333 lines
12 KiB
Rust
333 lines
12 KiB
Rust
use clippy_utils::consts::{constant, Constant};
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use clippy_utils::diagnostics::{span_lint, span_lint_and_then};
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use clippy_utils::sugg::Sugg;
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use if_chain::if_chain;
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use rustc_ast::ast::LitKind;
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use rustc_errors::Applicability;
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use rustc_hir::{BinOpKind, Expr, ExprKind};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_session::{declare_tool_lint, impl_lint_pass};
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use rustc_span::source_map::Span;
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declare_clippy_lint! {
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/// ### What it does
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/// 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?
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/// 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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/// ### Example
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/// ```rust
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/// # let x = 1;
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/// if (x & 1 == 2) { }
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/// ```
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pub BAD_BIT_MASK,
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correctness,
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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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declare_clippy_lint! {
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/// ### What it does
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/// 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?
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/// 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
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/// 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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/// # let x = 1;
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/// if (x | 1 > 3) { }
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/// ```
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pub INEFFECTIVE_BIT_MASK,
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correctness,
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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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declare_clippy_lint! {
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/// ### What it does
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/// Checks for bit masks that can be replaced by a call
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/// to `trailing_zeros`
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///
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/// ### Why is this bad?
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/// `x.trailing_zeros() > 4` is much clearer than `x & 15
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/// == 0`
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///
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/// ### Known problems
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/// llvm generates better code for `x & 15 == 0` on x86
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///
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/// ### Example
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/// ```rust
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/// # let x = 1;
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/// if x & 0b1111 == 0 { }
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/// ```
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pub VERBOSE_BIT_MASK,
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pedantic,
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"expressions where a bit mask is less readable than the corresponding method call"
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}
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#[derive(Copy, Clone)]
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pub struct BitMask {
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verbose_bit_mask_threshold: u64,
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}
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impl BitMask {
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#[must_use]
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pub fn new(verbose_bit_mask_threshold: u64) -> Self {
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Self {
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verbose_bit_mask_threshold,
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}
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}
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}
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impl_lint_pass!(BitMask => [BAD_BIT_MASK, INEFFECTIVE_BIT_MASK, VERBOSE_BIT_MASK]);
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impl<'tcx> LateLintPass<'tcx> for BitMask {
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fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
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if let ExprKind::Binary(cmp, left, right) = &e.kind {
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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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if_chain! {
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if let ExprKind::Binary(op, left, right) = &e.kind;
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if BinOpKind::Eq == op.node;
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if let ExprKind::Binary(op1, left1, right1) = &left.kind;
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if BinOpKind::BitAnd == op1.node;
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if let ExprKind::Lit(lit) = &right1.kind;
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if let LitKind::Int(n, _) = lit.node;
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if let ExprKind::Lit(lit1) = &right.kind;
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if let LitKind::Int(0, _) = lit1.node;
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if n.leading_zeros() == n.count_zeros();
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if n > u128::from(self.verbose_bit_mask_threshold);
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then {
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span_lint_and_then(cx,
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VERBOSE_BIT_MASK,
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e.span,
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"bit mask could be simplified with a call to `trailing_zeros`",
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|diag| {
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let sugg = Sugg::hir(cx, left1, "...").maybe_par();
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diag.span_suggestion(
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e.span,
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"try",
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format!("{}.trailing_zeros() >= {}", sugg, n.count_ones()),
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Applicability::MaybeIncorrect,
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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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#[must_use]
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fn invert_cmp(cmp: BinOpKind) -> BinOpKind {
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match cmp {
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BinOpKind::Eq => BinOpKind::Eq,
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BinOpKind::Ne => BinOpKind::Ne,
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BinOpKind::Lt => BinOpKind::Gt,
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BinOpKind::Gt => BinOpKind::Lt,
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BinOpKind::Le => BinOpKind::Ge,
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BinOpKind::Ge => BinOpKind::Le,
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_ => BinOpKind::Or, // Dummy
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}
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}
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fn check_compare(cx: &LateContext<'_>, bit_op: &Expr<'_>, cmp_op: BinOpKind, cmp_value: u128, span: Span) {
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if let ExprKind::Binary(op, left, right) = &bit_op.kind {
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if op.node != BinOpKind::BitAnd && op.node != BinOpKind::BitOr {
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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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#[allow(clippy::too_many_lines)]
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fn check_bit_mask(
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cx: &LateContext<'_>,
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bit_op: BinOpKind,
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cmp_op: BinOpKind,
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mask_value: u128,
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cmp_value: u128,
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span: Span,
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) {
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match cmp_op {
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BinOpKind::Eq | BinOpKind::Ne => match bit_op {
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BinOpKind::BitAnd => {
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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(
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cx,
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BAD_BIT_MASK,
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span,
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&format!(
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"incompatible bit mask: `_ & {}` can never be equal to `{}`",
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mask_value, cmp_value
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),
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);
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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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BinOpKind::BitOr => {
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if mask_value | cmp_value != cmp_value {
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span_lint(
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cx,
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BAD_BIT_MASK,
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span,
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&format!(
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"incompatible bit mask: `_ | {}` can never be equal to `{}`",
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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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BinOpKind::Lt | BinOpKind::Ge => match bit_op {
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BinOpKind::BitAnd => {
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if mask_value < cmp_value {
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span_lint(
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cx,
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BAD_BIT_MASK,
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span,
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&format!(
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"incompatible bit mask: `_ & {}` will always be lower than `{}`",
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mask_value, cmp_value
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),
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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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BinOpKind::BitOr => {
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if mask_value >= cmp_value {
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span_lint(
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cx,
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BAD_BIT_MASK,
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span,
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&format!(
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"incompatible bit mask: `_ | {}` will never be lower than `{}`",
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mask_value, cmp_value
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),
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);
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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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BinOpKind::BitXor => check_ineffective_lt(cx, span, mask_value, cmp_value, "^"),
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_ => (),
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},
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BinOpKind::Le | BinOpKind::Gt => match bit_op {
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BinOpKind::BitAnd => {
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if mask_value <= cmp_value {
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span_lint(
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cx,
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BAD_BIT_MASK,
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span,
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&format!(
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"incompatible bit mask: `_ & {}` will never be higher than `{}`",
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mask_value, cmp_value
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),
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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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BinOpKind::BitOr => {
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if mask_value > cmp_value {
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span_lint(
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cx,
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BAD_BIT_MASK,
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span,
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&format!(
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"incompatible bit mask: `_ | {}` will always be higher than `{}`",
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mask_value, cmp_value
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),
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);
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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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BinOpKind::BitXor => 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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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(
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cx,
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INEFFECTIVE_BIT_MASK,
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span,
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&format!(
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"ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
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op, m, c
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),
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);
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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(
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cx,
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INEFFECTIVE_BIT_MASK,
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span,
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&format!(
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"ineffective bit mask: `x {} {}` compared to `{}`, is the same as x compared directly",
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op, m, c
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),
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);
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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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match constant(cx, cx.typeck_results(), lit)?.0 {
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Constant::Int(n) => Some(n),
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_ => None,
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}
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}
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