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unused_assignments lint has trouble tracking that one match guard observes assignment from another match guard. #138069
Description
I tried this code (playground):
fn pnk(x: usize) -> &'static str {
let mut k1 = "k1";
let mut h1 = "h1";
match x & 3 {
3 if { k1 = "unused?"; false } => (),
_ if { h1 = k1; true } => (),
_ => (),
}
h1
}
fn main() {
dbg!(pnk(3));
}I expected to see this happen: No lint complaints about the assignment to k1 in the first match guard being "unused", since the assignment is subsequently observed by the second match guard.
Instead, this happened: The unused_assignments lint has a false positive on the code above:
warning: value assigned to `k1` is never read
--> src/main.rs:5:16
|
5 | 3 if { k1 = "unused?"; false } => (),
| ^^
|
= help: maybe it is overwritten before being read?
= note: `#[warn(unused_assignments)]` on by default
warning: `playground` (bin "playground") generated 1 warning
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.89s
Running `target/debug/playground`
[src/main.rs:13:5] pnk(3) = "unused?"
Meta
I'm testing on the playground, Rust stable 1.85.0
Reacted by Fridtjof Stoldt
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Area: Lints (warnings about flaws in source code) such as unused_mut.Relating to patterns and pattern matchingCategory: This is a bug.Call for participation: An issue has been fixed and does not reproduce, but no test has been added.Lint: False positive (should not have fired).Lint: unused_assignmentsRelevant to the compiler team, which will review and decide on the PR/issue.
Activity
pnkfelix commentedon Mar 5, 2025
Fun fact: This problem seems like it has been present for the entirety of Rust's support for assignments in pattern guards: I see the same false positive from the lint in Rust 1.39. (And in Rust 1.38, the code is rejected because at that time, assignments in pattern guards were illegal).
compiler-errors commentedon Mar 5, 2025
The liveness analysis that powers the unused assignments lint is pretty brittle. Ideally we'd reimplement it on the MIR with a real cfg-aware analysis so that we don't have disagreements between MIR's "uninitialized assignments" and this.
cjgillot commentedon Oct 13, 2025
Fixed by #142390
KaiTomotake commentedon Jan 18, 2026
@rustbot claim
add lint test
add lint test
Rollup merge of rust-lang#151302 - add-lint-test, r=lqd
Unrolled build for #151302