3 use syntax::ast::LitKind;
4 use syntax::codemap::Span;
5 use utils::{span_lint, span_lint_and_then};
7 use consts::{constant, Constant};
9 /// **What it does:** Checks for incompatible bit masks in comparisons.
11 /// The formula for detecting if an expression of the type `_ <bit_op> m
12 /// <cmp_op> c` (where `<bit_op>` is one of {`&`, `|`} and `<cmp_op>` is one of
13 /// {`!=`, `>=`, `>`, `!=`, `>=`, `>`}) can be determined from the following
16 /// |Comparison |Bit Op|Example |is always|Formula |
17 /// |------------|------|------------|---------|----------------------|
18 /// |`==` or `!=`| `&` |`x & 2 == 3`|`false` |`c & m != c` |
19 /// |`<` or `>=`| `&` |`x & 2 < 3` |`true` |`m < c` |
20 /// |`>` or `<=`| `&` |`x & 1 > 1` |`false` |`m <= c` |
21 /// |`==` or `!=`| `|` |`x | 1 == 0`|`false` |`c | m != c` |
22 /// |`<` or `>=`| `|` |`x | 1 < 1` |`false` |`m >= c` |
23 /// |`<=` or `>` | `|` |`x | 1 > 0` |`true` |`m > c` |
25 /// **Why is this bad?** If the bits that the comparison cares about are always
26 /// set to zero or one by the bit mask, the comparison is constant `true` or
27 /// `false` (depending on mask, compared value, and operators).
29 /// So the code is actively misleading, and the only reason someone would write
30 /// this intentionally is to win an underhanded Rust contest or create a
31 /// test-case for this lint.
33 /// **Known problems:** None.
37 /// if (x & 1 == 2) { … }
42 "expressions of the form `_ & mask == select` that will only ever return `true` or `false`"
45 /// **What it does:** Checks for bit masks in comparisons which can be removed
46 /// without changing the outcome. The basic structure can be seen in the
49 /// |Comparison| Bit Op |Example |equals |
50 /// |----------|---------|-----------|-------|
51 /// |`>` / `<=`|`|` / `^`|`x | 2 > 3`|`x > 3`|
52 /// |`<` / `>=`|`|` / `^`|`x ^ 1 < 4`|`x < 4`|
54 /// **Why is this bad?** Not equally evil as [`bad_bit_mask`](#bad_bit_mask),
55 /// but still a bit misleading, because the bit mask is ineffective.
57 /// **Known problems:** False negatives: This lint will only match instances
58 /// where we have figured out the math (which is for a power-of-two compared
59 /// value). This means things like `x | 1 >= 7` (which would be better written
60 /// as `x >= 6`) will not be reported (but bit masks like this are fairly
65 /// if (x | 1 > 3) { … }
68 pub INEFFECTIVE_BIT_MASK,
70 "expressions where a bit mask will be rendered useless by a comparison, e.g. `(x | 1) > 2`"
73 /// **What it does:** Checks for bit masks that can be replaced by a call
74 /// to `trailing_zeros`
76 /// **Why is this bad?** `x.trailing_zeros() > 4` is much clearer than `x & 15
79 /// **Known problems:** llvm generates better code for `x & 15 == 0` on x86
88 "expressions where a bit mask is less readable than the corresponding method call"
91 #[derive(Copy, Clone)]
93 verbose_bit_mask_threshold: u64,
97 pub fn new(verbose_bit_mask_threshold: u64) -> Self {
99 verbose_bit_mask_threshold,
104 impl LintPass for BitMask {
105 fn get_lints(&self) -> LintArray {
106 lint_array!(BAD_BIT_MASK, INEFFECTIVE_BIT_MASK, VERBOSE_BIT_MASK)
110 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BitMask {
111 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
112 if let ExprBinary(ref cmp, ref left, ref right) = e.node {
113 if cmp.node.is_comparison() {
114 if let Some(cmp_opt) = fetch_int_literal(cx, right) {
