This error indicates that an `#[inline(..)]` attribute was incorrectly placed
on something other than a function or method.
Examples of erroneous code:
```compile_fail,E0518
#[inline(always)]
struct Foo;
#[inline(never)]
impl Foo {
// ...
}
```
`#[inline]` hints the compiler whether or not to attempt to inline a method or
function. By default, the compiler does a pretty good job of figuring this out
itself, but if you feel the need for annotations, `#[inline(always)]` and
`#[inline(never)]` can override or force the compiler's decision.
If you wish to apply this attribute to all methods in an impl, manually annotate
each method; it is not possible to annotate the entire impl with an `#[inline]`
attribute.