Tidy up `const_ops`, rename to `const_arith_ops` #143949

clarfonthey · 2025-07-15T01:03:23Z

Tracking issue: #143802

This is split into three commits for ease of reviewability:

Updates the forward_ref_* macros to accept multiple attributes (in anticipation of needing rust_const_unstable attributes) and also require attributes in these macros. Since the default attribute only helps for the initial implementations, it means it's easy to get wrong for future implementations, as shown for the saturating implementations which were incorrect before.
Renames const_ops feature to const_arith_ops and adds constness and trivial implementations to the remaining traits in core::ops::arith.
Constifies the rest of the reasonable implementations.

A few random other notes on the implementation specifically:

Unfortunately, without Tracking Issue for macro_named_capture_groups #142999 implemented, we can't make the constness in the forward_ref_* macros work nicely without copy-pasting the macro body. Instead of doing that, I decided to go for a cursed extra const argument, which is then used for the actual const keyword. I explicitly mention named macro capture groups as a way of doing this better in the comments.
I unindented the attributes that were passed to the forward_ref_* macro calls because in some places rustfmt wanted them to be unindented, and in others it was allowed because they were themselves inside of macro bodies. I chose the consistent indenting even though I (personally) think it looks worse.

As far as the actual changes go, this renames the const_ops feature to const_arith_ops (anticipating, e.g., const_bit_ops) and constifies the following additional traits:

Neg
AddAssign
SubAssign
MulAssign
DivAssign
RemAssign

In terms of constified implementations of these traits, it adds the reference-forwarded versions of all the arithmetic operators, which are defined by the macros in library/core/src/internal_macros.rs. I'm not going to fully enumerate these because we'd be here all day, but sufficed to say, it effectively allows adding an & to one or both sides of an operator for primitives.

Additionally, I constified the implementations for Wrapping, Saturating, and Duration as well, since all of them forward to already-const-stable methods. Specifically for Duration, the ref-forwarding is not present.

Caution

Concerns (1 active)

May break Clippy

Managed by @rustbot—see help for details.

…ibutes explicitly

rustbot · 2025-07-15T01:03:27Z

r? @ibraheemdev

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rustbot · 2025-07-15T01:03:31Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

hkBst · 2025-07-15T19:23:43Z

src/tools/clippy/tests/ui/assign_ops.rs

@@ -75,6 +76,7 @@ impl MulAssign<i64> for Wrap {
    }
 }

+#[clippy::msrv = "1.88.0"]


What is this for?

This hides a real issue: to know whether it can replace "a = a OP b" by "a OP= b" (where OP is, for example, "+"), Clippy looks for the const-stability of the sole associated item of the trait corresponding to "OP=". Here, AddAssign is now identified as const-stable, and Clippy attempts to replace a = a + b; by a += b; in a const context, when a is of a generic type implementing AddAssign.

Yes: I'm not quite sure whether this code should even remain on Clippy's side after this PR, since I'm pretty sure that "MSRV" doesn't account for nightly compilers, and we definitely won't want to ever stabilise an operator without its corresponding assign version. I chose the easiest approach to get it to pass without making too much of a modification, since it makes more sense to edit this in the clippy repo directly instead of via the subtree.

Filed rust-lang/rust-clippy#15285 to keep track of this.

In the meantime, you could go with https://rfc1149.pastes.sh/patch-assign-op-pattern.diff and remove the msrv attribute, and we'll take care of reintroducing the proper code in Clippy.

@rustbot concern Breaks clippy

The problem in commenting the test code but letting the code in Clippy as-is is that starting from the nightly after the PR is merged, Clippy (as part of the nightly release of Rust) will start giving bogus suggestions in const contexts, including in CI runs for people using nightly. And this will stay for at least two sync cycles (up to one month) and may even enter the next Rust beta release.

Applying the patch I referenced above will not only prevent Clippy from giving bad suggestions, but it will also still pass all the tests without altering them.

What exactly do you mean as far as bogus suggestions? It actually is correct; if you enable the const_trait_impls feature and the const_arith_ops feature, as are required for the add operator to work in const context, then it will correctly suggest that you can replace x = x + y with x += y. I'm a bit confused what the issue is, here.

In the test, res = res + Self::ONE; cannot be replaced by res += Self::ONE in a const context because T (the type of res and Self::ONE) implements AddAssign, not [const] AddAssign (and it implements [const] Add, hence the valid + operation in the const context). But Clippy, if it stays unpatched, will suggest it because it only checks the constness of the AddAssign trait declaration, not its implementation.

If I am mistaken and this suggestion is valid, then instead of commenting the test, just add a //~^ assign_op_pattern below the res = res + Self::ONE; line and bless the results, but I think you'll get an invalid .fixed file.

Oh, you're totally correct here; I was mistaken. So, this is a genuine bug, but it's unrelated to the actual issue at hand, IMHO: it should also suggest to replace [const] Add with [const] AddAssign, or add it. Or at least make it not an automatically applicable thing. It has nothing to do with the const stability of the trait, but the const bound on the trait.

library/core/src/ops/arith.rs

rustbot · 2025-07-16T22:03:02Z

Only organization members can add or resolve concerns.

rust-log-analyzer · 2025-07-16T22:13:56Z

The job aarch64-gnu-llvm-19-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

