Handle ADTs with implicit lifetime params resulting a lifetime mismatch in fn definition

[TomasTomecek]

I have this function:

use git2::Reference;
fn resolve_symbolic_reference(&self, reference: Option<Reference>) -> Option<Reference> {
    return reference;
    // here's more logic, this is just shortened for clarity
}

Here's the source of the struct.

And I get this from compiler:

error[E0308]: mismatched types
  --> src/backend.rs:71:16
   |
71 |         return reference;
   |                ^^^^^^^^^ lifetime mismatch
   |
   = note: expected type `std::option::Option<git2::Reference<'_>>`
              found type `std::option::Option<git2::Reference<'_>>`
note: the anonymous lifetime #2 defined on the body at 70:92...
  --> src/backend.rs:70:93
   |
70 |     fn resolve_symbolic_reference(&self, reference: Option<Reference>) -> Option<Reference> {
   |                                                                                             ^
note: ...does not necessarily outlive the anonymous lifetime #1 defined on the body at 70:92
  --> src/backend.rs:70:93
   |
70 |     fn resolve_symbolic_reference(&self, reference: Option<Reference>) -> Option<Reference> {
   |                                                                                             ^

error: aborting due to previous error

First of all: I don't understand what's wrong with my code: isn't the lifetime of the function argument problem of the caller of my function?
Second: the compiler's error message is not helpful, especially:

   = note: expected type `std::option::Option<git2::Reference<'_>>`
              found type `std::option::Option<git2::Reference<'_>>`

I run rust unstable installed via rustup:

$ rustc 1.17.0-nightly (0aeb9c129 2017-03-15)
binary: rustc
commit-hash: 0aeb9c12979e6da753701a798d04105b6b1a8c28
commit-date: 2017-03-15
host: x86_64-unknown-linux-gnu
release: 1.17.0-nightly
LLVM version: 3.9

I have also reported this to the library's upstream.

[TomasTomecek]

So the original issue is resolved: I still think that it would be very helpful if compiler provided precise info about lifetimes.

[estebank]

The current output:

error[E0623]: lifetime mismatch
 --> src/lib.rs:8:12
  |
7 | fn resolve_symbolic_reference(&self, reference: Option<Reference>) -> Option<Reference> {
  |                                                        ---------      -----------------
  |                                                        |
  |                                                        this parameter and the return type are declared with different lifetimes...
8 |     return reference;
  |            ^^^^^^^^^ ...but data from `reference` is returned here

It should suggest adding a common lifetime argument to both References.

With nll enabled:

error: lifetime may not live long enough
  --> src/lib.rs:10:12
   |
9  | fn resolve_symbolic_reference(&self, reference: Option<Reference>) -> Option<Reference> {
   |                               -      --------- has type `std::option::Option<Reference<'1>>`
   |                               |
   |                               let's call the lifetime of this reference `'2`
10 |     return reference;
   |            ^^^^^^^^^ function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`

[rylev]

Triage: this still returns the same error above (the one not labeled as nll).

This issue also stems from the fact that by default, borrowed return types in methods get the same lifetime as &self which is distinct from the lifetime of arguments. Adding a note to this effect can also help. Something like:

error[E0623]: lifetime mismatch
 --> src/lib.rs:5:16
  |
4 |     fn resolve_symbolic_reference(&self, reference: Option<Reference>) -> Option<Reference> {
  |                                                            ---------      -----------------
  |                                                            |
  |                                                            this parameter and the return type are declared with different lifetimes...
5 |         return reference;
  |                ^^^^^^^^^ ...but data from `reference` is returned here
  = note: by default the lifetime of the return type and `&self` are the same while the method arguments have a different lifetime.
  = note: consider giving `&self`, the arguments, and the return type explicit lifetime annotations.   

[estebank]

We now emit the nll output. It should suggest &'a self and Reference<'a>.