Clarify precedence with ^ in chained comparison

[varkor]

fn foo(x: i32, y: i32, z: i32) -> bool {
    x < y ^ y < z // error: chained comparison operators require parentheses
}

Actual output:

error: chained comparison operators require parentheses
 --> src/lib.rs:2:7
  |
2 |     x < y ^ y < z
  |       ^^^^^^^^^^^
  |
  = help: use `::<...>` instead of `<...>` if you meant to specify type arguments
  = help: or use `(...)` if you meant to specify fn arguments

error[E0308]: mismatched types
 --> src/lib.rs:2:17
  |
2 |     x < y ^ y < z
  |                 ^ expected bool, found i32

error: aborting due to 2 previous errors

It would be helpful to clarify that ^ has a higher precedence than < and suggest that the user parenthesises (x < y) ^ (y < z), which would result in the correct expected type.

[estebank]

Current output:

error: comparison operators cannot be chained
 --> src/lib.rs:2:7
  |
2 |     x < y ^ y < z // error: chained comparison operators require parentheses
  |       ^^^^^^^^^
  |
  = help: use `::<...>` instead of `<...>` to specify type arguments
  = help: or use `(...)` if you meant to specify fn arguments
help: split the comparison into two...
  |
2 |     x < y ^ y && y ^ y < z // error: chained comparison operators require parentheses
  |     ^^^^^^^^^^^^^^^^^^^^
help: ...or parenthesize one of the comparisons
  |
2 |     (x < y ^ y) < z // error: chained comparison operators require parentheses
  |     ^^^^^^^^^^^^^

[pierwill]

@rustbot claim

[pierwill]

Current output:

   Compiling playground v0.0.1 (/playground)
error: comparison operators cannot be chained
 --> src/lib.rs:2:7
  |
2 |     x < y ^ y < z
  |       ^       ^
  |
  = help: use `::<...>` instead of `<...>` to specify type or const arguments
  = help: or use `(...)` if you meant to specify fn arguments
help: split the comparison into two
  |
2 |     x < y ^ y && y ^ y < z
  |               ++++++++

error: could not compile `playground` due to previous error

We've lost the nice parentheses suggestion. 🤔

[pierwill]

The relevant code is here:
https://github.com/rust-lang/rust/blob/0cad49666f395119e6d46811ef46a74f0366a595/compiler/rustc_parse/src/parser/diagnostics.rs#L757-L760

https://github.com/rust-lang/rust/blob/0cad49666f395119e6d46811ef46a74f0366a595/compiler/rustc_parse/src/parser/diagnostics.rs#L774-L783

[pierwill]

Are there any other operators in place of ^ that would make x < y ^ y < z valid if the suggestion were to parenthesize both outer pairs, like this (x < y) ^ (y < z)?

If not, I think x < y ^ y < z could be added as its own case in attempt_chained_comparison_suggestion. Unless I'm misunderstanding the code, or there's something general I'm missing.

[estebank]

I guess | and & would be the only other ones.

[pierwill]

@rustbot release-assignment