use rustc::hir::*;
use rustc::hir::def::{Def, PathResolution};
use rustc::lint::*;
use rustc_const_eval::lookup_const_by_id;
use syntax::ast::LitKind;
use syntax::codemap::Span;
use utils::span_lint;
/// **What it does:** This lint 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:** `x & 1 == 2` (also see table above)
declare_lint! {
pub BAD_BIT_MASK,
Warn,
"expressions of the form `_ & mask == select` that will only ever return `true` or `false` \
(because in the example `select` containing bits that `mask` doesn't have)"
}
/// **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:
///
/// |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:** `x | 1 > 3` (also see table above)
declare_lint! {
pub INEFFECTIVE_BIT_MASK,
Warn,
"expressions where a bit mask will be rendered useless by a comparison, e.g. `(x | 1) > 2`"
}
/// Checks for incompatible bit masks in comparisons, e.g. `x & 1 == 2`.
/// This cannot work because the bit that makes up the value two was
/// zeroed out by the bit-and with 1. So 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` |
///
/// This lint is **deny** by default
///
/// There is also a lint that warns on ineffective masks that is *warn*
/// by default.
///
/// |Comparison|Bit Op |Example |equals |Formula|
/// |`>` / `<=`|`|` / `^`|`x | 2 > 3`|`x > 3`|`¹ && m <= c`|
/// |`<` / `>=`|`|` / `^`|`x ^ 1 < 4`|`x < 4`|`¹ && m < c` |
///
/// `¹ power_of_two(c + 1)`
#[derive(Copy,Clone)]
pub struct BitMask;
impl LintPass for BitMask {
fn get_lints(&self) -> LintArray {
lint_array!(BAD_BIT_MASK, INEFFECTIVE_BIT_MASK)
}
}
impl LateLintPass for BitMask {
fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
if let ExprBinary(ref cmp, ref left, ref right) = e.node {
if cmp.node.is_comparison() {
fetch_int_literal(cx, right).map_or_else(|| {
fetch_int_literal(cx, left).map_or((), |cmp_val| {
check_compare(cx,
right,
invert_cmp(cmp.node),
cmp_val,
&e.span)
})
},
|cmp_opt| check_compare(cx, left, cmp.node, cmp_opt, &e.span))
}
}
}
}
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: u64, 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: u64, cmp_value: u64, 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: u64, c: u64, 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: u64, c: u64, 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<u64> {
match lit.node {
ExprLit(ref lit_ptr) => {
if let LitKind::Int(value, _) = lit_ptr.node {
Some(value) //TODO: Handle sign
} else {
None
}
}
ExprPath(_, _) => {
{
// Important to let the borrow expire before the const lookup to avoid double
// borrowing.
let def_map = cx.tcx.def_map.borrow();
match def_map.get(&lit.id) {
Some(&PathResolution { base_def: Def::Const(def_id), ..}) => Some(def_id),
_ => None,
}
}
.and_then(|def_id| lookup_const_by_id(cx.tcx, def_id, None))
.and_then(|(l, _ty)| fetch_int_literal(cx, l))
}
_ => None,
}
}