Is there a way to obtain elided lifetime parameters from the Rust compiler?

Question

Given a Rust program, which compiles correctly, can I get the compiler to tell me what the elided lifetimes were inferred to be?

Waldheim

Actually, it is called lifetime elision and you can find the rules here: doc.rust-lang.org/stable/nomicon/lifetime-elision.html – Samurai

@TiborBenke Surely "elision" is what the code author does; the compiler can infer the elided values. In any case, I didn't mean "which lifetimes are elided" - that should be obvious by their absence, I meant what are the values of those lifetimes? Updated the question to reflect that – Waldheim

@TiborBenke I guess what you are really saying is that the elision rules are so simple that the lifetimes can be deduced trivially: there is only one lifetime to consider and it is used in all lifetime positions. – Waldheim

Yes, the compiler follows these rules to infer/expand the concrete lifetimes. – Samurai

Answer 1

The cases where the compiler (currently¹) can allow elided lifetimes are actually so simple that there isn't much the compiler could tell you about what it inferred:

Given a function, all elided lifetimes have the same value.

The compiler doesn't accept elided lifetimes in cases where it would have a choice to make. The exception is in methods, but tying all lifetimes to self is nearly always what is intended, so it makes sense for it to make this assumption.

[1] If a future version of Rust performed more sophisticated inference on elided lifetimes, then this question might have a far less trivial answer. For example the compiler could analyse the entire codebase to deduce a coherent set of lifetimes for all functions (or impls or structs if elision was permitted there too).

Waldheim

If a future version of Rust [...] — This is very unlikely, as Rust tends to favor explicit code. The reason that lifetime inference was added is because it had a small number of simple rules and those rules cleaned up something like 87% of cases that used lifetimes. – Martita