Unused trait bound with lifetime causes compile error

[lo48576]

Summary

What happens

• Unused trait bound causes compile error.
  • In non-(3c) case, Into::<Arc<B>>::into() is completely unused, but &'a B: Into<Arc<B>> causes compile error.
  • I think this may be a compiler issue.
• With either of these small modifications, they successfully compile.
  • Changing &a B: Into<Arc<B>> to for<'b> &'b B: Into<Arc<B>>.
  • Changing &self to &'a self (or wrap: &OwnedWrap<B> to wrap: &'a OwnedWrap<B>).

Expected behavior

• Unused trait bound does not cause compile error.
• With where B: 'a + ToOwned + ?Sized, &'a B: Into<Arc<B>>,
  let b: &B = owned.borrow(); successfully compiles.
  • I think let b: &B = owned.borrow(); does not require b to outlive lifetime of &self nor 'a,
    so it should not be error.
  • owned.borrow().into() might be error though, but I think it is OK because owned.borrow() does not outlive 'a but Into<Arc<B>> requires 'a &B.

Meta

• Compiler version: 1.28.0 (stable)

Code to reproduce

Simplified cases 1: in impl

Basic code: https://play.rust-lang.org/?gist=40cb0fa71f3bd32091fbc3e144db2de1&version=stable&mode=debug&edition=2015

• error
  • (1a-2a-3a)
  • (1a-2a-3c)
• success
  • (1a-2b-3a)
  • (1a-2b-3b)
  • (1a-2b-3c)
  • (1b-2a-3a)
  • (1b-2a-3b)
  • (1b-2a-3c)
  • (1b-2b-3a)
  • (1b-2b-3b)
  • (1b-2b-3c)
• intermediate
  • (1a-2a-3b)
    • Compiles without NLL, but fails to compile with NLL.
    • I filed the difference as another issue. See Borrow::borrow() causes compile error with NLL, but compiles without NLL #53151.

//#![feature(nll)]

use std::borrow::Borrow;
use std::sync::Arc;

struct OwnedWrap<B: ToOwned + ?Sized>(<B as ToOwned>::Owned);

impl<'a, B> OwnedWrap<B>
where
    B: 'a + ToOwned + ?Sized,
    &'a B: Into<Arc<B>>, // (1a) does not compile.
    //for<'b> &'b B: Into<Arc<B>>, // (1b) successfully compiles.
{
    pub fn to_arc(&self) -> Arc<B> { // (2a) does not compile.
    //pub fn to_shared(&'a self) -> Arc<B> { // (2b) `successfully compiles.
        let OwnedWrap(ref owned) = *self;
        let b: &B = owned.borrow(); // (3a) does not compile.
        //let _: &B = owned.borrow(); // (3b) error with NLL, compiles without NLL.

        //owned.borrow().into() // (3c) What I want to do.
        unimplemented!() // comment in when using 3c.
    }
}

fn main() {}

Error for (1a-2a-3a) without NLL:

   Compiling playground v0.0.1 (file:///playground)
error[E0495]: cannot infer an appropriate lifetime for pattern due to conflicting requirements
  --> src/main.rs:16:23
   |
16 |         let OwnedWrap(ref owned) = *self;
   |                       ^^^^^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 14:5...
  --> src/main.rs:14:5
   |
14 | /     pub fn to_arc(&self) -> Arc<B> { // (2a) does not compile.
15 | |     //pub fn to_shared(&'a self) -> Arc<B> { // (2b) `successfully compiles.
16 | |         let OwnedWrap(ref owned) = *self;
17 | |         let b: &B = owned.borrow(); // (3a) does not compile.
...  |
21 | |         unimplemented!() // comment in when using 3c.
22 | |     }
   | |_____^
note: ...so that reference does not outlive borrowed content
  --> src/main.rs:16:23
   |
16 |         let OwnedWrap(ref owned) = *self;
   |                       ^^^^^^^^^
note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 8:1...
  --> src/main.rs:8:1
   |
8  | / impl<'a, B> OwnedWrap<B>
9  | | where
10 | |     B: 'a + ToOwned + ?Sized,
11 | |     &'a B: Into<Arc<B>>, // (1a) does not compile.
...  |
22 | |     }
23 | | }
   | |_^
   = note: ...so that the types are compatible:
           expected std::marker::Sized
              found std::marker::Sized

error: aborting due to previous error

For more information about this error, try `rustc --explain E0495`.
error: Could not compile `playground`.

