Duration's constructors should not trigger or_fun_call

[asomers-ax]

Lint name: or_fun_call

I tried this code:

let mut tick_rate = cli.time.unwrap_or(Duration::from_secs(1));

I expected to see this happen: nothing, since Duration::from_secs is a const fn. it's cheaper to use unwrap_or than unwrap_or_else, which requires creating a closure. I understand the rationale for closing #6943: Clippy can't be certain that the function will be evaluated at compile time. However, I would argue that since Clippy can neither be certain that the function will be evaluated at runtime, it should not make a recommendation one way or the other in this case. This is a case where the programmer probably knows more than Clippy does.

Instead, this happened:

warning: use of `unwrap_or` followed by a function call
   --> src/main.rs:358:34
    |
358 |     let mut tick_rate = cli.time.unwrap_or(Duration::from_secs(1));
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| Duration::from_secs(1))`

Meta

• cargo clippy -V: e.g. clippy 0.0.212 (f455e46 2020-06-20)
• rustc -Vv:

rustc 1.55.0-nightly (952fdf2a1 2021-07-05)
binary: rustc
commit-hash: 952fdf2a1119affa1b37bcacb0c49cf9f0168ac8
commit-date: 2021-07-05
host: x86_64-unknown-freebsd
release: 1.55.0-nightly
LLVM version: 12.0.1

[Jarcho]

This will be partially fixed in #7639. const functions for which all arguments are constants will no longer suggest switching to or from a closure. In cases where the arguments are not constants a closure will still be suggested. e.g.

let x = 0;
foo.unwrap(Duration::from_secs(x));

Will still suggest the use of unwrap_or_else