Add a custom source code formatter to replace rust-fmt

[cuddlefishie]

Generated code looks very messy when not formatted,
rustfmt take a lot of time to format the code, so this crate
implements a reduced formatter that's supposed to run fast enough
that it can be part of the regular build process.

TODOs:

• change StopWatch so the timing summary makes sense again
• more "hard numbers" for the performance of the new formatter (MB/s?)

[bluenote10]

Just wondering, would prettyplease fit in here?

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-303

[Bromeon]

Just wondering, would prettyplease fit in here?

We considered it, but it takes a syn syntax tree, not TokenStream -- so we'd need to add a heavy dependency and do full-blown Rust parsing. We also looked at rust_format, but from what I can see, it's mostly an abstraction around rustfmt or prettyplease.

[Bromeon]

Thanks so much for this! The output code looks really much nicer 🙂

The test-cases directory is mostly for benchmarking, right? Do the 5 files differ substantially, or could we do with just 3 or so? (Asking because they add 0.5 MB to the repo forever; even if compressed in Git).

Also, I don't see the formatting time output in *stats.txt files, would it be possible to include that again?

[cuddlefishie]

I addressed the review comments! Thanks!

Because of the way StopWatch is set up and the way the SubmitFn is performing the formatting (or not) immediately, it's harder to get direct timing.

Comparing the total time in codegen-stats.txt with and without formatting gave me inconsistent results. Sometimes running with formatting was faster (?!), sometimes the other way around. So if somebody else can test this, that would be good :)

[Bromeon]

Thanks a lot for this! The benchmarking and StopWatch measurements can be addressed at a later time.

bors r+

[bors]

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