Error for (1a2a-3a) with NLL:

   Compiling playground v0.0.1 (file:///playground)
warning: unused variable: `b`
  --> src/main.rs:17:13
   |
17 |         let b: &B = owned.borrow(); // (3a) does not compile.
   |             ^ help: consider using `_b` instead
   |
   = note: #[warn(unused_variables)] on by default

error: unsatisfied lifetime constraints
  --> src/main.rs:17:21
   |
8  | impl<'a, B> OwnedWrap<B>
   |      -- lifetime `'a` defined here
...
14 |     pub fn to_arc(&self) -> Arc<B> { // (2a) does not compile.
   |                   - let's call the lifetime of this reference `'1`
...
17 |         let b: &B = owned.borrow(); // (3a) does not compile.
   |                     ^^^^^^^^^^^^^^ requires that `'1` must outlive `'a`

error: aborting due to previous error

error: Could not compile `playground`.

Simplified cases 2: toplevel fn

Basic code: https://play.rust-lang.org/?gist=649f1a00835b68d4b51ef4a78a5ac857&version=stable&mode=debug&edition=2015

• error
  • (1a-2a-3a)
  • (1a-2a-3c)
• success
  • (1a-2b-3a)
  • (1a-2b-3b)
  • (1a-2b-3c)
  • (1b-2a-3a)
  • (1b-2a-3b)
  • (1b-2a-3c)
  • (1b-2b-3a)
  • (1b-2b-3b)
  • (1b-2b-3c)
• intermediate
  • (1a-2a-3b)
    • Compiles without NLL, but fails to compile with NLL.
    • I filed the difference as another issue. See Borrow::borrow() causes compile error with NLL, but compiles without NLL #53151.

//#![feature(nll)]

use std::borrow::Borrow;
use std::sync::Arc;

struct OwnedWrap<B: ToOwned + ?Sized>(<B as ToOwned>::Owned);

fn to_arc<'a, B>(wrap: &OwnedWrap<B>) -> Arc<B> // (2a) does not compile.
//fn to_arc<'a, B>(wrap: &'a OwnedWrap<B>) -> Arc<B> // (2b) successfully compiles.
where
    B: 'a + ToOwned + ?Sized,
    &'a B: Into<Arc<B>>, // (1a) does not compile.
    //for<'b> &'b B: Into<Arc<B>>, // (1b) successfully compiles.
{
        let OwnedWrap(ref owned) = wrap;
        let b: &B = owned.borrow(); // (3a) does not compile.
        //let _: &B = owned.borrow(); // (3b) error with NLL, compiles without NLL.

        //owned.borrow().into() // (3c) What I want to do.
        unimplemented!() // comment out when using 3c.

}

fn main() {}

FYI: What I've wanted to do

This code is more meaningful than simplified cases.
https://play.rust-lang.org/?gist=2b304fda2af1102dff60b72b5681a945&version=nightly&mode=debug&edition=2015

[nikomatsakis]

This is basically #21974, I believe. We may want to close as a duplicate.

[nikomatsakis]

But maybe it's useful to keep this specific case around.

[nikomatsakis]

I'm not really sure how to fix this — I think that chalk integration is a sort of pre-requisite.

[istankovic]

Both examples from the description compile successfully (with unused code warnings, though) with

rustc 1.75.0-nightly (0039d739d 2023-10-18)