Replace some matches! with if let or ==

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[rustbot]

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Some changes occurred in exhaustiveness checking

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rustc_errors::emitter was changed

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Some changes occurred in check-cfg diagnostics

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Some changes occurred in compiler/rustc_passes/src/check_attr.rs

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Some changes occurred in need_type_info.rs

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Some changes occurred in compiler/rustc_ast/src/expand/autodiff_attrs.rs

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[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] unicode_segmentation test:false 0.259
[RUSTC-TIMING] askama_derive test:false 1.271
    Checking askama v0.14.0
[RUSTC-TIMING] askama test:false 0.165
error[E0369]: binary operation `==` cannot be applied to type `cfg::Format`
   --> src/librustdoc/clean/cfg.rs:228:33
    |
228 |         let mut msg = if format == Format::LongHtml {
    |                          ------ ^^ ---------------- cfg::Format
    |                          |
    |                          cfg::Format
    |
note: an implementation of `std::cmp::PartialEq` might be missing for `cfg::Format`
   --> src/librustdoc/clean/cfg.rs:403:1

[RalfJung]

What is the reason for this change? What I like about matches! is that I can use it without worrying about whether the type implements PartialEq. Also, with if let I have to carefully check the condition to see if anything is being bound; matches! makes it clear that nothing is bound.

So while I don't really mind any of these diffs, it also doesn't look like a clear improvement that's worth a sweeping change, and new "unnecessary" matches! will inevitably be added again.

[jdonszelmann]

agreed, not sure this is necessarily better.

[nnethercote]

I don't mind the change. if let/== is shorter than matches!, and using built-in features rather than macros is nice.

I do think a PR touching 82 files deserves a description that explains the how and the why, though. What prompted this? Did you automate any of it? When did you use == vs if let?