Wrong elided lifetimes for Commit and Signature

[bcmyers]

This issue is very similar to #299, which was resolved in this PR: https://github.com/rust-lang/git2-rs/pull/302

In addition to filing this issue, I'm also submitting a new PR to address it.

Motivating example

This code should compile

struct Wrapper<'repo> {
    inner: git2::Commit<'repo>,
}

impl<'repo> Wrapper<'repo> {
    fn email(&self) -> Option<&'repo str> {
        let signature = self.inner.author();
        let email = signature.email();
        email
    }
}

but fails with

error: lifetime may not live long enough
 --> src/lib.rs:9:9
  |
5 | impl<'repo> Wrapper<'repo> {
  |      ----- lifetime `'repo` defined here
6 |     fn email(&self) -> Option<&'repo str> {
  |              - let's call the lifetime of this reference `'1`
...
9 |         email
  |         ^^^^^ associated function was supposed to return data with lifetime `'repo` but it is returning data with lifetime `'1`

error[E0515]: cannot return value referencing local variable `signature`
 --> src/lib.rs:9:9
  |
8 |         let email = signature.email();
  |                     ----------------- `signature` is borrowed here
9 |         email
  |         ^^^^^ returns a value referencing data owned by the current function

For more information about this error, try `rustc --explain E0515`.

Underlying issue

On the Commit struct, there are several "getter" methods where the lifetime of the return object is tied to a borrow of Self:

• pub fn author(&self) -> Signature<'_> {
• pub fn body(&self) -> Option<&str> {
• Etc.

In other words, written without elision, these signatures all look like

• pub fn author<'a>(&'a self) -> Signature<'a> {
• pub fn body<'a>(&'a self) -> Option<&'a str> {
• Etc.

But the pointers that are returned by these methods all live as long as the 'repo lifetime of the Commit struct itself if you look at the C code. In other words, they should all have these (more flexible) signatures:

struct Commit<'repo> { ... }

impl<'repo> Commit<'repo> {
    pub fn author(&self) -> Signature<'repo> { .. } 
    pub fn body(&self) -> Option<&'repo str> { .. }
    // Etc.
}

There is a very similar problem happening for the "getter" methods of the Signature struct.

The C code

The equivalent "getter" methods are defined in the underlying C code is here:

• https://github.com/libgit2/libgit2/blob/a3841af5eecc6301e87f8302c7fdce6555e39247/src/libgit2/commit.c#L521

The pointers they return live here:

• https://github.com/libgit2/libgit2/blob/main/src/libgit2/commit.h#L19

I believe all these pointers live as long as this raw *mut raw::git_commit pointer here:

• https://github.com/rust-lang/git2-rs/blob/master/src/commit.rs#L15

which has the 'repo lifetime.