation performance
Documentation / comments

m-ou-se self-assigned this

Nov 5, 2025

m-ou-se added T-compiler

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A-fmtArea: `core::fmt` labels

Nov 5, 2025

rustbot added the S-waiting-on-authorStatus: This is awaiting some action (such as code changes or more information) from the author. label

Nov 5, 2025

m-ou-se added7 commits

November 5, 2025 15:47

Don't produce empty literal pieces in inline_literals.

a62287e

Expose expr_unsafe in LoweringContext.

bde1eaa

Don't use fmt::Arguments implementation details in assert_unsafe_prec…

98678ec

…ondition.

New fmt::Arguments implementation.

6a68f1f

Formatting.

c9a4bc6

Optimize fmt::Arguments for the static string case.

2205bcd

fmt::Arguments can now store a &str without any indirection andadditional storage: There is now a fmt::Arguments::from_str() function.

Bless tests.

4136c71

m-ou-se force-pushed thenew-fmt-args branch from4e314c3 to4136c71Compare

November 5, 2025 14:48

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m-ou-se commentedNov 5, 2025

@bors try@rust-timer queue

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Nov 5, 2025

Auto merge of#148529- m-ou-se:new-fmt-args, r=<try>

89ea077

Experiment: New fmt::Arguments implementation

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Nov 5, 2025

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Fix typo.

400931e

This was referencedNov 5, 2025

Tracking issue for improving std::fmt::Arguments and format_args!()#99012

Open

Optimizepanic!(<str lit>) into method call#148375

Open

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Make clippy happy.

4f55e11

rustbot added the T-clippyRelevant to the Clippy team. label

Nov 5, 2025

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rust-log-analyzer commentedNov 5, 2025

The jobaarch64-gnu-llvm-20-1 failed! Check out the build log:(web)(plain enhanced)(plain)

Click to see the possible cause of the failure (guessed by this bot)

