1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
use rustc::hir::*;
use rustc::hir::def::Def;
use rustc::lint::*;
use rustc_const_eval::lookup_const_by_id;
use syntax::ast::LitKind;
use syntax::codemap::Span;
use utils::{span_lint, span_lint_and_then};
use utils::sugg::Sugg;
/// **What it does:** Checks for incompatible bit masks in comparisons.
///
/// The formula for detecting if an expression of the type `_ <bit_op> m
/// <cmp_op> c` (where `<bit_op>` is one of {`&`, `|`} and `<cmp_op>` is one of
/// {`!=`, `>=`, `>`, `!=`, `>=`, `>`}) can be determined from the following
/// table:
///
/// |Comparison |Bit Op|Example |is always|Formula |
/// |------------|------|------------|---------|----------------------|
/// |`==` or `!=`| `&` |`x & 2 == 3`|`false` |`c & m != c` |
/// |`<` or `>=`| `&` |`x & 2 < 3` |`true` |`m < c` |
/// |`>` or `<=`| `&` |`x & 1 > 1` |`false` |`m <= c` |
/// |`==` or `!=`| `|` |`x | 1 == 0`|`false` |`c | m != c` |
/// |`<` or `>=`| `|` |`x | 1 < 1` |`false` |`m >= c` |
/// |`<=` or `>` | `|` |`x | 1 > 0` |`true` |`m > c` |
///
/// **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).
///
/// 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.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// if (x & 1 == 2) { … }
/// ```
declare_lint! {
pub BAD_BIT_MASK,
Warn,
"expressions of the form `_ & mask == select` that will only ever return `true` or `false`"
}
/// **What it does:** Checks for bit masks in comparisons which can be removed
/// without changing the outcome. The basic structure can be seen in the
/// following table:
///
/// |Comparison| Bit Op |Example |equals |
/// |----------|---------|-----------|-------|
/// |`>` / `<=`|`|` / `^`|`x | 2 > 3`|`x > 3`|
/// |`<` / `>=`|`|` / `^`|`x ^ 1 < 4`|`x < 4`|
///
/// **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.
///
/// **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).
///
/// **Example:**
/// ```rust
/// if (x | 1 > 3) { … }
/// ```
declare_lint! {
pub INEFFECTIVE_BIT_MASK,
Warn,
"expressions where a bit mask will be rendered useless by a comparison, e.g. `(x | 1) > 2`"
}
/// **What it does:** Checks for bit masks that can be replaced by a call
/// to `trailing_zeros`
///
/// **Why is this bad?** `x.trailing_zeros() > 4` is much clearer than `x & 15
/// == 0`
///
/// **Known problems:** llvm generates better code for `x & 15 == 0` on x86
///
/// **Example:**
/// ```rust
/// x & 0x1111 == 0
/// ```
declare_lint! {
pub VERBOSE_BIT_MASK,
Warn,
"expressions where a bit mask is less readable than the corresponding method call"
}
#[derive(Copy, Clone)]
pub struct BitMask;
impl LintPass for BitMask {
fn get_lints(&self) -> LintArray {
lint_array!(BAD_BIT_MASK, INEFFECTIVE_BIT_MASK, VERBOSE_BIT_MASK)
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BitMask {
fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
if let ExprBinary(ref cmp, ref left, ref right) = e.node {
if cmp.node.is_comparison() {
if let Some(cmp_opt) = fetch_int_literal(cx, right) {
check_compare(cx, left, cmp.node, cmp_opt, &e.span)
} else if let Some(cmp_val) = fetch_int_literal(cx, left) {
check_compare(cx, right, invert_cmp(cmp.node), cmp_val, &e.span)
}
}
}
if_let_chain!{[
let Expr_::ExprBinary(ref op, ref left, ref right) = e.node,
BinOp_::BiEq == op.node,
let Expr_::ExprBinary(ref op1, ref left1, ref right1) = left.node,
BinOp_::BiBitAnd == op1.node,
let Expr_::ExprLit(ref lit) = right1.node,
let LitKind::Int(n, _) = lit.node,
let Expr_::ExprLit(ref lit1) = right.node,
let LitKind::Int(0, _) = lit1.node,
n.leading_zeros() == n.count_zeros(),
], {
span_lint_and_then(cx,
VERBOSE_BIT_MASK,
e.span,
"bit mask could be simplified with a call to `trailing_zeros`",
|db| {
let sugg = Sugg::hir(cx, left1, "...").maybe_par();
