audio: Replace inferred array lengths with BUFFER_SIZE

[atomkernel0]

Summary

Fix stable Rust builds (E0658) by replacing _-inferred array lengths with explicit BUFFER_SIZE in crates/audio/src/audio.rs. No behavior changes.

Context

Stable Rust doesn’t allow _ as an array length in type position (E0658; see rust-lang/rust#85077), causing two compile errors in audio.rs.

I got these two compilation errors :

error[E0658]: using `_` for array lengths is unstable
   --> crates/audio/src/audio.rs:131:40
    |
131 |                     let mut buf: [i16; _] = buffer.map(|s| s.to_sample());
    |                                        ^
    |
    = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information

error[E0658]: using `_` for array lengths is unstable
   --> crates/audio/src/audio.rs:172:43
    |
172 |                 let mut int_buffer: [i16; _] = buffer.map(|s| s.to_sample());
    |                                           ^
    |
    = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `audio` (lib) due to 2 previous errors
warning: build failed, waiting for other jobs to finish...

BUFFER_SIZE matches the surrounding const generics:

• inspect_buffer::<BUFFER_SIZE, _>(...)
• process_buffer::<BUFFER_SIZE, _>(...)

Testing

Built successfully on stable:

• rustc 1.88.0 (6b00bc388 2025-06-23), LLVM 19.1.4
• target: x86_64-unknown-linux-gnu (Void Linux)

Impact

Build-only fix; no functional changes.

Release notes

Fix: Restore stable Rust build by replacing _ array lengths with BUFFER_SIZE in crates/audio/src/audio.rs (E0658).

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[atomkernel0]

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[maxdeviant]

We already build on Rust stable, so this seems like something specific to your environment.

Built successfully on stable:

• rustc 1.88.0 (6b00bc388 2025-06-23), LLVM 19.1.4

I see you're building with Rust 1.88, but we have Rust 1.89 in our rust-toolchain.toml:

zed/rust-toolchain.toml

Line 2 in ffa23d2

channel = "1.89"

What happens if you build on Rust 1.89?

Edit: It looks like feature(generic_arg_infer) was stabilized in Rust 1.89: https://releases.rs/docs/1.89.0/:

• Stabilize explicitly inferred const arguments (feature(generic_arg_infer))

[maxdeviant]

Going to close this, as the feature has been stabilized in Rust 1.89, and that is the minimum version of Rust we support building Zed with.