NLL regresses diagnostic for impl-trait/static-return-lifetime-infered.rs

[pnkfelix]

Consider this code from static-return-lifetime-infered.rs (play):

// #![feature(nll)]

struct A {
    x: [(u32, u32); 10]
}

impl A {
    fn iter_values_anon(&self) -> impl Iterator<Item=u32> {
        self.x.iter().map(|a| a.0)
    }
    //~^^ ERROR cannot infer an appropriate lifetime
    fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> {
        self.x.iter().map(|a| a.0)
    }
    //~^^ ERROR cannot infer an appropriate lifetime
}

fn main() {}

Part of the diagnostic info you get is this useful help message:

help: you can add a constraint to the return type to make it last less than `'static` and match the anonymous lifetime #1 defined on the method body at 8:5
  |
8 |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + '_ {
  |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

---

Unfortunately, under #![feature(nll)], we fail to produce that help diagnostic (play):

error: unsatisfied lifetime constraints
 --> src/main.rs:9:9
  |
8 |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> {
  |                         - let's call the lifetime of this reference `'1`
9 |         self.x.iter().map(|a| a.0)
  |         ^^^^^^^^^^^^^ requires that `'1` must outlive `'static`

error: unsatisfied lifetime constraints
  --> src/main.rs:13:9
   |
12 |     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> {
   |                    -- lifetime `'a` defined here
13 |         self.x.iter().map(|a| a.0)
   |         ^^^^^^^^^^^^^ requires that `'a` must outlive `'static`

error: aborting due to 2 previous errors

---

"Luckily", this does not need to be a high priority issue for us to resolve, because in the special "migration" mode for NLL (which you can observe today by opting into the 2018 edition), we do produce the help message (play):

error: cannot infer an appropriate lifetime
 --> src/main.rs:9:16
  |
8 |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> {
  |                                   ----------------------- this return type evaluates to the `'static` lifetime...
9 |         self.x.iter().map(|a| a.0)
  |         ------ ^^^^
  |         |
  |         ...but this borrow...
  |
note: ...can't outlive the anonymous lifetime #1 defined on the method body at 8:5
 --> src/main.rs:8:5
  |
8 | /     fn iter_values_anon(&self) -> impl Iterator<Item=u32> {
9 | |         self.x.iter().map(|a| a.0)
10| |     }
  | |_____^
help: you can add a constraint to the return type to make it last less than `'static` and match the anonymous lifetime #1 defined on the method body at 8:5
  |
8 |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + '_ {
  |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: cannot infer an appropriate lifetime
  --> src/main.rs:13:16
   |
12 |     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> {
   |                                     ----------------------- this return type evaluates to the `'static` lifetime...
13 |         self.x.iter().map(|a| a.0)
   |         ------ ^^^^
   |         |
   |         ...but this borrow...
   |
note: ...can't outlive the lifetime 'a as defined on the method body at 12:20
  --> src/main.rs:12:20
   |
12 |     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> {
   |                    ^^
help: you can add a constraint to the return type to make it last less than `'static` and match the lifetime 'a as defined on the method body at 12:20
   |
12 |     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> + 'a {
   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

[estebank]

For reference, that suggestion was implemented in #51444, the NLL suggestion will have to follow the same/similar logic.