db.span_suggestion(e.span, "try", format!("{}.trailing_zeros() >= {}", sugg, n.count_ones()));
});
}}
}
}
fn invert_cmp(cmp: BinOp_) -> BinOp_ {
match cmp {
BiEq => BiEq,
BiNe => BiNe,
BiLt => BiGt,
BiGt => BiLt,
BiLe => BiGe,
BiGe => BiLe,
_ => BiOr, // Dummy
}
}
fn check_compare(cx: &LateContext, bit_op: &Expr, cmp_op: BinOp_, cmp_value: u128, span: &Span) {
if let ExprBinary(ref op, ref left, ref right) = bit_op.node {
if op.node != BiBitAnd && op.node != BiBitOr {
return;
}
fetch_int_literal(cx, right)
.or_else(|| fetch_int_literal(cx, left))
.map_or((), |mask| check_bit_mask(cx, op.node, cmp_op, mask, cmp_value, span))
}
}
fn check_bit_mask(cx: &LateContext, bit_op: BinOp_, cmp_op: BinOp_, mask_value: u128, cmp_value: u128, span: &Span) {
match cmp_op {
BiEq | BiNe => {
match bit_op {
BiBitAnd => {
if mask_value & cmp_value != cmp_value {
if cmp_value != 0 {
span_lint(
cx,
BAD_BIT_MASK,
*span,
&format!(
"incompatible bit mask: `_ & {}` can never be equal to `{}`",
mask_value,
cmp_value
),
);
}
} else if mask_value == 0 {
span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
}
},
BiBitOr => {
if mask_value | cmp_value != cmp_value {
span_lint(
cx,
BAD_BIT_MASK,
*span,
&format!(
"incompatible bit mask: `_ | {}` can never be equal to `{}`",
mask_value,
cmp_value
),
);
}
},
_ => (),
}
},
BiLt | BiGe => {
match bit_op {
BiBitAnd => {
if mask_value < cmp_value {
span_lint(
cx,
BAD_BIT_MASK,
*span,
&format!(
"incompatible bit mask: `_ & {}` will always be lower than `{}`",
mask_value,
cmp_value
),
);
} else if mask_value == 0 {
span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
}
},
BiBitOr => {
if mask_value >= cmp_value {
span_lint(
cx,
BAD_BIT_MASK,
*span,
&format!(
"incompatible bit mask: `_ | {}` will never be lower than `{}`",
mask_value,
cmp_value
),
);
} else {
check_ineffective_lt(cx, *span, mask_value, cmp_value, "|");
}
},
BiBitXor => check_ineffective_lt(cx, *span, mask_value, cmp_value, "^"),
_ => (),
}
},
BiLe | BiGt => {
match bit_op {
BiBitAnd => {
if mask_value <= cmp_value {
span_lint(
cx,
BAD_BIT_MASK,
*span,
&format!(
"incompatible bit mask: `_ & {}` will never be higher than `{}`",
mask_value,
cmp_value
),
);
} else if mask_value == 0 {
span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
}
},
BiBitOr => {
if mask_value > cmp_value {
span_lint(
cx,
BAD_BIT_MASK,
*span,
&format!(
"incompatible bit mask: `_ | {}` will always be higher than `{}`",
mask_value,
cmp_value
),
);
} else {
check_ineffective_gt(cx, *span, mask_value, cmp_value, "|");
}
},
BiBitXor => check_ineffective_gt(cx, *span, mask_value, cmp_value, "^"),
_ => (),
}
},
_ => (),
}
}
fn check_ineffective_lt(cx: &LateContext, span: Span, m: u128, c: u128, op: &str) {
if c.is_power_of_two() && m < c {
span_lint(
cx,
INEFFECTIVE_BIT_MASK,
span,
&format!(
"ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
op,
m,
c
),
);
}
}
fn check_ineffective_gt(cx: &LateContext, span: Span, m: u128, c: u128, op: &str) {
if (c + 1).is_power_of_two() && m <= c {
span_lint(
cx,
INEFFECTIVE_BIT_MASK,
span,
&format!(
"ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
op,
m,
c
),
);
}
}
fn fetch_int_literal(cx: &LateContext, lit: &Expr) -> Option<u128> {
use rustc::ty::subst::Substs;
match lit.node {
ExprLit(ref lit_ptr) => {
if let LitKind::Int(value, _) = lit_ptr.node {
Some(value) // TODO: Handle sign
} else {
None
}
},
ExprPath(ref qpath) => {
let def = cx.tables.qpath_def(qpath, lit.hir_id);
if let Def::Const(def_id) = def {
lookup_const_by_id(cx.tcx, cx.param_env.and((def_id, Substs::empty()))).and_then(|(l, _ty)| {
let body = if let Some(id) = cx.tcx.hir.as_local_node_id(l) {
cx.tcx.mir_const_qualif(def_id);
cx.tcx.hir.body(cx.tcx.hir.body_owned_by(id))
} else {
cx.tcx.sess.cstore.item_body(cx.tcx, def_id)
};
fetch_int_literal(cx, &body.value)
})
} else {
None
}
},
_ => None,
}
}