Possible single_use_lifetime false positive

[leonardo-m]

Perhaps here I am missing something and it's just a mistake of mine, but even in such case, other programmers could trip like me and they could enjoy a better error message.

I have this small program, it compiles without warnings and works:

#![feature(underscore_lifetimes, in_band_lifetimes)]

//#![warn(single_use_lifetime)]
#![allow(dead_code)]

fn reverse_digits(mut n: u32, digs: &mut [u32]) -> &[u32] {
    if n == 0 {
        digs[0] = 0;
        return &digs[0 .. 1];
    }
    let mut pos = 0;
    while n > 0 {
        digs[pos] = n % 10;
        n /= 10;
        pos += 1
    }
    &digs[.. pos]
}

fn rotations<'a, 'b>(n: u32, digs: &'a mut [u32], rots: &'b mut [u32]) -> &'b [u32] {
    let digits = reverse_digits(n, digs);
    let ln = digits.len();
    let mut pos = 0;
    for i in 0 .. ln {
        let mut m = 0;
        for j in (0 .. ln).rev() {
            m = m * 10 + digits[(j + i) % ln];
        }
        rots[pos] = m;
        pos += 1;
    }
    &rots[.. pos]
}

fn main() {}

I've tried to use the new single_use_lifetime lint, if I uncomment the #![warn(single_use_lifetime)] line, I see the warnings/errors:

warning: lifetime name `'b` only used once
  --> C:\lavoro\bugs\test2.rs:20:18
   |
20 | fn rotations<'a, 'b>(n: u32, digs: &'a mut [u32], rots: &'b mut [u32]) -> &'b [u32] {
   |                  ^^
   |
note: lint level defined here
  --> C:\lavoro\bugs\test2.rs:3:9
   |
3  | #![warn(single_use_lifetime)]
   |         ^^^^^^^^^^^^^^^^^^^

warning: lifetime name `'a` only used once
  --> C:\lavoro\bugs\test2.rs:20:14
   |
20 | fn rotations<'a, 'b>(n: u32, digs: &'a mut [u32], rots: &'b mut [u32]) -> &'b [u32] {
   |              ^^

Using in_band_lifetimes and leaving just 'a gives a similar warning:

warning: lifetime name `'a` only used once
  --> C:\lavoro\bugs\test2.rs:20:48
   |
20 | fn rotations(n: u32, digs: & mut [u32], rots: &'a mut [u32]) -> &'a [u32] {
   |                                                ^^

Not using in_band_lifetimes and leaving just 'a gives the same:

warning: lifetime name `'a` only used once
  --> C:\lavoro\bugs\test2.rs:20:14
   |
20 | fn rotations<'a>(n: u32, digs: & mut [u32], rots: &'a mut [u32]) -> &'a [u32] {
   |              ^^

The warning seems to suggest to use the 'a name too, but it doesn't compile:

20 | fn rotations(n: u32, digs: &mut [u32], rots: &mut [u32]) -> &[u32] {
   |                                                             ^ expected lifetime parameter
   |
   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `digs` or `rots`

If I try to use an underscore the error is similar:

20 | fn rotations(n: u32, digs: & mut [u32], rots: &'_ mut [u32]) -> &'_ [u32] {
   |                                                                  ^^ expected lifetime parameter
   |
   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `digs` or `rots`

So is the message given to me by single_use_lifetime correct? If it's correct then I don't see how to fix the code to remove the warning

[Manishearth]

It wants you to say

fn rotations(n: u32, digs: &mut [u32], rots: &'rots mut [u32]) -> &'rots [u32] {

[Manishearth]

Filed a specific issue on improving the diagnostics at #46944

[leonardo-m]

Perhaps I'm still missing something, but I am still seeing warnings:

warning: lifetime name `'rots` only used once
  --> ...\test.rs:20:47
   |
20 | fn rotations(n: u32, digs: &mut [u32], rots: &'rots mut [u32]) -> &'rots [u32] {
   |                                               ^^^^^
   |
note: lint level defined here
  --> ...\test.rs:3:9
   |
3  | #![warn(single_use_lifetime)]
   |         ^^^^^^^^^^^^^^^^^^^

warning: lifetime name `'rots` only used once
  --> ...\test.rs:20:47
   |
20 | fn rotations(n: u32, digs: &mut [u32], rots: &'rots mut [u32]) -> &'rots [u32] {
   |                                               ^^^^^

Another possible solution:

warning: lifetime name `'rots` only used once
  --> src/main.rs:20:62
   |
20 | fn rotations(n: u32, digs: &mut [u32], rots: &mut [u32]) -> &'rots [u32] {
   |                                                              ^^^^^
   |
note: lint level defined here
  --> src/main.rs:3:9
   |
3  | #![warn(single_use_lifetime)]
   |         ^^^^^^^^^^^^^^^^^^^

error[E0621]: explicit lifetime required in the type of `rots`
  --> src/main.rs:32:5
   |
20 | fn rotations(n: u32, digs: &mut [u32], rots: &mut [u32]) -> &'rots [u32] {
   |                                        ---- consider changing the type of `rots` to `&'rots mut [u32]`
...
32 |     &rots[.. pos]
   |     ^^^^^^^^^^^^^ lifetime `'rots` required

[leonardo-m]

How can I reopen this issue?

[Manishearth]

Ah, looks like I tested it with the thing commented out. My bad.

Reopening.

https://play.rust-lang.org/?gist=73158fdb88e83c6a6f592838e3b066b9&version=nightly

(In the future please give playpen link for bug reports)

[zackmdavis]

This is fixed now.