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Fix float_cmp false positive when comparing signum #4275

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Jul 16, 2019
Merged

Fix float_cmp false positive when comparing signum #4275

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8adbf7f
Fix float_cmp false positive when comparing signum
Jul 15, 2019
8a8eedf
Lint
Jul 15, 2019
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25 changes: 25 additions & 0 deletions clippy_lints/src/misc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -364,6 +364,12 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MiscLints {
if is_allowed(cx, left) || is_allowed(cx, right) {
return;
}

// Allow comparing the results of signum()
if is_signum(cx, left) && is_signum(cx, right) {
return;
}

if let Some(name) = get_item_name(cx, expr) {
let name = name.as_str();
if name == "eq"
Expand Down Expand Up @@ -493,6 +499,25 @@ fn is_allowed<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) -> bool {
}
}

// Return true if `expr` is the result of `signum()` invoked on a float value.
fn is_signum(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
// The negation of a signum is still a signum
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Nice, good catch!

Lint idea (for another PR):

// for
f1.signum() == -f2.signum()
// suggest
f1.signum() != f2.signum()

Same goes for -sig==+sig and -sig==-sig and the != variants.

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Thanks! I'm not sure that's the best way to simplify f1.signum() == -f2.signum(), since if both f1 and f2 are 0, then also f1.signum() == f2.signum() == 0.

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Oh your right, in this case we can't really lint this. Never mind then :D

if let ExprKind::Unary(UnNeg, ref child_expr) = expr.node {
return is_signum(cx, &child_expr);
}

if_chain! {
if let ExprKind::MethodCall(ref method_name, _, ref expressions) = expr.node;
if sym!(signum) == method_name.ident.name;
// Check that the receiver of the signum() is a float (expressions[0] is the receiver of
// the method call)
then {
return is_float(cx, &expressions[0]);
}
}
false
}

fn is_float(cx: &LateContext<'_, '_>, expr: &Expr) -> bool {
matches!(walk_ptrs_ty(cx.tables.expr_ty(expr)).sty, ty::Float(_))
}
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19 changes: 19 additions & 0 deletions tests/ui/float_cmp.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -77,4 +77,23 @@ fn main() {
let b: *const f32 = xs.as_ptr();

assert_eq!(a, b); // no errors

// no errors - comparing signums is ok
let x32 = 3.21f32;
1.23f32.signum() == x32.signum();
1.23f32.signum() == -(x32.signum());
1.23f32.signum() == 3.21f32.signum();

1.23f32.signum() != x32.signum();
1.23f32.signum() != -(x32.signum());
1.23f32.signum() != 3.21f32.signum();

let x64 = 3.21f64;
1.23f64.signum() == x64.signum();
1.23f64.signum() == -(x64.signum());
1.23f64.signum() == 3.21f64.signum();

1.23f64.signum() != x64.signum();
1.23f64.signum() != -(x64.signum());
1.23f64.signum() != 3.21f64.signum();
}