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assert upgrades!

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Lokathor 2018-12-15 21:11:13 -07:00
parent 3d2dbbf214
commit 9e3b3e15f1
1 changed files with 13 additions and 8 deletions
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21
book/src/01-quirks/05-const_asserts.md
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@ -72,15 +72,20 @@ macro_rules! const_assert {
Technically, written like this, the expression can be anything with a
`core::ops::Not` implementation that can also be `as` cast into `usize`. That's
`bool`, but also basically all the other number types.
`bool`, but also basically all the other number types. Since we want to ensure
that we get proper looking type errors when things go wrong, we can use
`($condition && true)` to enforce that we get a `bool` (thanks to `Talchas` for
that particular suggestion).
It doesn't really hurt if you want to `const_assert!` a number I guess. I mean,
any number other than the `MAX` value of an unsigned type or the `-1` value of
an unsigned type will fail such an assertion, but I bet you'll notice that you
did something wrong pretty quick. We could use the
[type_ascription](https://github.com/rust-lang/rust/issues/23416) feature to
really force a `bool`, but it's not that critical, so we'll avoid using a
feature that we don't need until it's stable.
```rust
macro_rules! const_assert {
($condition:expr) => {
#[deny(const_err)]
#[allow(dead_code)]
const _: usize = 0 - !($condition && true) as usize;
}
}
```
## Asserting Something