match_single_binding mangels `match match match` expr

[matthiaskrgr]

Using the following flags

--force-warn clippy::match_single_binding

this code:

fn r#match() {
    let val: () = match match match match match () {
        () => ()
    } {
        () => ()
    } {
        () => ()
    } {
        () => ()
    } {
        () => ()
    };
    assert_eq!(val, ());
}

caused the following diagnostics:

    Checking _a v0.1.0 (/tmp/icemaker_global_tempdir.DAwWpheccItk/icemaker_clippyfix_tempdir.zY2bBPDwHH24/_a)
warning: this match could be written as a `let` statement
  --> src/lib.rs:2:5
   |
2  | /     let val: () = match match match match match () {
3  | |         () => ()
4  | |     } {
5  | |         () => ()
...  |
11 | |         () => ()
12 | |     };
   | |______^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_single_binding
   = note: requested on the command line with `--force-warn clippy::match-single-binding`
help: consider using a `let` statement
   |
2  ~     let () = match match match match () {
3  +         () => ()
4  +     } {
5  +         () => ()
6  +     } {
7  +         () => ()
8  +     } {
9  +         () => ()
10 +     };
11 +     let val = ();
   |

warning: this match could be written as a `let` statement
  --> src/lib.rs:2:25
   |
2  |       let val: () = match match match match match () {
   |  _________________________^
3  | |         () => ()
4  | |     } {
5  | |         () => ()
...  |
9  | |         () => ()
10 | |     } {
   | |_____^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_single_binding
help: consider using a `let` statement
   |
2  ~     let val: () = match let () = match match match () {
3  +         () => ()
4  +     } {
5  +         () => ()
6  +     } {
7  +         () => ()
8  +     };
9  ~     () {
   |

warning: this match could be written as a `let` statement
 --> src/lib.rs:2:31
  |
2 |       let val: () = match match match match match () {
  |  _______________________________^
3 | |         () => ()
4 | |     } {
5 | |         () => ()
6 | |     } {
7 | |         () => ()
8 | |     } {
  | |_____^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_single_binding
help: consider using a `let` statement
  |
2 ~     let val: () = match match let () = match match () {
3 +         () => ()
4 +     } {
5 +         () => ()
6 +     };
7 ~     () {
  |

warning: this match could be written as a `let` statement
 --> src/lib.rs:2:37
  |
2 |       let val: () = match match match match match () {
  |  _____________________________________^
3 | |         () => ()
4 | |     } {
5 | |         () => ()
6 | |     } {
  | |_____^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_single_binding
help: consider using a `let` statement
  |
2 ~     let val: () = match match match let () = match () {
3 +         () => ()
4 +     };
5 ~     () {
  |

warning: this match could be written as a `let` statement
 --> src/lib.rs:2:43
  |
2 |       let val: () = match match match match match () {
  |  ___________________________________________^
3 | |         () => ()
4 | |     } {
  | |_____^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_single_binding
help: consider using a `let` statement
  |
2 ~     let val: () = match match match match let () = ();
3 ~     () {
  |

warning: `_a` (lib) generated 5 warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s

However after applying these diagnostics, the resulting code:

fn r#match() {
    let val: () = match match   let () = ();
    () {
        () => ()
    } {
        () => ()
    } {
        () => ()
    } {
        () => ()
    };
    assert_eq!(val, ());
}

no longer compiled:

    Checking _a v0.1.0 (/tmp/icemaker_global_tempdir.DAwWpheccItk/icemaker_clippyfix_tempdir.zY2bBPDwHH24/_a)
warning: failed to automatically apply fixes suggested by rustc to crate `_a`

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

  * src/lib.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: expected expression, found `let` statement
 --> src/lib.rs:2:28
  |
2 |     let val: () = match    let () = ();
  |                            ^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions

error: expected one of `.`, `?`, `{`, or an operator, found `;`
 --> src/lib.rs:2:39
  |
2 |     let val: () = match    let () = ();
  |                   -----               ^ expected one of `.`, `?`, `{`, or an operator
  |                   |
  |                   while parsing this `match` expression
  |                   help: try removing this `match`

error: aborting due to 2 previous errors

Original diagnostics will follow.

error: expected expression, found `let` statement
 --> src/lib.rs:2:33
  |
2 |     let val: () = match match   let () = ();
  |                                 ^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions

error: expected one of `.`, `?`, `{`, or an operator, found `;`
 --> src/lib.rs:2:44
  |
2 |     let val: () = match match   let () = ();
  |                   ----- -----              ^ expected one of `.`, `?`, `{`, or an operator
  |                   |     |
  |                   |     while parsing this `match` expression
  |                   |     help: try removing this `match`
  |                   while parsing this `match` expression

error: could not compile `_a` (lib test) due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
warning: failed to automatically apply fixes suggested by rustc to crate `_a`

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

  * src/lib.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: expected expression, found `let` statement
 --> src/lib.rs:2:23
  |
2 |     let val: () =     let () = ();
  |                       ^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions

error: expected one of `.`, `;`, `?`, `}`, or an operator, found `{`
 --> src/lib.rs:3:8
  |
3 |     () {
  |        ^ expected one of `.`, `;`, `?`, `}`, or an operator

error: aborting due to 2 previous errors

Original diagnostics will follow.

error: expected expression, found `let` statement
 --> src/lib.rs:2:28
  |
2 |     let val: () = match    let () = ();
  |                            ^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions

error: expected one of `.`, `?`, `{`, or an operator, found `;`
 --> src/lib.rs:2:39
  |
2 |     let val: () = match    let () = ();
  |                   -----               ^ expected one of `.`, `?`, `{`, or an operator
  |                   |
  |                   while parsing this `match` expression
  |                   help: try removing this `match`

error: could not compile `_a` (lib) due to 2 previous errors

Version:

rustc 1.90.0-nightly (d41e12f1f 2025-06-27)
binary: rustc
commit-hash: d41e12f1f4e4884c356f319b881921aa37040de5
commit-date: 2025-06-27
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7

[samueltardieu]

This will be partially fixed by #15060 (only partially because suggestions will overlap and will not be auto-applicable).