rust-lang / rust-lang/rust-clippy
`unnecessary_lazy_evaluations` fires when the closure has a `match`
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Description
Summary
I'd argue that match is not completely free, so it shouldn't be made eager? Though I guess it's a bit less expensive in my case, because the enum variants don't actually contain values that are then used in the match arms.
Lint Name
unnecessary_lazy_evaluations
Reproducer
I tried this code:
#![allow(unused)]
#![warn(clippy::unnecessary_lazy_evaluations)]
enum MyEnum {
Foo,
Bar,
Baz,
}
fn foo(opt: Option<u32>, e: MyEnum) -> Option<u32> {
opt.or_else(|| match e {
MyEnum::Foo => Some(7),
MyEnum::Bar => Some(13),
MyEnum::Baz => None,
})
}
I saw this happen:
warning: unnecessary closure used to substitute value for `Option::None`
--> src/main.rs:11:5
|
11 | / opt.or_else(|| match e {
12 | | MyEnum::Foo => Some(7),
13 | | MyEnum::Bar => Some(13),
14 | | MyEnum::Baz => None,
15 | | })
| |______^
|
I expected to see this happen:
No lint
Version
rustc 1.92.0-nightly (3d8c1c1fc 2025-10-06)
binary: rustc
commit-hash: 3d8c1c1fc077d04658de63261d8ce2903546db13
commit-date: 2025-10-06
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.2
Additional Labels
No response
Contributor guide
First steps
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- Open a pull request that references the issue number.
Research direction
Start by locating the implementation of the unnecessary_lazy_evaluations lint and read how it handles closures containing match expressions. Run the supplied Rust reproducer, then verify that the lint no longer fires for this case while its other intended diagnostics remain unchanged.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- tooling
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Clearly specified
- Newbie friendliness
- 55/100