Stop instantiating fix_position so many times

[dtolnay]

The lambda here unfortunately seems to capture the V type parameter so it is instantiated lots of times. This tiny function contributes more LLVM IR than all but 5 much larger functions in #313 (comment). It should be instantiated only once per R type parameter on your deserializer.

json/src/de.rs

Line 259 in c5f30c3

Err(err) => Err(err.fix_position(|code| self.error(code))),

@Rufflewind or @Coding-Doctors would you be interested in working on this?

[Mark-Simulacrum]

If we think that the lambda shouldn't function the V type parameter and be instantiated multiple times in IR, maybe this isn't really a Serde bug and more of a rustc bug; while potentially worth fixing here, it might be better to file at rust-lang/rust instead?

[dtolnay]

Seems reasonable. I can't do it right now but I will put together a self-contained repro case and report it if someone else hasn't done it by then.