Description
Summary
When the array-size-threshold configuration value is small or the array elements are large, clippy::large_stack_arrays will lint on a vec! expression. It then proposes using vec![...].into_boxed_slice(), which is unhelpful: the boxed slice in question is itself inside the vec! macro expansion, so continuing to use vec![] but with more conversions won't do anything.
Additionally, this suggestion seems like it cannot help even a normal array construction, since even if this were a literal array, switching to vec![] would not take it off the stack totally, just after construction, so the lint would continue to fire (correctly) — unless we are using [value; len] syntax. In that case only, changing to vec! would avoid putting any large array on the stack.
Lint Name
large_stack_arrays
Reproducer
I tried this code:
fn foo() {
#![warn(clippy::large_stack_arrays)]
let x = [0u8; 65000];
let y = vec![x, x, x, x, x, x, x, x];
}I saw this happen:
warning: allocating a local array larger than 512000 bytes
--> src/lib.rs:5:13
|
5 | let y = vec![x, x, x, x, x, x, x, x];
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= help: consider allocating on the heap with `vec![$($x),+].into_boxed_slice()`
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#large_stack_arrays
note: the lint level is defined here
--> src/lib.rs:2:13
|
2 | #![warn(clippy::large_stack_arrays)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
= note: this warning originates in the macro `vec` (in Nightly builds, run with -Z macro-backtrace for more info)
I expected to see this happen: Either no recommendation, or a recommendation that actually avoids the array going on the stack.
Version
rustc 1.79.0-nightly (c9f8f3438 2024-03-27)
binary: rustc
commit-hash: c9f8f3438a8134a413aa5d4903e0196e44e37bbc
commit-date: 2024-03-27
host: x86_64-apple-darwin
release: 1.79.0-nightly
LLVM version: 18.1.2
Additional Labels
No response