2 use rustc::hir::def::{Def, PathResolution};
4 use rustc_const_eval::lookup_const_by_id;
5 use syntax::ast::LitKind;
6 use syntax::codemap::Span;
9 /// **What it does:** This lint checks for incompatible bit masks in comparisons.
11 /// The formula for detecting if an expression of the type `_ <bit_op> m <cmp_op> c` (where `<bit_op>`
12 /// is one of {`&`, `|`} and `<cmp_op>` is one of {`!=`, `>=`, `>`, `!=`, `>=`, `>`}) can be determined from the following table:
14 /// |Comparison |Bit Op|Example |is always|Formula |
15 /// |------------|------|------------|---------|----------------------|
16 /// |`==` or `!=`| `&` |`x & 2 == 3`|`false` |`c & m != c` |
17 /// |`<` or `>=`| `&` |`x & 2 < 3` |`true` |`m < c` |
18 /// |`>` or `<=`| `&` |`x & 1 > 1` |`false` |`m <= c` |
19 /// |`==` or `!=`| `|` |`x | 1 == 0`|`false` |`c | m != c` |
20 /// |`<` or `>=`| `|` |`x | 1 < 1` |`false` |`m >= c` |
21 /// |`<=` or `>` | `|` |`x | 1 > 0` |`true` |`m > c` |
23 /// **Why is this bad?** If the bits that the comparison cares about are always set to zero or one by the bit mask, the comparison is constant `true` or `false` (depending on mask, compared value, and operators).
25 /// So the code is actively misleading, and the only reason someone would write this intentionally is to win an underhanded Rust contest or create a test-case for this lint.
27 /// **Known problems:** None
31 /// if (x & 1 == 2) { … }
36 "expressions of the form `_ & mask == select` that will only ever return `true` or `false` \
37 (because in the example `select` containing bits that `mask` doesn't have)"
40 /// **What it does:** This lint checks for bit masks in comparisons which can be removed without changing the outcome. The basic structure can be seen in the following table:
42 /// |Comparison| Bit Op |Example |equals |
43 /// |----------|---------|-----------|-------|
44 /// |`>` / `<=`|`|` / `^`|`x | 2 > 3`|`x > 3`|
45 /// |`<` / `>=`|`|` / `^`|`x ^ 1 < 4`|`x < 4`|
47 /// **Why is this bad?** Not equally evil as [`bad_bit_mask`](#bad_bit_mask), but still a bit misleading, because the bit mask is ineffective.
49 /// **Known problems:** False negatives: This lint will only match instances where we have figured out the math (which is for a power-of-two compared value). This means things like `x | 1 >= 7` (which would be better written as `x >= 6`) will not be reported (but bit masks like this are fairly uncommon).
53 /// if (x | 1 > 3) { … }
56 pub INEFFECTIVE_BIT_MASK,
58 "expressions where a bit mask will be rendered useless by a comparison, e.g. `(x | 1) > 2`"
61 /// Checks for incompatible bit masks in comparisons, e.g. `x & 1 == 2`.
