manual_slice_fill wrongly suggest non-const initializers

[samueltardieu]

Summary

manual_slice_fill does not check:

• that the loop variable is used to index the slice being initialized
• that the expression doesn't reference the slice content (which may have been modified earlier in the loop) or the index (which is modified for every element)

Lint Name

manual_slice_fill

Reproducer

I tried this code:

fn main() {
    let mut tmp = vec![0; 3];

    for i in 0..tmp.len() {
        tmp[i] = i;
    }

    for i in 0..tmp.len() {
        tmp[0] = i;
    }
}

I saw this happen, for both loops:

warning: manually filling a slice
  --> /tmp/t.rs:8:5
   |
8  | /     for i in 0..tmp.len() {
9  | |         tmp[0] = i;
10 | |     }
   | |_____^ help: try: `tmp.fill(i);`

I expected to see this happen: no lint, since in the first loop the initializer depends on the loop variable, and in the second loop the loop variable is not used to index the slice.

Version

rustc 1.86.0-nightly (a9730c3b5 2025-02-05)
binary: rustc
commit-hash: a9730c3b5f84a001c052c60c97ed0765e9ceac04
commit-date: 2025-02-05
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.7

Additional Labels

@rustbot label +I-suggestion-causes-error

[samueltardieu]

Ping @lapla-cogito

Note that #14191 fixes another bug, when the slice is initialized from an iterator.

[lapla-cogito]

Thank you for letting me know, I submitted #14193.

[samueltardieu]

That was fast!

[marxin]

Don't know how it's possible, but it seems this issue somehow sneaked into 1.86.0:

❯ cargo +stable clippy
warning: manually filling a slice
   --> linker-utils/src/elf.rs:719:9
    |
719 | /         for b in &mut mask {
720 | |             *b = !*b;
721 | |         }
    | |_________^ help: try: `mask.fill(!*b);`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_slice_fill
    = note: `#[warn(clippy::manual_slice_fill)]` on by default

CI run: https://github.com/davidlattimore/wild/actions/runs/14258683395/job/39965815664?pr=642

[y21]

Don't know how it's possible, but it seems this issue somehow sneaked into 1.86.0:

1.86 branched from master on Feb 14, and the fix only made it into the rust repository on Mar 2, so it missed the 1.86 release. It should be fixed on beta and will be in the next release 1.87.

[marxin]

I can confirm it's OK with the current nightly. I think the issue is pretty common, and I would consider backporting the fix to 1.86.1, what do you think?