Explicit lifetimes

[nrc]

Currently in Rust lifetimes must be inferred. It would be nice to be able to name all lifetimes and be able to pass explicit, actual lifetime parameters to functions and data types. This is useful for 'debugging' type/borrow check errors. For lexically scoped lifetimes, this probably just means naming scopes. For non-lexically scoped lifetimes, this will be more challenging. We might not be able to do better than naming upper and lower bounds for the lifetime with named scopes.

[steveklabnik]

All must be inferred?

[nrc]

@steveklabnik sorry, I can't parse your question :-p If you are asking for clarification, consider a function example fn foo<'a>(x: &'a int) { ... }, when we call this the actual lifetime parameter substituted for 'a is inferred by the compiler from the calling context (in this case from the type of the actual parameter for x) it is impossible for the programmer to specify the actual lifetime parameter. I would like to be able to write something like <'static>::foo(...) in the same way as we can for type parameters.

[steveklabnik]

I guess I'm misunderstanding the terminology, yes. What is the 'actual lifetime parameter' vs 'a?

[nrc]

Actual parameters are the values the caller provides. Formal parameters are parameters the callee declares. In my example 'a is the formal lifetime parameter and the corresponding actual parameter would be the lifetime which instantiates it when foo is called.

[pnkfelix]

(I've also seen the terms "argument" versus "parameter" used for this distinction, and in my own writing will often incorporate both distinguishing tokens: "actual argument" versus "formal parameter"; this way people with either of the two background terminologies can understand what I am saying.)