Inferred Const Generic Arguments: Call for testing!

posts/inside-rust/2025-03-05-inferred-const-generic-arguments.md

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+---
+layout: post
+title: "Inferred const generic arguments: Call for Testing!"
+author: BoxyUwU
+team: The Const Generics Project Group <https://rust-lang.github.io/project-const-generics/>
+---
+
+We are excited to announce that `feature(generic_arg_infer)` is nearing the point of stabilization. In this post we'd like to talk a bit about what this feature does, and what comes next for it.
+
+## What is `feature(generic_arg_infer)`
+
+When `feature(min_const_generics)` was [stabilized in early 2021](https://github.com/rust-lang/rust/pull/79135) it did not include the ability to use `_` as an explicit const argument:
+```rust
+fn foo() {
+  // This errors due to `_` as an array length being unsupported
+  let a: [u8; _] = [Default::default()];
+  // This is legal as `_` is permitted as a type argument
+  let b: [_; 1] = a;
+}
+```
+
+This is entirely a syntactic limitation; it is possible to entirely elide generic argument listings that may involve const arguments:
+```rust
+fn foo<const N: usize>(_: [u8; N]) {}
+
+fn bar() {
+  // This errors due to `_` as a const argument being unsupported
+  foo::<_>([1]);
+  // This is legal as even though the const argument is *inferred*
+  // there is no explicit `_` written.
+  foo([1]);
+}
+```
+
+The compiler has always been able to infer values for const generic parameters, only the ability to explicitly ask for a const argument to be inferred is unstable.
+
+It is currently also not possible to the infer the length of a repeat expression. Doing so would require moving the expression into a separate function generic over the array length.
+
+```rust
+fn foo() {
+    // This errors due to `_` as a repeat count being unsupported
+    let a: [_; 1] = [String::new(); _];
+}
+```
+
+With `feature(generic_arg_infer)` all of the previous examples compile. This should hopefully feel like something that should "obviously" be supported by Rust.
+
+## What comes next
+
+We have [significantly reworked the implementation](https://github.com/rust-lang/rust/pull/135272) of this recently and it should now be ready for stabilization. We'd love for you to try it out on a recent nightly and report any issues you encounter.
+
+## Acknowledgements
+
+My recent push to make this feature ready for testing would not have been possible without the help of many others.
+
+A big thank you to [@lcnr][lcnr] and [@JulianKnodt][JulianKnodt] for the initial implementation of `generic_arg_infer`, [@camelid][camelid] for refactoring our representation of const generic arguments to be more flexible, [@voidc][voidc] for helping unify the way we operate on array lengths and const generic arguments, [@lcnr][lcnr] for design work on abstracting away differences between inferred type/const/generic arguments, and finally [@compiler-errors][compiler-errors] for reviewing many PRs and implementation decisions made as part of work on this feature.
+
+[lcnr]: https://github.com/lcnr
+[JulianKnodt]: https://github.com/JulianKnodt
+[camelid]: https://github.com/camelid
+[voidc]: https://github.com/voidc
+[compiler-errors]: https://github.com/compiler-errors