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by Karellen
1146 days ago
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Not parent, but not a rust programmer: > If I write a Rust program which relies on this for a constant, it works fine, because it is constant, but if I try to compile it with Rust 1.68 (from earlier this year) it won't work. How would it not work? If your program compiles and runs with 1.69, then surely that's because it never tries to modify the value. But if your program never tries to modify the value, how would it fail with 1.68? |
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In this case, the fn `std::net::SocketAddr::new` was made a const fn recently--but was a regular fn before.
>that's because it never tries to modify the value
That's something different. If you want variables that are read-only, you don't need any extra keyword. It's the normal case:
let x = 5;
If you want a mutable variable, you add a `mut` keyword.
let mut x = 5;
But if you want a compile-time constant, you use `const` instead.
const x: u32 = 5;
The latter will be evaluated by the compiler at compile time (so interpreted) and substituted BEFORE the user runs the program. This is new-ish stuff and is slowly worming its way into the standard library.