unit_arg breaks code

[matthiaskrgr]

Summary

.

Lint Name

unit_arg

Reproducer

I tried this code:

#![allow(dead_code)]
#![allow(unused_assignments)]
#![allow(unused_variables)]
#![allow(unreachable_code)]

trait UnitDefault {
    fn default() -> Self;
}

impl UnitDefault for () {
    fn default() -> () {
        panic!()
    }
}

fn match_arm() {
    let _x = match Ok(UnitDefault::default()) {
        Ok(v) => v,
        Err(()) => return,
    };
}

fn match_arm_rev() {
    let _x = match Ok(UnitDefault::default()) {
        Err(()) => return,
        Ok(v) => v,
    };
}

fn main() {}

I saw this happen:

after fixes were automatically applied the compiler reported errors within these files:

  * src/main.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/rust/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
  --> src/main.rs:18:9
   |
7  |     fn default() -> Self;
   |     --------------------- `UnitDefault::default` defined here
...
18 |         UnitDefault::default();
   |         ^^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait

error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
  --> src/main.rs:28:9
   |
7  |     fn default() -> Self;
   |     --------------------- `UnitDefault::default` defined here
...
28 |         UnitDefault::default();
   |         ^^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0790`.
Original diagnostics will follow.

warning: passing a unit value to a function
  --> src/main.rs:17:20
   |
17 |     let _x = match Ok(UnitDefault::default()) {
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unit_arg
   = note: requested on the command line with `-W clippy::unit-arg`
help: move the expression in front of the call and replace it with the unit literal `()`
   |
17 ~     let _x = match {
18 +         UnitDefault::default();
19 +         Ok(())
20 ~     } {
   |

warning: passing a unit value to a function
  --> src/main.rs:24:20
   |
24 |     let _x = match Ok(UnitDefault::default()) {
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unit_arg
help: move the expression in front of the call and replace it with the unit literal `()`
   |
24 ~     let _x = match {
25 +         UnitDefault::default();
26 +         Ok(())
27 ~     } {
   |

Version

rustc 1.67.0-nightly (c97b539e4 2022-11-30)
binary: rustc
commit-hash: c97b539e408ea353f4fde2f9251d598291fec421
commit-date: 2022-11-30
host: x86_64-unknown-linux-gnu
release: 1.67.0-nightly
LLVM version: 15.0.4

Additional Labels

No response

[teor2345]

We get a similar error when trying to fix https://github.com/ZcashFoundation/zebra/blob/d501d07898be9e5a77fc89908822b3890e9d687d/zebra-rpc/src/server/tests/vectors.rs#L57

I've included one example of each error, it is repeated multiple times in the file:

warning: failed to automatically apply fixes suggested by rustc to crate `zebra_rpc`                                                                                                                       
                                                                                                                                                                                                           
after fixes were automatically applied the compiler reported errors within these files:                                                                                                                    
                                                                                                                                                                                                           
  * zebra-rpc/src/server/tests/vectors.rs                                                                                                                                                                  
                                                                                                                                                                                                           
This likely indicates a bug in either rustc or cargo itself,                                                                                                                                               
and we would appreciate a bug report! You're likely to see                                                                                                                                                 
a number of compiler warnings after this message which cargo                                                                                                                                               
attempted to fix but failed. If you could open an issue at                                                                                                                                                 
https://github.com/rust-lang/rust-clippy/issues                                                                                                                                                            
quoting the full output of this command we'd be very appreciative!                                                                                                                                         
Note that you may be able to make some more progress in the near-term                                                                                                                                      
fixing code with the `--broken-code` flag                                                                                                                                                                  
                                                                                                                                                                                                           
The following errors were reported:                                                                                                                                                                        
error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type                                                                                                    
  --> zebra-rpc/src/server/tests/vectors.rs:58:13                                                                                                                                                          
   |                                                                                                                                                                                                       
58 |             Default::default();                                                                                                                                                                       
   |             ^^^^^^^^^^^^^^^^ cannot call associated function of trait                                                                                                                                 
   |                                                                                                                                                                                                       
help: use a fully-qualified path to a specific available implementation (1046 found)                                                                                                                       
   |                                                                                                                                                                                                       
58 |             <std::sync::LazyLock<T> as Default>::default();                                                                                                                                           
   |             ++++++++++++++++++++++++++        +                                         
...
error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0790`.
Original diagnostics will follow.

warning: passing a unit value to a function
  --> zebra-rpc/src/server/tests/vectors.rs:57:79
   |
57 |           let (rpc_server_task_handle, rpc_tx_queue_task_handle, _rpc_server) = RpcServer::spawn(
   |  _______________________________________________________________________________^
58 | |             config,
59 | |             Default::default(),
60 | |             "RPC server test",
...  |
67 | |             Mainnet,
68 | |         );
   | |_________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unit_arg
   = note: `#[warn(clippy::unit_arg)]` on by default
help: move the expression in front of the call and replace it with the unit literal `()`
   |
57 ~         let (rpc_server_task_handle, rpc_tx_queue_task_handle, _rpc_server) = {
58 +             Default::default();
59 +             RpcServer::spawn(
60 +                 config,
61 +                 (),
62 +                 "RPC server test",
63 +                 Buffer::new(mempool.clone(), 1),
64 +                 Buffer::new(state.clone(), 1),
65 +                 Buffer::new(chain_verifier.clone(), 1),
66 +                 MockSyncStatus::default(),
67 +                 MockAddressBookPeers::default(),
68 +                 NoChainTip,
69 +                 Mainnet,
70 +             )
71 ~         };
   |
...