Use slice patterns in rustc

[oli-obk]

After #61541 is merged, we should have a look at

rust/src/libsyntax/diagnostics/plugin.rs

Line 36 in 817d2fe

let code = match (token_tree.len(), token_tree.get(0)) {

and replace all such workaround matches like that with matches that use slice patterns.

More examples in https://github.com/rust-lang/rust/blob/master/src/libsyntax/parse/mod.rs

[Electron-libre]

I can give it a try.
I think that searching through code for such workaround will take most of the time.

[oli-obk]

Search for get(0), get(1) and such. Outside of tests these often can be a sign of workarounds.

[Electron-libre]

Ok, I am on it.

[awaitlink]

Shouldn't this be closed because #61654 is now merged?