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 `_ m c` (where `` /// is one of {`&`, `|`} and `` 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 `_ m c` (where `` /// is one of {`&`, '|'} and `` 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 { 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, } }