test [mir-opt] tests/mir-opt/unusual_item_types.rs ... okfailures:---- [mir-opt] tests/mir-opt/sroa/lifetimes.rs stdout ----18       let mut _18: core::fmt::rt::Argument<'_>;19       let mut _19: &u32;20       let mut _20: core::fmt::rt::Template<'_>;-       let mut _21: &[core::fmt::rt::Piece; 7];-       let _22: &[core::fmt::rt::Piece; 7];-       let _23: [core::fmt::rt::Piece; 7];+       let mut _21: &[core::fmt::rt::Piece; 6];+       let _22: &[core::fmt::rt::Piece; 6];+       let _23: [core::fmt::rt::Piece; 6];24       let mut _24: core::fmt::rt::Piece;25       let mut _25: core::fmt::rt::Piece;26       let mut _26: core::fmt::rt::Piece;27       let mut _27: core::fmt::rt::Piece;28       let mut _28: core::fmt::rt::Piece;29       let mut _29: core::fmt::rt::Piece;-       let mut _30: core::fmt::rt::Piece;-       let mut _31: &[core::fmt::rt::Argument<'_>; 2];-       let _32: &[core::fmt::rt::Argument<'_>; 2];-       let mut _34: &std::boxed::Box<dyn std::fmt::Display>;-       let mut _35: &u32;-       let mut _36: bool;+       let mut _30: &[core::fmt::rt::Argument<'_>; 2];+       let _31: &[core::fmt::rt::Argument<'_>; 2];+       let mut _33: &std::boxed::Box<dyn std::fmt::Display>;+       let mut _34: &u32;+       let mut _35: bool;+       let mut _36: isize;36       let mut _37: isize;37       let mut _38: isize;-       let mut _39: isize;- +     let _40: std::result::Result<std::boxed::Box<dyn std::fmt::Display>, <T as Err>::Err>;- +     let _41: u32;+ +     let _39: std::result::Result<std::boxed::Box<dyn std::fmt::Display>, <T as Err>::Err>;+ +     let _40: u32;41       scope 1 {42 -         debug foo => _1;- +         debug ((foo: Foo<T>).0: std::result::Result<std::boxed::Box<dyn std::fmt::Display>, <T as Err>::Err>) => _40;- +         debug ((foo: Foo<T>).1: u32) => _41;+ +         debug ((foo: Foo<T>).0: std::result::Result<std::boxed::Box<dyn std::fmt::Display>, <T as Err>::Err>) => _39;+ +         debug ((foo: Foo<T>).1: u32) => _40;45           let _5: std::result::Result<std::boxed::Box<dyn std::fmt::Display>, <T as Err>::Err>;46           scope 2 {47               debug x => _5;52                       debug x => _8;53                       let _8: std::boxed::Box<dyn std::fmt::Display>;54                       let _12: (&std::boxed::Box<dyn std::fmt::Display>, &u32);- +                     let _42: &std::boxed::Box<dyn std::fmt::Display>;- +                     let _43: &u32;+ +                     let _41: &std::boxed::Box<dyn std::fmt::Display>;+ +                     let _42: &u32;57                       scope 5 {58 -                         debug args => _12;- +                         debug ((args: (&Box<dyn std::fmt::Display>, &u32)).0: &std::boxed::Box<dyn std::fmt::Display>) => _42;- +                         debug ((args: (&Box<dyn std::fmt::Display>, &u32)).1: &u32) => _43;+ +                         debug ((args: (&Box<dyn std::fmt::Display>, &u32)).0: &std::boxed::Box<dyn std::fmt::Display>) => _41;+ +                         debug ((args: (&Box<dyn std::fmt::Display>, &u32)).1: &u32) => _42;61                           let _15: [core::fmt::rt::Argument<'_>; 2];62                           scope 6 {63                               debug args => _15;-                               let mut _33: &[core::fmt::rt::Piece; 7];+                               let mut _32: &[core::fmt::rt::Piece; 6];65                           }66                       }67                   }70       }---77 +         nop;78           StorageLive(_2);79           StorageLive(_3);87           _2 = Result::<Box<dyn std::fmt::Display>, <T as Err>::Err>::Ok(move _3);88           StorageDead(_3);89 -         _1 = Foo::<T> { x: move _2, y: const 7_u32 };- +         _40 = move _2;- +         _41 = const 7_u32;+ +         _39 = move _2;+ +         _40 = const 7_u32;92 +         nop;93           StorageDead(_2);94           StorageLive(_5);-           _36 = const true;+           _35 = const true;96 -         _5 = move (_1.0: std::result::Result<std::boxed::Box<dyn std::fmt::Display>, <T as Err>::Err>);- +         _5 = move _40;+ +         _5 = move _39;98           StorageLive(_6);99 -         _6 = copy (_1.1: u32);- +         _6 = copy _41;+ +         _6 = copy _40;101           _7 = discriminant(_5);102           switchInt(move _7) -> [0: bb2, otherwise: bb8];103       }109           StorageLive(_10);110           StorageLive(_11);111 -         StorageLive(_12);---115           StorageLive(_13);116           _13 = &_8;117           StorageLive(_14);118           _14 = &_6;119 -         _12 = (move _13, move _14);- +         _42 = move _13;- +         _43 = move _14;+ +         _41 = move _13;+ +         _42 = move _14;122 +         nop;123           StorageDead(_14);124           StorageDead(_13);125           StorageLive(_15);126           StorageLive(_16);127           StorageLive(_17);- -         _34 = copy (_12.0: &std::boxed::Box<dyn std::fmt::Display>);- +         _34 = copy _42;-           _17 = &(*_34);+ -         _33 = copy (_12.0: &std::boxed::Box<dyn std::fmt::Display>);+ +         _33 = copy _41;+           _17 = &(*_33);131           _16 = core::fmt::rt::Argument::<'_>::new_display::<Box<dyn std::fmt::Display>>(move _17) -> [return: bb3, unwind unreachable];132       }133   135           StorageDead(_17);136           StorageLive(_18);137           StorageLive(_19);- -         _35 = copy (_12.1: &u32);- +         _35 = copy _43;-           _19 = &(*_35);+ -         _34 = copy (_12.1: &u32);+ +         _34 = copy _42;+           _19 = &(*_34);141           _18 = core::fmt::rt::Argument::<'_>::new_display::<u32>(move _19) -> [return: bb4, unwind unreachable];142       }143   149           StorageLive(_20);150           StorageLive(_21);151           StorageLive(_22);-           _33 = const foo::<T>::promoted[0];-           _22 = &(*_33);+           _32 = const foo::<T>::promoted[0];+           _22 = &(*_32);154           _21 = &(*_22);-           _20 = core::fmt::rt::Template::<'_>::new::<7>(move _21) -> [return: bb5, unwind unreachable];+           _20 = core::fmt::rt::Template::<'_>::new::<6>(move _21) -> [return: bb5, unwind unreachable];156       }157   158       bb5: {159           StorageDead(_22);160           StorageDead(_21);+           StorageLive(_30);161           StorageLive(_31);-           StorageLive(_32);-           _32 = &_15;-           _31 = &(*_32);-           _11 = core::fmt::rt::<impl Arguments<'_>>::new::<2>(move _20, move _31) -> [return: bb6, unwind unreachable];+           _31 = &_15;+           _30 = &(*_31);+           _11 = core::fmt::rt::<impl Arguments<'_>>::new::<2>(move _20, move _30) -> [return: bb6, unwind unreachable];166       }167   168       bb6: {-           StorageDead(_32);170           StorageDead(_31);+           StorageDead(_30);171           StorageDead(_20);172           _10 = _eprint(move _11) -> [return: bb7, unwind unreachable];173       }176           StorageDead(_11);177           StorageDead(_15);178 -         StorageDead(_12);---198   199       bb10: {200           StorageDead(_6);-           _37 = discriminant(_5);-           switchInt(move _37) -> [0: bb11, otherwise: bb12];+           _36 = discriminant(_5);+           switchInt(move _36) -> [0: bb11, otherwise: bb12];203       }204   205       bb11: {-           _36 = const false;---212           return;213       }thread '[mir-opt] tests/mir-opt/sroa/lifetimes.rs' panicked at src/tools/compiletest/src/runtest/mir_opt.rs:84:21:Actual MIR output differs from expected MIR output /checkout/tests/mir-opt/sroa/lifetimes.foo.ScalarReplacementOfAggregates.diffstack backtrace:   5: __rustc::rust_begin_unwind             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/std/src/panicking.rs:698:5   6: core::panicking::panic_fmt             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/core/src/panicking.rs:80:14