62 /// This cannot work because the bit that makes up the value two was
63 /// zeroed out by the bit-and with 1. So the formula for detecting if an
64 /// expression of the type `_ <bit_op> m <cmp_op> c` (where `<bit_op>`
65 /// is one of {`&`, '|'} and `<cmp_op>` is one of {`!=`, `>=`, `>` ,
66 /// `!=`, `>=`, `>`}) can be determined from the following table:
68 /// |Comparison |Bit Op|Example |is always|Formula |
69 /// |------------|------|------------|---------|----------------------|
70 /// |`==` or `!=`| `&` |`x & 2 == 3`|`false` |`c & m != c` |
71 /// |`<` or `>=`| `&` |`x & 2 < 3` |`true` |`m < c` |
72 /// |`>` or `<=`| `&` |`x & 1 > 1` |`false` |`m <= c` |
73 /// |`==` or `!=`| `|` |`x | 1 == 0`|`false` |`c | m != c` |
74 /// |`<` or `>=`| `|` |`x | 1 < 1` |`false` |`m >= c` |
75 /// |`<=` or `>` | `|` |`x | 1 > 0` |`true` |`m > c` |
77 /// This lint is **deny** by default
79 /// There is also a lint that warns on ineffective masks that is *warn*
82 /// |Comparison|Bit Op |Example |equals |Formula|
83 /// |`>` / `<=`|`|` / `^`|`x | 2 > 3`|`x > 3`|`¹ && m <= c`|
84 /// |`<` / `>=`|`|` / `^`|`x ^ 1 < 4`|`x < 4`|`¹ && m < c` |
86 /// `¹ power_of_two(c + 1)`
90 impl LintPass for BitMask {
91 fn get_lints(&self) -> LintArray {
92 lint_array!(BAD_BIT_MASK, INEFFECTIVE_BIT_MASK)
96 impl LateLintPass for BitMask {
97 fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
98 if let ExprBinary(ref cmp, ref left, ref right) = e.node {
99 if cmp.node.is_comparison() {
100 if let Some(cmp_opt) = fetch_int_literal(cx, right) {
101 check_compare(cx, left, cmp.node, cmp_opt, &e.span)
102 } else if let Some(cmp_val) = fetch_int_literal(cx, left) {
103 check_compare(cx, right, invert_cmp(cmp.node), cmp_val, &e.span)
110 fn invert_cmp(cmp: BinOp_) -> BinOp_ {
123 fn check_compare(cx: &LateContext, bit_op: &Expr, cmp_op: BinOp_, cmp_value: u64, span: &Span) {
124 if let ExprBinary(ref op, ref left, ref right) = bit_op.node {
125 if op.node != BiBitAnd && op.node != BiBitOr {
128 fetch_int_literal(cx, right)
129 .or_else(|| fetch_int_literal(cx, left))
130 .map_or((), |mask| check_bit_mask(cx, op.node, cmp_op, mask, cmp_value, span))
134 fn check_bit_mask(cx: &LateContext, bit_op: BinOp_, cmp_op: BinOp_, mask_value: u64, cmp_value: u64, span: &Span) {
139 if mask_value & cmp_value != cmp_value {
144 &format!("incompatible bit mask: `_ & {}` can never be equal to `{}`",
148 } else if mask_value == 0 {
149 span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
153 if mask_value | cmp_value != cmp_value {
157 &format!("incompatible bit mask: `_ | {}` can never be equal to `{}`",
168 if mask_value < cmp_value {
172 &format!("incompatible bit mask: `_ & {}` will always be lower than `{}`",
175 } else if mask_value == 0 {
176 span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
180 if mask_value >= cmp_value {
184 &format!("incompatible bit mask: `_ | {}` will never be lower than `{}`",
188 check_ineffective_lt(cx, *span, mask_value, cmp_value, "|");
191 BiBitXor => check_ineffective_lt(cx, *span, mask_value, cmp_value, "^"),
198 if mask_value <= cmp_value {
202 &format!("incompatible bit mask: `_ & {}` will never be higher than `{}`",
205 } else if mask_value == 0 {
206 span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
210 if mask_value > cmp_value {
214 &format!("incompatible bit mask: `_ | {}` will always be higher than `{}`",
218 check_ineffective_gt(cx, *span, mask_value, cmp_value, "|");
221 BiBitXor => check_ineffective_gt(cx, *span, mask_value, cmp_value, "^"),
229 fn check_ineffective_lt(cx: &LateContext, span: Span, m: u64, c: u64, op: &str) {
230 if c.is_power_of_two() && m < c {
232 INEFFECTIVE_BIT_MASK,
234 &format!("ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
241 fn check_ineffective_gt(cx: &LateContext, span: Span, m: u64, c: u64, op: &str) {
242 if (c + 1).is_power_of_two() && m <= c {
244 INEFFECTIVE_BIT_MASK,
246 &format!("ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
253 fn fetch_int_literal(cx: &LateContext, lit: &Expr) -> Option<u64> {
255 ExprLit(ref lit_ptr) => {
256 if let LitKind::Int(value, _) = lit_ptr.node {
257 Some(value) //TODO: Handle sign
264 // Important to let the borrow expire before the const lookup to avoid double
266 let def_map = cx.tcx.def_map.borrow();
267 match def_map.get(&lit.id) {
268 Some(&PathResolution { base_def: Def::Const(def_id), .. }) => Some(def_id),
272 .and_then(|def_id| lookup_const_by_id(cx.tcx, def_id, None))
273 .and_then(|(l, _ty)| fetch_int_literal(cx, l))