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Fix false positive in cast_possible_truncation #12722

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Merged
merged 1 commit into from
Apr 28, 2024
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Fix false positive in cast_possible_truncation #12722

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7 changes: 6 additions & 1 deletion clippy_lints/src/casts/cast_possible_truncation.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -40,9 +40,14 @@ fn apply_reductions(cx: &LateContext<'_>, nbits: u64, expr: &Expr<'_>, signed: b
get_constant_bits(cx, right).map_or(0, |b| b.saturating_sub(1))
})
},
BinOpKind::Rem | BinOpKind::BitAnd => get_constant_bits(cx, right)
BinOpKind::Rem => get_constant_bits(cx, right)
.unwrap_or(u64::MAX)
.min(apply_reductions(cx, nbits, left, signed)),
BinOpKind::BitAnd => get_constant_bits(cx, right)
.unwrap_or(u64::MAX)
.min(get_constant_bits(cx, left).unwrap_or(u64::MAX))
.min(apply_reductions(cx, nbits, right, signed))
.min(apply_reductions(cx, nbits, left, signed)),
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We should apply reductions to right as well, e.g. in

fn x() -> u64 {
    u64::MAX
}

let _ = ((x() & 255) & 999999) as u8;
let _ = (999999 & (x() & 255)) as u8; // it would stop this one linting

That & the above added to the tests should be good to go, if you could squash the commits that would be great also

A bit of a nit but a commit message that doesn't require looking up the issue # would be ideal

BinOpKind::Shr => apply_reductions(cx, nbits, left, signed)
.saturating_sub(constant_int(cx, right).map_or(0, |s| u64::try_from(s).unwrap_or_default())),
_ => nbits,
Expand Down
21 changes: 20 additions & 1 deletion tests/ui/cast.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,7 +13,8 @@
clippy::cast_abs_to_unsigned,
clippy::no_effect,
clippy::unnecessary_operation,
clippy::unnecessary_literal_unwrap
clippy::unnecessary_literal_unwrap,
clippy::identity_op
)]

fn main() {
Expand Down Expand Up @@ -479,3 +480,21 @@ fn issue12506() -> usize {
let bar: Result<Option<i64>, u32> = Ok(Some(10));
bar.unwrap().unwrap() as usize
}

fn issue12721() {
fn x() -> u64 {
u64::MAX
}

// Don't lint.
(255 & 999999u64) as u8;
// Don't lint.
let _ = ((x() & 255) & 999999) as u8;
// Don't lint.
let _ = (999999 & (x() & 255)) as u8;

(256 & 999999u64) as u8;
//~^ ERROR: casting `u64` to `u8` may truncate the value
(255 % 999999u64) as u8;
//~^ ERROR: casting `u64` to `u8` may truncate the value
}
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