268 lines
10 KiB
Rust
268 lines
10 KiB
Rust
use rustc::hir::*;
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use rustc::hir::def::Def;
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use rustc::lint::*;
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use rustc_const_eval::lookup_const_by_id;
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use syntax::ast::LitKind;
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use syntax::codemap::Span;
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use utils::span_lint;
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/// **What it does:** Checks for incompatible bit masks in comparisons.
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///
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/// The formula for detecting if an expression of the type `_ <bit_op> m
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/// <cmp_op> c` (where `<bit_op>` is one of {`&`, `|`} and `<cmp_op>` is one of
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/// {`!=`, `>=`, `>`, `!=`, `>=`, `>`}) can be determined from the following
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/// table:
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///
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/// |Comparison |Bit Op|Example |is always|Formula |
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/// |------------|------|------------|---------|----------------------|
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/// |`==` or `!=`| `&` |`x & 2 == 3`|`false` |`c & m != c` |
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/// |`<` or `>=`| `&` |`x & 2 < 3` |`true` |`m < c` |
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/// |`>` or `<=`| `&` |`x & 1 > 1` |`false` |`m <= c` |
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/// |`==` or `!=`| `|` |`x | 1 == 0`|`false` |`c | m != c` |
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/// |`<` or `>=`| `|` |`x | 1 < 1` |`false` |`m >= c` |
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/// |`<=` or `>` | `|` |`x | 1 > 0` |`true` |`m > c` |
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///
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/// **Why is this bad?** If the bits that the comparison cares about are always
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/// set to zero or one by the bit mask, the comparison is constant `true` or
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/// `false` (depending on mask, compared value, and operators).
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///
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/// So the code is actively misleading, and the only reason someone would write
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/// this intentionally is to win an underhanded Rust contest or create a
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/// test-case for this lint.
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///
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/// **Known problems:** None.
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///
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/// **Example:**
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/// ```rust
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/// if (x & 1 == 2) { … }
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/// ```
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declare_lint! {
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pub BAD_BIT_MASK,
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Warn,
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"expressions of the form `_ & mask == select` that will only ever return `true` or `false`"
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}
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/// **What it does:** Checks for bit masks in comparisons which can be removed
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/// without changing the outcome. The basic structure can be seen in the
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/// following table:
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///
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/// |Comparison| Bit Op |Example |equals |
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/// |----------|---------|-----------|-------|
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/// |`>` / `<=`|`|` / `^`|`x | 2 > 3`|`x > 3`|
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/// |`<` / `>=`|`|` / `^`|`x ^ 1 < 4`|`x < 4`|
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///
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/// **Why is this bad?** Not equally evil as [`bad_bit_mask`](#bad_bit_mask),
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/// but still a bit misleading, because the bit mask is ineffective.
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///
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/// **Known problems:** False negatives: This lint will only match instances
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/// where we have figured out the math (which is for a power-of-two compared
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/// value). This means things like `x | 1 >= 7` (which would be better written
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/// as `x >= 6`) will not be reported (but bit masks like this are fairly
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/// uncommon).
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///
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/// **Example:**
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/// ```rust
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/// if (x | 1 > 3) { … }
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/// ```
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declare_lint! {
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pub INEFFECTIVE_BIT_MASK,
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Warn,
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"expressions where a bit mask will be rendered useless by a comparison, e.g. `(x | 1) > 2`"
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}
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#[derive(Copy,Clone)]
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pub struct BitMask;
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impl LintPass for BitMask {
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fn get_lints(&self) -> LintArray {
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lint_array!(BAD_BIT_MASK, INEFFECTIVE_BIT_MASK)
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}
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}
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BitMask {
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fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
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if let ExprBinary(ref cmp, ref left, ref right) = e.node {
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if cmp.node.is_comparison() {
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if let Some(cmp_opt) = fetch_int_literal(cx, right) {
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check_compare(cx, left, cmp.node, cmp_opt, &e.span)
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} else if let Some(cmp_val) = fetch_int_literal(cx, left) {
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check_compare(cx, right, invert_cmp(cmp.node), cmp_val, &e.span)
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}
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}
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}
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}
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}
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fn invert_cmp(cmp: BinOp_) -> BinOp_ {
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match cmp {
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BiEq => BiEq,
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BiNe => BiNe,
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BiLt => BiGt,
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BiGt => BiLt,
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BiLe => BiGe,
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BiGe => BiLe,
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_ => BiOr, // Dummy
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}
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}
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fn check_compare(cx: &LateContext, bit_op: &Expr, cmp_op: BinOp_, cmp_value: u128, span: &Span) {
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if let ExprBinary(ref op, ref left, ref right) = bit_op.node {
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if op.node != BiBitAnd && op.node != BiBitOr {
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return;
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}
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fetch_int_literal(cx, right)
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.or_else(|| fetch_int_literal(cx, left))
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.map_or((), |mask| check_bit_mask(cx, op.node, cmp_op, mask, cmp_value, span))
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}
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}
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fn check_bit_mask(cx: &LateContext, bit_op: BinOp_, cmp_op: BinOp_, mask_value: u128, cmp_value: u128, span: &Span) {
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match cmp_op {
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BiEq | BiNe => {
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match bit_op {
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BiBitAnd => {
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if mask_value & cmp_value != cmp_value {
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if cmp_value != 0 {
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span_lint(cx,
