On Sat, Feb 01, 2014 at 03:42:45PM -0800, Vadim wrote:
> Since &'a Foo<T> currently means "the return value is a reference into
> something that has lifetime 'a", 'a Foo<T> feels like a natural extension
> for saying "the return value is a reference-like thing whose safety depends
> on something that has lifetime 'a still being around".
> Foo<'a,T>, of the other hand... it is not obvious to me why would it
> necessarily mean that.
It does not, in fact, *necessarily* mean that, though certainly it
most commonly does. It will depend on the definition of the `Foo` type
and how the lifetime parameter is used within the type, as you say. It
seems then that you did not mean for `'a Foo<T>` to be syntactic sugar
for `Foo<'a, T>` but rather a new kind of type: kind of a "by value
that is limited to 'a".
> I've been around Rust for almost a year now, and certainly since the time
> the current lifetime notation has been introduced, and I *still *could not
> explain to somebody, why a lifetime parameter appearing among the type
> parameters of a trait or a struct refers to the lifetime of that trait or
> struct.
As I wrote above, a lifetime parameter does not by itself have any
effect, just like a type parameter. Both type and lifetime parameters
are simply substituted into the struct body, and any limitations arise
from there. That is, if I have
struct Foo<'a> {
x: &'a int
}
then `Foo<'xyz>` is limited to the lifetime `'xyz` because `Foo`
contains a field `x` whose type is (after substitution) `&'xyz int`,
and that *field* cannot escape the lifetime `'xyz`.
Thus, there are in fact corner cases where the lifetime parameter has
no effect, and so it is not the case that `SomeType<'xyz>` is
necessarily limited to `'xyz` (the most obvious being when 'xyz is
unused within the struct body, as you suggest).
Niko
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