clippy_lints/src/implicit_saturating_sub.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::{higher, is_integer_literal, peel_blocks_with_stmt, SpanlessEq};
   3 use if_chain::if_chain;
   4 use rustc_ast::ast::LitKind;
   5 use rustc_errors::Applicability;
   6 use rustc_hir::{BinOpKind, Expr, ExprKind, QPath};
   7 use rustc_lint::{LateContext, LateLintPass};
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9
  10 declare_clippy_lint! {
  11     /// ### What it does
  12     /// Checks for implicit saturating subtraction.
  13     ///
  14     /// ### Why is this bad?
  15     /// Simplicity and readability. Instead we can easily use an builtin function.
  16     ///
  17     /// ### Example
  18     /// ```rust
  19     /// # let end: u32 = 10;
  20     /// # let start: u32 = 5;
  21     /// let mut i: u32 = end - start;
  22     ///
  23     /// if i != 0 {
  24     ///     i -= 1;
  25     /// }
  26     /// ```
  27     ///
  28     /// Use instead:
  29     /// ```rust
  30     /// # let end: u32 = 10;
  31     /// # let start: u32 = 5;
  32     /// let mut i: u32 = end - start;
  33     ///
  34     /// i = i.saturating_sub(1);
  35     /// ```
  36     #[clippy::version = "1.44.0"]
  37     pub IMPLICIT_SATURATING_SUB,
  38     style,
  39     "Perform saturating subtraction instead of implicitly checking lower bound of data type"
  40 }
  41
  42 declare_lint_pass!(ImplicitSaturatingSub => [IMPLICIT_SATURATING_SUB]);
  43
  44 impl<'tcx> LateLintPass<'tcx> for ImplicitSaturatingSub {
  45     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
  46         if expr.span.from_expansion() {
  47             return;
  48         }
  49         if_chain! {
  50             if let Some(higher::If { cond, then, r#else: None }) = higher::If::hir(expr);
  51
  52             // Check if the conditional expression is a binary operation
  53             if let ExprKind::Binary(ref cond_op, cond_left, cond_right) = cond.kind;
  54
  55             // Ensure that the binary operator is >, !=, or <
  56             if BinOpKind::Ne == cond_op.node || BinOpKind::Gt == cond_op.node || BinOpKind::Lt == cond_op.node;
  57
  58             // Check if assign operation is done
  59             if let Some(target) = subtracts_one(cx, then);
  60
  61             // Extracting out the variable name
  62             if let ExprKind::Path(QPath::Resolved(_, ares_path)) = target.kind;
  63
  64             then {
  65                 // Handle symmetric conditions in the if statement
  66                 let (cond_var, cond_num_val) = if SpanlessEq::new(cx).eq_expr(cond_left, target) {
  67                     if BinOpKind::Gt == cond_op.node || BinOpKind::Ne == cond_op.node {
  68                         (cond_left, cond_right)
  69                     } else {
  70                         return;
  71                     }
  72                 } else if SpanlessEq::new(cx).eq_expr(cond_right, target) {
  73                     if BinOpKind::Lt == cond_op.node || BinOpKind::Ne == cond_op.node {
  74                         (cond_right, cond_left)
  75                     } else {
  76                         return;
  77                     }
  78                 } else {
  79                     return;
  80                 };
  81
  82                 // Check if the variable in the condition statement is an integer
  83                 if !cx.typeck_results().expr_ty(cond_var).is_integral() {
  84                     return;
  85                 }
  86
  87                 // Get the variable name
  88                 let var_name = ares_path.segments[0].ident.name.as_str();
  89                 match cond_num_val.kind {
  90                     ExprKind::Lit(ref cond_lit) => {
  91                         // Check if the constant is zero
  92                         if let LitKind::Int(0, _) = cond_lit.node {
  93                             if cx.typeck_results().expr_ty(cond_left).is_signed() {
  94                             } else {
  95                                 print_lint_and_sugg(cx, var_name, expr);
  96                             };
  97                         }
  98                     },
  99                     ExprKind::Path(QPath::TypeRelative(_, name)) => {
 100                         if_chain! {
 101                             if name.ident.as_str() == "MIN";
 102                             if let Some(const_id) = cx.typeck_results().type_dependent_def_id(cond_num_val.hir_id);
 103                             if let Some(impl_id) = cx.tcx.impl_of_method(const_id);
 104                             if let None = cx.tcx.impl_trait_ref(impl_id); // An inherent impl
 105                             if cx.tcx.type_of(impl_id).is_integral();
 106                             then {
 107                                 print_lint_and_sugg(cx, var_name, expr)
 108                             }
 109                         }
 110                     },
 111                     ExprKind::Call(func, []) => {
 112                         if_chain! {
 113                             if let ExprKind::Path(QPath::TypeRelative(_, name)) = func.kind;
 114                             if name.ident.as_str() == "min_value";
 115                             if let Some(func_id) = cx.typeck_results().type_dependent_def_id(func.hir_id);
 116                             if let Some(impl_id) = cx.tcx.impl_of_method(func_id);
 117                             if let None = cx.tcx.impl_trait_ref(impl_id); // An inherent impl
 118                             if cx.tcx.type_of(impl_id).is_integral();
 119                             then {
 120                                 print_lint_and_sugg(cx, var_name, expr)
 121                             }
 122                         }
 123                     },
 124                     _ => (),
 125                 }
 126             }
 127         }
 128     }
 129 }
 130
 131 fn subtracts_one<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<&'a Expr<'a>> {
 132     match peel_blocks_with_stmt(expr).kind {
 133         ExprKind::AssignOp(ref op1, target, value) => {
 134             // Check if literal being subtracted is one
 135             (BinOpKind::Sub == op1.node && is_integer_literal(value, 1)).then_some(target)
 136         },
 137         ExprKind::Assign(target, value, _) => {
 138             if_chain! {
 139                 if let ExprKind::Binary(ref op1, left1, right1) = value.kind;
 140                 if BinOpKind::Sub == op1.node;
 141
 142                 if SpanlessEq::new(cx).eq_expr(left1, target);
 143
 144                 if is_integer_literal(right1, 1);
 145                 then {
 146                     Some(target)
 147                 } else {
 148                     None
 149                 }
 150             }
 151         },
 152         _ => None,
 153     }
 154 }
 155
 156 fn print_lint_and_sugg(cx: &LateContext<'_>, var_name: &str, expr: &Expr<'_>) {
 157     span_lint_and_sugg(
 158         cx,
 159         IMPLICIT_SATURATING_SUB,
 160         expr.span,
 161         "implicitly performing saturating subtraction",
 162         "try",
 163         format!("{var_name} = {var_name}.saturating_sub({});", '1'),
 164         Applicability::MachineApplicable,
 165     );
 166 }