“Truly Hygienic” Let Statements in Rust (sabrinajewson.org)

submitted 1 month ago by hallettj@leminal.space to c/rust@programming.dev

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A short post on how variable names can leak out of macros if there is a name collision with a constant. I thought this was a delightful read!

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[-] ExperimentalGuy@programming.dev 17 points 1 month ago

That was such a cute lil post

[-] livingcoder@programming.dev 7 points 1 month ago

This was a great post, but is the last state of the macro actually bad for performance in any way? I get that it's ugly (and we should only choose to make code less readable like this when there's actually an issue) but is it worse for runtime performance?

[-] TehPers@beehaw.org 9 points 1 month ago

Adding a single unused function should no effect on runtime performance. The compiler removes dead code during compilation, and there's no concept at runtime anyway of "creating a function" since it's just a compile-time construct to group reusable code (generally speaking - yes the pedants will be right when they say functions appear in the compiled output, to some extent).

Anyway, this can all be tested on Godbolt anyway if you want to verify yourself. Make a function with and without a nested unused function and check the output.

[-] arendjr@programming.dev 1 points 1 month ago

Runtime performance is entirely unaffected by the use of macros. It can have a negative impact on compile-time performance though, if you overdo it.

[-] livingcoder@programming.dev 2 points 1 month ago

I understand that the macro only affects compile time but I'm talking about the extra function that's included in the resulting source code when the macro is expanded during compile. Based on other feedback, it looks like the unused function is optimized away.

[-] arendjr@programming.dev 2 points 1 month ago

Ah yes, exactly.

[+] BB_C@programming.dev -7 points 1 month ago

Maybe a good idea for a post. But the amount of reaches required makes this icky.

Pretending people write:

let Ok(x) = read_input() else { return Err(Error) };

instead of

 let x = read_input().map_err(|_| ...)?;

Pretending people write:

 const x: &str = "...";

instead of

 const X: &str = "...";

Pretending there exist people who have such knowledge of rust macros hygiene, ident namespaces, etc, but somehow don't know about how macro code expands (the "shock" about the compile error).

Maybe there is a reason after all why almost no one (maybe no one, period) was ever in that situation.