Downgrade trivially_copy_pass_by_ref to pedantic

[dtolnay]

The rationale for this lint is documented as:

In many calling conventions instances of structs will be passed through registers if they fit into two or less general purpose registers.

I think the purported performance benefits of clippy's recommendation are overstated. This isn't worth asking people to sprinkle code with more *​*​&​*​& to chase the alleged performance.

This should be a pedantic lint that is disabled by default and opted in if some specific performance sensitive codebase determines that it is worthwhile.

As a reminder, a typical place that a reference to a primitive would come up is if the function is used as a filter. Triggering a performance-oriented lint on this type of code is the definition of pedantic.

fn filter(_n: &i32) -> bool {
    true
}

fn main() {
    let v = vec![1, 2, 3];
    v.iter().copied().filter(filter).for_each(drop);
}

warning: this argument (4 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> src/main.rs:1:15
  |
1 | fn filter(_n: &i32) -> bool {
  |               ^^^^ help: consider passing by value instead: `i32`

changelog: Remove trivially_copy_pass_by_ref from default set of enabled lints

[flip1995]

This is one of the lints, that I hit the most when I'm using Clippy and I think, that it makes the code more readable overall. That is mainly because I can save a & in every function call for small copy types.

IMO, having a filter function is a special case, which should be just allowed.

So for me this lint is more about style than perf, and I'd agree to "downgrade" it to style and adapt the documentation accordingly. But I'd hate it to be disabled by default.

Marking as C-needs-discussion. cc @rust-lang/clippy

[dtolnay]

Reframing this as a style lint is a slight improvement but I think it would still be better to disable by default. According to #5418, as a style lint this would be the 3rd most commonly suppressed style lint which indicates that the community has consciously decided that they don't share Clippy's opinion on the style of this.

[flip1995]

Thanks for the analysis in #5418. Seeing, that this lint is really often allowed, I agree to turning it into a pedantic lint.

[flip1995]

@bors r+ rollup

[bors]

📌 Commit 94154ca has been approved by flip1995

[bors]

🌲 The tree is currently closed for pull requests below priority 2, this pull request will be tested once the tree is reopened