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clarification to const_assert

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Lokathor 2018-12-15 20:43:01 -07:00
parent 51d3915dea
commit 09c16ef1b0
1 changed files with 13 additions and 7 deletions
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20
book/src/01-quirks/05-static_asserts.md
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@ -52,11 +52,11 @@ name.
const _: usize = 0 - 1;
```
Now we wrap this in a macro where we give an expression for a bool. We negate
the bool then cast it to a `usize`, meaning that `true` negates into `false`,
which becomes `0usize`, and then there's no underflow error. Or if the input was
`false`, it negates into `true`, then becomes `1usize`, and then the underflow
error fires.
Now we wrap this in a macro where we give a `bool` expression as input. We
negate the bool then cast it to a `usize`, meaning that `true` negates into
`false`, which becomes `0usize`, and then there's no underflow error. Or if the
input was `false`, it negates into `true`, then becomes `1usize`, and then the
underflow error fires.
```rust
macro_rules! const_assert {
@ -68,8 +68,14 @@ macro_rules! const_assert {
}
```
This allows anything which supports `core::ops::Not` and can then can cast into
`usize`, which technically isn't just `bool` values, but close enough.
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.
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.
## Asserting Something