Missing lifetimes in pointer signatures

[yyny]

Commit c44a7aa removes the lifetimes on pointer and pointer_mut, which makes the inferred lifetimes too restrictive:

fn bug(val: &mut serde_json::Value, pointer: &str) -> &mut serde_json::Value {
    match val.pointer_mut(pointer) {
        Some(v) => v,
        None => val,
    }
}

 error[E0106]: missing lifetime specifier
 --> bug.rs:1:55
  |
1 | fn bug(val: &mut serde_json::Value, pointer: &str) -> &mut serde_json_Value {
  |             ----------------------           ----     ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `val` or `pointer`
help: consider introducing a named lifetime parameter
  |
1 | fn bug<'a>(val: &'a mut serde_json::Value, pointer: &'a str) -> &'a mut serde_json_Value {
  |       ++++       ++                                  ++          ++

---

fn bug<'a>(val: &'a mut serde_json::Value, pointer: &str) -> &'a mut serde_json::Value {
    match val.pointer_mut(pointer) {
        Some(v) => v,
        None => val,
    }
}

error[E0505]: cannot move out of `val` because it is borrowed
 --> bug.rs:4:17
  |
1 |   fn bug<'a>(val: &'a mut serde_json::Value, pointer: &str) -> &'a mut serde_json::Value {
  |          --  --- binding `val` declared here
  |          |
  |          lifetime `'a` defined here
2 |       match val.pointer_mut(pointer) {
  |       -     --- borrow of `*val` occurs here
  |  _____|
  | |
3 | |         Some(v) => v,
4 | |         None => val,
  | |                 ^^^ move out of `val` occurs here
5 | |     }
  | |_____- returning this value requires that `*val` is borrowed for `'a`

---

fn bug<'a>(val: &'a mut serde_json::Value, pointer: &str) -> &'a mut serde_json::Value {
    match unsafe { std::mem::transmute::<Option<&mut serde_json::Value, &'a mut serde_json::Value>(val.pointer_mut(pointer)) } {
        Some(v) => v,
        None => val,
    }
}

[dtolnay]

The lifetime error in your second snippet is rust-lang/rust#21906, and the lifetime error in your first snippet is unrelated to any signatures in serde_json. The 5-year-old commit you linked doesn't have relevance to either one.

[yyny]

Well, the first snippet is just to show why the second snippet is necessary, but you're right, this looks like a compiler bug, thanks for the link.