[RUSTC-TIMING] alloctests test:true 16.282
error: `std::ops::Add` is not yet stable as a const trait
##[error]   --> coretests/tests/floats/mod.rs:260:23
    |
260 |         assert_biteq!(ten.add(two), ten + two);
    |                       ^^^^^^^^^^^^
    |
help: add `#![feature(const_arith_ops)]` to the crate attributes to enable
   --> coretests/tests/lib.rs:109:1
    |
109 + #![feature(const_arith_ops)]
    |

error: `std::ops::Sub` is not yet stable as a const trait
##[error]   --> coretests/tests/floats/mod.rs:261:23
    |
261 |         assert_biteq!(ten.sub(two), ten - two);
    |                       ^^^^^^^^^^^^
    |
help: add `#![feature(const_arith_ops)]` to the crate attributes to enable
   --> coretests/tests/lib.rs:109:1
    |
109 + #![feature(const_arith_ops)]
    |

error: `std::ops::Mul` is not yet stable as a const trait
##[error]   --> coretests/tests/floats/mod.rs:262:23
    |
262 |         assert_biteq!(ten.mul(two), ten * two);
    |                       ^^^^^^^^^^^^
    |
help: add `#![feature(const_arith_ops)]` to the crate attributes to enable
   --> coretests/tests/lib.rs:109:1
    |
109 + #![feature(const_arith_ops)]
    |

error: `std::ops::Div` is not yet stable as a const trait
##[error]   --> coretests/tests/floats/mod.rs:263:23
    |
263 |         assert_biteq!(ten.div(two), ten / two);
    |                       ^^^^^^^^^^^^
    |
help: add `#![feature(const_arith_ops)]` to the crate attributes to enable
   --> coretests/tests/lib.rs:109:1
    |
109 + #![feature(const_arith_ops)]
    |

error: `std::ops::Rem` is not yet stable as a const trait
##[error]   --> coretests/tests/floats/mod.rs:278:23
    |
278 |         assert_biteq!(ten.rem(two), ten % two);
    |                       ^^^^^^^^^^^^
    |
help: add `#![feature(const_arith_ops)]` to the crate attributes to enable
   --> coretests/tests/lib.rs:109:1
    |
109 + #![feature(const_arith_ops)]
    |

For more information about this error, try `rustc --explain E0635`.
[RUSTC-TIMING] coretests test:true 47.421
error: could not compile `coretests` (test "coretests") due to 6 previous errors
env -u RUSTC_WRAPPER CARGO_ENCODED_RUSTDOCFLAGS="-Csymbol-mangling-version=v0\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}--check-cfg=cfg(llvm_enzyme)\u{1f}-Dwarnings\u{1f}-Wrustdoc::invalid_codeblock_attributes\u{1f}--crate-version\u{1f}1.90.0-nightly\t(29b1640c5\t2025-07-16)" CARGO_ENCODED_RUSTFLAGS="-Csymbol-mangling-version=v0\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}--check-cfg=cfg(llvm_enzyme)\u{1f}-Zmacro-backtrace\u{1f}-Csplit-debuginfo=off\u{1f}-Clink-arg=-L/usr/lib/llvm-19/lib\u{1f}-Cllvm-args=-import-instr-limit=10\u{1f}-Clink-args=-Wl,-z,origin\u{1f}-Clink-args=-Wl,-rpath,$ORIGIN/../lib\u{1f}-Alinker-messages\u{1f}--cap-lints=allow\u{1f}--cfg\u{1f}randomized_layouts" RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools/cg_clif/dist/rustc-clif" RUSTDOC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools/cg_clif/dist/rustdoc-clif" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "test" "--manifest-path" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools/cg_clif/build/sysroot_tests/Cargo.toml" "--target-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools/cg_clif/build/sysroot_tests_target" "--locked" "--target" "aarch64-unknown-linux-gnu" "-p" "coretests" "-p" "alloctests" "--tests" "--" "-q" exited with status ExitStatus(unix_wait_status(25856))
Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:13:17
  local time: Wed Jul 16 22:13:42 UTC 2025
  network time: Wed, 16 Jul 2025 22:13:43 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

samueltardieu · 2025-07-16T22:27:11Z

@rustbot concern May break Clippy

(checking if concerns work as top-level comments only, real concern was written here, and discussion about the error message is on Zulip)

Let forward_ref_* macros accept multiple attributes, and require attr…

500a041

…ibutes explicitly

rustbot assigned ibraheemdev Jul 15, 2025

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clarfonthey force-pushed the const-arith-ops branch from 97d85a1 to d7a8770 Compare July 15, 2025 02:31

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clarfonthey force-pushed the const-arith-ops branch from d7a8770 to ad7b7c4 Compare July 15, 2025 04:48

hkBst reviewed Jul 15, 2025

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samueltardieu reviewed Jul 15, 2025

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library/core/src/ops/arith.rs Show resolved Hide resolved

clarfonthey mentioned this pull request Jul 16, 2025

Custom handling for const-stability of assign operators in assign_op_pattern lint should be removed rust-lang/rust-clippy#15285

Open

clarfonthey added 2 commits July 16, 2025 17:50

Rename const_ops to const_arith_ops, add to remaining arith traits

49c6561

Constify missing reasonable implementations

071b5f0

clarfonthey force-pushed the const-arith-ops branch from ad7b7c4 to 071b5f0 Compare July 16, 2025 21:50

rustbot added the S-waiting-on-concerns Status: Awaiting concerns to be addressed by the author label Jul 16, 2025

samueltardieu mentioned this pull request Jul 16, 2025

Misleading error message when concern is left in a file comment rust-lang/triagebot#2114

Open

Tidy up const_ops, rename to const_arith_ops #143949

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Tidy up const_ops, rename to const_arith_ops #143949

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Tidy up `const_ops`, rename to `const_arith_ops` #143949

Tidy up `const_ops`, rename to `const_arith_ops` #143949

clarfonthey commented Jul 15, 2025 •

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