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☀️ Try build successful (CI)
Build commit:89ea077 (89ea077499c3fe06868ef1d91f2c47c8af941dce, parent:53efb3d4f3b67d189a0c72eb475a52017d79d609)

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rust-timer commentedNov 6, 2025

Finished benchmarking commit (89ea077):comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with@rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.1%, 8.4%]	95
Regressions ❌ (secondary)	1.1%	[0.0%, 12.0%]	37
Improvements ✅ (primary)	-0.6%	[-2.2%, -0.1%]	39
Improvements ✅ (secondary)	-2.1%	[-35.3%, -0.0%]	55
All ❌✅ (primary)	0.5%	[-2.2%, 8.4%]	134

Max RSS (memory usage)

Results (primary 1.6%, secondary -0.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.7%	[1.0%, 5.5%]	22
Regressions ❌ (secondary)	4.3%	[2.2%, 8.9%]	7
Improvements ✅ (primary)	-2.6%	[-6.5%, -0.6%]	6
Improvements ✅ (secondary)	-3.6%	[-5.6%, -1.5%]	12
All ❌✅ (primary)	1.6%	[-6.5%, 5.5%]	28

Cycles

Results (primary 4.5%, secondary -4.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	6.8%	[1.7%, 11.0%]	13
Regressions ❌ (secondary)	5.8%	[2.2%, 11.1%]	7
Improvements ✅ (primary)	-3.1%	[-5.8%, -2.1%]	4
Improvements ✅ (secondary)	-8.8%	[-36.8%, -2.4%]	16
All ❌✅ (primary)	4.5%	[-5.8%, 11.0%]	17

Binary size

Results (primary -0.2%, secondary -0.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.0%, 2.2%]	24
Regressions ❌ (secondary)	3.7%	[0.1%, 19.5%]	26
Improvements ✅ (primary)	-0.4%	[-2.2%, -0.0%]	100
Improvements ✅ (secondary)	-1.2%	[-11.2%, -0.0%]	105
All ❌✅ (primary)	-0.2%	[-2.2%, 2.2%]	124

Bootstrap: 473.384s -> 471.178s (-0.47%)
Artifact size: 390.72 MiB -> 387.77 MiB (-0.75%)

rustbot added the perf-regressionPerformance regression. label

Nov 6, 2025

rustbot removed the S-waiting-on-perfStatus: Waiting on a perf run to be completed. label

Nov 6, 2025

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Overall result: ❌✅ regressions and improvements - please read the text below

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Overall result: ❌✅ regressions and improvements - please read the text below

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