Document / adjust operator precedence lint

[dhardy]

Description

The precedence lint flags some stuff which is (IMO) unintuitive:

`1 << 2 + 3`

But also some stuff which is (again, IMO) clearer:

`u64::from(data[1]) << 32 | u64::from(data[0])`

The lint does not flag some stuff which is widely understood:

`2 * 3 - 1`

---

The lint documentation could be more precise about which operator combinations get flagged, with a link to the reference on operator expression precedence.

Further, the lint could be disagglomerated or adjusted to allow the bit-shift with binary-and/or/xor case above.

Version

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[dhardy]

Sorry, I missed that this was changed recently: #13743.

Would you consider moving the extended precedence checks for bitmasking and shift to a new lint?

[dhardy]

Also, is this lint deprecated, while still being reported?

$ cargo clippy --all-targets -- -A precedence
    Checking rand_core v0.9.0-beta.1 (/home/dhardy/projects/rand/rand/rand_core)
warning: lint name `precedence` is deprecated and may not have an effect in the future
  |
  = help: change it to clippy::precedence
  = note: requested on the command line with `-A precedence`
  = note: `#[warn(renamed_and_removed_lints)]` on by default

warning: operator precedence can trip the unwary
   --> rand_core/src/block.rs:200:13
    |
200 |             u64::from(data[1]) << 32 | u64::from(data[0])
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider parenthesizing your expression: `(u64::from(data[1]) << 32) | u64::from(data[0])`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#precedence
    = note: `#[warn(clippy::precedence)]` on by default