clippy_lints/src/casts/cast_possible_truncation.rs

   1 use clippy_utils::consts::{constant, Constant};
   2 use clippy_utils::diagnostics::span_lint;
   3 use clippy_utils::expr_or_init;
   4 use clippy_utils::ty::{get_discriminant_value, is_isize_or_usize};
   5 use rustc_hir::def::{DefKind, Res};
   6 use rustc_hir::{BinOpKind, Expr, ExprKind};
   7 use rustc_lint::LateContext;
   8 use rustc_middle::ty::{self, FloatTy, Ty};
   9 use rustc_target::abi::IntegerType;
  10
  11 use super::{utils, CAST_ENUM_TRUNCATION, CAST_POSSIBLE_TRUNCATION};
  12
  13 fn constant_int(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<u128> {
  14     if let Some((Constant::Int(c), _)) = constant(cx, cx.typeck_results(), expr) {
  15         Some(c)
  16     } else {
  17         None
  18     }
  19 }
  20
  21 fn get_constant_bits(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<u64> {
  22     constant_int(cx, expr).map(|c| u64::from(128 - c.leading_zeros()))
  23 }
  24
  25 fn apply_reductions(cx: &LateContext<'_>, nbits: u64, expr: &Expr<'_>, signed: bool) -> u64 {
  26     match expr_or_init(cx, expr).kind {
  27         ExprKind::Cast(inner, _) => apply_reductions(cx, nbits, inner, signed),
  28         ExprKind::Block(block, _) => block.expr.map_or(nbits, |e| apply_reductions(cx, nbits, e, signed)),
  29         ExprKind::Binary(op, left, right) => match op.node {
  30             BinOpKind::Div => {
  31                 apply_reductions(cx, nbits, left, signed).saturating_sub(if signed {
  32                     // let's be conservative here
  33                     0
  34                 } else {
  35                     // by dividing by 1, we remove 0 bits, etc.
  36                     get_constant_bits(cx, right).map_or(0, |b| b.saturating_sub(1))
  37                 })
  38             },
  39             BinOpKind::Rem | BinOpKind::BitAnd => get_constant_bits(cx, right)
  40                 .unwrap_or(u64::max_value())
  41                 .min(apply_reductions(cx, nbits, left, signed)),
  42             BinOpKind::Shr => apply_reductions(cx, nbits, left, signed)
  43                 .saturating_sub(constant_int(cx, right).map_or(0, |s| u64::try_from(s).expect("shift too high"))),
  44             _ => nbits,
  45         },
  46         ExprKind::MethodCall(method, left, [right], _) => {
  47             if signed {
  48                 return nbits;
  49             }
  50             let max_bits = if method.ident.as_str() == "min" {
  51                 get_constant_bits(cx, right)
  52             } else {
  53                 None
  54             };
  55             apply_reductions(cx, nbits, left, signed).min(max_bits.unwrap_or(u64::max_value()))
  56         },
  57         ExprKind::MethodCall(method, _, [lo, hi], _) => {
  58             if method.ident.as_str() == "clamp" {
  59                 //FIXME: make this a diagnostic item
  60                 if let (Some(lo_bits), Some(hi_bits)) = (get_constant_bits(cx, lo), get_constant_bits(cx, hi)) {
  61                     return lo_bits.max(hi_bits);
  62                 }
  63             }
  64             nbits
  65         },
  66         ExprKind::MethodCall(method, _value, [], _) => {
  67             if method.ident.name.as_str() == "signum" {
  68                 0 // do not lint if cast comes from a `signum` function
  69             } else {
  70                 nbits
  71             }
  72         },
  73         _ => nbits,
  74     }
  75 }
  76
  77 pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
  78     let msg = match (cast_from.kind(), cast_to.is_integral()) {
  79         (ty::Int(_) | ty::Uint(_), true) => {
  80             let from_nbits = apply_reductions(
  81                 cx,
  82                 utils::int_ty_to_nbits(cast_from, cx.tcx),
  83                 cast_expr,
  84                 cast_from.is_signed(),
  85             );
  86             let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
  87
  88             let (should_lint, suffix) = match (is_isize_or_usize(cast_from), is_isize_or_usize(cast_to)) {
  89                 (true, true) | (false, false) => (to_nbits < from_nbits, ""),
  90                 (true, false) => (
  91                     to_nbits <= 32,
  92                     if to_nbits == 32 {
  93                         " on targets with 64-bit wide pointers"
  94                     } else {
  95                         ""
  96                     },
  97                 ),
  98                 (false, true) => (from_nbits == 64, " on targets with 32-bit wide pointers"),
  99             };
 100
 101             if !should_lint {
 102                 return;
 103             }
 104
 105             format!("casting `{cast_from}` to `{cast_to}` may truncate the value{suffix}",)
 106         },
 107
 108         (ty::Adt(def, _), true) if def.is_enum() => {
 109             let (from_nbits, variant) = if let ExprKind::Path(p) = &cast_expr.kind
 110                 && let Res::Def(DefKind::Ctor(..), id) = cx.qpath_res(p, cast_expr.hir_id)
 111             {
 112                 let i = def.variant_index_with_ctor_id(id);
 113                 let variant = def.variant(i);
 114                 let nbits = utils::enum_value_nbits(get_discriminant_value(cx.tcx, *def, i));
 115                 (nbits, Some(variant))
 116             } else {
 117                 (utils::enum_ty_to_nbits(*def, cx.tcx), None)
 118             };
 119             let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);
 120
 121             let cast_from_ptr_size = def.repr().int.map_or(true, |ty| matches!(ty, IntegerType::Pointer(_),));
 122             let suffix = match (cast_from_ptr_size, is_isize_or_usize(cast_to)) {
 123                 (false, false) if from_nbits > to_nbits => "",
 124                 (true, false) if from_nbits > to_nbits => "",
 125                 (false, true) if from_nbits > 64 => "",
 126                 (false, true) if from_nbits > 32 => " on targets with 32-bit wide pointers",
 127                 _ => return,
 128             };
 129
 130             if let Some(variant) = variant {
 131                 span_lint(
 132                     cx,
 133                     CAST_ENUM_TRUNCATION,
 134                     expr.span,
 135                     &format!(
 136                         "casting `{cast_from}::{}` to `{cast_to}` will truncate the value{suffix}",
 137                         variant.name,
 138                     ),
 139                 );
 140                 return;
 141             }
 142             format!("casting `{cast_from}` to `{cast_to}` may truncate the value{suffix}",)
 143         },
 144
 145         (ty::Float(_), true) => {
 146             format!("casting `{cast_from}` to `{cast_to}` may truncate the value")
 147         },
 148
 149         (ty::Float(FloatTy::F64), false) if matches!(cast_to.kind(), &ty::Float(FloatTy::F32)) => {
 150             "casting `f64` to `f32` may truncate the value".to_string()
 151         },
 152
 153         _ => return,
 154     };
 155
 156     span_lint(cx, CAST_POSSIBLE_TRUNCATION, expr.span, &msg);
 157 }