115 check_compare(cx, left, cmp.node, cmp_opt, &e.span)
116 } else if let Some(cmp_val) = fetch_int_literal(cx, left) {
117 check_compare(cx, right, invert_cmp(cmp.node), cmp_val, &e.span)
122 if let Expr_::ExprBinary(ref op, ref left, ref right) = e.node;
123 if BinOp_::BiEq == op.node;
124 if let Expr_::ExprBinary(ref op1, ref left1, ref right1) = left.node;
125 if BinOp_::BiBitAnd == op1.node;
126 if let Expr_::ExprLit(ref lit) = right1.node;
127 if let LitKind::Int(n, _) = lit.node;
128 if let Expr_::ExprLit(ref lit1) = right.node;
129 if let LitKind::Int(0, _) = lit1.node;
130 if n.leading_zeros() == n.count_zeros();
131 if n > u128::from(self.verbose_bit_mask_threshold);
133 span_lint_and_then(cx,
136 "bit mask could be simplified with a call to `trailing_zeros`",
138 let sugg = Sugg::hir(cx, left1, "...").maybe_par();
139 db.span_suggestion(e.span, "try", format!("{}.trailing_zeros() >= {}", sugg, n.count_ones()));
146 fn invert_cmp(cmp: BinOp_) -> BinOp_ {
159 fn check_compare(cx: &LateContext, bit_op: &Expr, cmp_op: BinOp_, cmp_value: u128, span: &Span) {
160 if let ExprBinary(ref op, ref left, ref right) = bit_op.node {
161 if op.node != BiBitAnd && op.node != BiBitOr {
164 fetch_int_literal(cx, right)
165 .or_else(|| fetch_int_literal(cx, left))
166 .map_or((), |mask| check_bit_mask(cx, op.node, cmp_op, mask, cmp_value, span))
170 fn check_bit_mask(cx: &LateContext, bit_op: BinOp_, cmp_op: BinOp_, mask_value: u128, cmp_value: u128, span: &Span) {
172 BiEq | BiNe => match bit_op {
173 BiBitAnd => if mask_value & cmp_value != cmp_value {
180 "incompatible bit mask: `_ & {}` can never be equal to `{}`",
186 } else if mask_value == 0 {
187 span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
189 BiBitOr => if mask_value | cmp_value != cmp_value {
195 "incompatible bit mask: `_ | {}` can never be equal to `{}`",
203 BiLt | BiGe => match bit_op {
204 BiBitAnd => if mask_value < cmp_value {
210 "incompatible bit mask: `_ & {}` will always be lower than `{}`",
215 } else if mask_value == 0 {
216 span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
218 BiBitOr => if mask_value >= cmp_value {
224 "incompatible bit mask: `_ | {}` will never be lower than `{}`",
230 check_ineffective_lt(cx, *span, mask_value, cmp_value, "|");
232 BiBitXor => check_ineffective_lt(cx, *span, mask_value, cmp_value, "^"),
235 BiLe | BiGt => match bit_op {
236 BiBitAnd => if mask_value <= cmp_value {
242 "incompatible bit mask: `_ & {}` will never be higher than `{}`",
247 } else if mask_value == 0 {
248 span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
250 BiBitOr => if mask_value > cmp_value {
256 "incompatible bit mask: `_ | {}` will always be higher than `{}`",
262 check_ineffective_gt(cx, *span, mask_value, cmp_value, "|");
264 BiBitXor => check_ineffective_gt(cx, *span, mask_value, cmp_value, "^"),
271 fn check_ineffective_lt(cx: &LateContext, span: Span, m: u128, c: u128, op: &str) {
272 if c.is_power_of_two() && m < c {
275 INEFFECTIVE_BIT_MASK,
278 "ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
287 fn check_ineffective_gt(cx: &LateContext, span: Span, m: u128, c: u128, op: &str) {
288 if (c + 1).is_power_of_two() && m <= c {
291 INEFFECTIVE_BIT_MASK,
294 "ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
303 fn fetch_int_literal(cx: &LateContext, lit: &Expr) -> Option<u128> {
304 match constant(cx, lit)?.0 {
305 Constant::Int(n) => Some(n),