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BAD_BIT_MASK,
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*span,
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&format!("incompatible bit mask: `_ & {}` can never be equal to `{}`",
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mask_value,
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cmp_value));
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}
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} else if mask_value == 0 {
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span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
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}
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},
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BiBitOr => {
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if mask_value | cmp_value != cmp_value {
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span_lint(cx,
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BAD_BIT_MASK,
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*span,
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&format!("incompatible bit mask: `_ | {}` can never be equal to `{}`",
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mask_value,
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cmp_value));
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}
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},
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_ => (),
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}
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},
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BiLt | BiGe => {
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match bit_op {
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BiBitAnd => {
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if mask_value < cmp_value {
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span_lint(cx,
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BAD_BIT_MASK,
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*span,
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&format!("incompatible bit mask: `_ & {}` will always be lower than `{}`",
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mask_value,
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cmp_value));
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} else if mask_value == 0 {
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span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
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}
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},
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BiBitOr => {
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if mask_value >= cmp_value {
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span_lint(cx,
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BAD_BIT_MASK,
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*span,
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&format!("incompatible bit mask: `_ | {}` will never be lower than `{}`",
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mask_value,
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cmp_value));
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} else {
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check_ineffective_lt(cx, *span, mask_value, cmp_value, "|");
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}
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},
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BiBitXor => check_ineffective_lt(cx, *span, mask_value, cmp_value, "^"),
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_ => (),
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}
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},
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BiLe | BiGt => {
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match bit_op {
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BiBitAnd => {
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if mask_value <= cmp_value {
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span_lint(cx,
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BAD_BIT_MASK,
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*span,
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&format!("incompatible bit mask: `_ & {}` will never be higher than `{}`",
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mask_value,
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cmp_value));
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} else if mask_value == 0 {
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span_lint(cx, BAD_BIT_MASK, *span, "&-masking with zero");
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}
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},
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BiBitOr => {
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if mask_value > cmp_value {
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span_lint(cx,
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BAD_BIT_MASK,
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*span,
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&format!("incompatible bit mask: `_ | {}` will always be higher than `{}`",
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mask_value,
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cmp_value));
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} else {
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check_ineffective_gt(cx, *span, mask_value, cmp_value, "|");
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}
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},
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BiBitXor => check_ineffective_gt(cx, *span, mask_value, cmp_value, "^"),
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_ => (),
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}
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},
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_ => (),
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}
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}
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fn check_ineffective_lt(cx: &LateContext, span: Span, m: u128, c: u128, op: &str) {
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if c.is_power_of_two() && m < c {
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span_lint(cx,
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INEFFECTIVE_BIT_MASK,
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span,
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&format!("ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
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op,
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m,
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c));
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}
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}
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fn check_ineffective_gt(cx: &LateContext, span: Span, m: u128, c: u128, op: &str) {
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if (c + 1).is_power_of_two() && m <= c {
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span_lint(cx,
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INEFFECTIVE_BIT_MASK,
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span,
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&format!("ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
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op,
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m,
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c));
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}
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}
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fn fetch_int_literal(cx: &LateContext, lit: &Expr) -> Option<u128> {
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use rustc::ty::subst::Substs;
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match lit.node {
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ExprLit(ref lit_ptr) => {
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if let LitKind::Int(value, _) = lit_ptr.node {
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Some(value) // TODO: Handle sign
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} else {
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None
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}
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},
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ExprPath(ref qpath) => {
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let def = cx.tables.qpath_def(qpath, lit.id);
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if let Def::Const(def_id) = def {
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lookup_const_by_id(cx.tcx, def_id, Substs::empty()).and_then(|(l, _ty)| {
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let body = if let Some(id) = cx.tcx.hir.as_local_node_id(l) {
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cx.tcx.mir_const_qualif(def_id);
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cx.tcx.hir.body(cx.tcx.hir.body_owned_by(id))
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} else {
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cx.tcx.sess.cstore.item_body(cx.tcx, def_id)
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};
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fetch_int_literal(cx, &body.value)
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})
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} else {
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None
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}
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},
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_ => None,
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}
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}
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