2025-01-16 04:18:12 -05:00
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---
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minutes: 10
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---
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2024-03-15 18:31:41 +05:30
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# `const`
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Constants are evaluated at compile time and their values are inlined wherever
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they are used:
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<!-- mdbook-xgettext: skip -->
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```rust,editable
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const DIGEST_SIZE: usize = 3;
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const ZERO: Option<u8> = Some(42);
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fn compute_digest(text: &str) -> [u8; DIGEST_SIZE] {
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let mut digest = [ZERO.unwrap_or(0); DIGEST_SIZE];
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for (idx, &b) in text.as_bytes().iter().enumerate() {
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digest[idx % DIGEST_SIZE] = digest[idx % DIGEST_SIZE].wrapping_add(b);
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}
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digest
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}
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fn main() {
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let digest = compute_digest("Hello");
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println!("digest: {digest:?}");
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}
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```
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According to the [Rust RFC Book][1] these are inlined upon use.
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Only functions marked `const` can be called at compile time to generate `const`
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values. `const` functions can however be called at runtime.
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<details>
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- Mention that `const` behaves semantically similar to C++'s `constexpr`
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- It isn't super common that one would need a runtime evaluated constant, but it
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is helpful and safer than using a static.
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</details>
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[1]: https://rust-lang.github.io/rfcs/0246-const-vs-static.html
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