clippy_lints/src/implicit_saturating_sub.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::{in_macro, match_qpath, 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, StmtKind};
   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:** Checks for implicit saturating subtraction.
  12     ///
  13     /// **Why is this bad?** Simplicity and readability. Instead we can easily use an builtin function.
  14     ///
  15     /// **Known problems:** None.
  16     ///
  17     /// **Example:**
  18     ///
  19     /// ```rust
  20     /// let end: u32 = 10;
  21     /// let start: u32 = 5;
  22     ///
  23     /// let mut i: u32 = end - start;
  24     ///
  25     /// // Bad
  26     /// if i != 0 {
  27     ///     i -= 1;
  28     /// }
  29     ///
  30     /// // Good
  31     /// i = i.saturating_sub(1);
  32     /// ```
  33     pub IMPLICIT_SATURATING_SUB,
  34     pedantic,
  35     "Perform saturating subtraction instead of implicitly checking lower bound of data type"
  36 }
  37
  38 declare_lint_pass!(ImplicitSaturatingSub => [IMPLICIT_SATURATING_SUB]);
  39
  40 impl<'tcx> LateLintPass<'tcx> for ImplicitSaturatingSub {
  41     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
  42         if in_macro(expr.span) {
  43             return;
  44         }
  45         if_chain! {
  46             if let ExprKind::If(cond, then, None) = &expr.kind;
  47
  48             // Check if the conditional expression is a binary operation
  49             if let ExprKind::Binary(ref cond_op, ref cond_left, ref cond_right) = cond.kind;
  50
  51             // Ensure that the binary operator is >, != and <
  52             if BinOpKind::Ne == cond_op.node || BinOpKind::Gt == cond_op.node || BinOpKind::Lt == cond_op.node;
  53
  54             // Check if the true condition block has only one statement
  55             if let ExprKind::Block(ref block, _) = then.kind;
  56             if block.stmts.len() == 1 && block.expr.is_none();
  57
  58             // Check if assign operation is done
  59             if let StmtKind::Semi(ref e) = block.stmts[0].kind;
  60             if let Some(target) = subtracts_one(cx, e);
  61
  62             // Extracting out the variable name
  63             if let ExprKind::Path(QPath::Resolved(_, ref ares_path)) = target.kind;
  64
  65             then {
  66                 // Handle symmetric conditions in the if statement
  67                 let (cond_var, cond_num_val) = if SpanlessEq::new(cx).eq_expr(cond_left, target) {
  68                     if BinOpKind::Gt == cond_op.node || BinOpKind::Ne == cond_op.node {
  69                         (cond_left, cond_right)
  70                     } else {
  71                         return;
  72                     }
  73                 } else if SpanlessEq::new(cx).eq_expr(cond_right, target) {
  74                     if BinOpKind::Lt == cond_op.node || BinOpKind::Ne == cond_op.node {
  75                         (cond_right, cond_left)
  76                     } else {
  77                         return;
  78                     }
  79                 } else {
  80                     return;
  81                 };
  82
  83                 // Check if the variable in the condition statement is an integer
  84                 if !cx.typeck_results().expr_ty(cond_var).is_integral() {
  85                     return;
  86                 }
  87
  88                 // Get the variable name
  89                 let var_name = ares_path.segments[0].ident.name.as_str();
  90                 const INT_TYPES: [&str; 5] = ["i8", "i16", "i32", "i64", "i128"];
  91
  92                 match cond_num_val.kind {
  93                     ExprKind::Lit(ref cond_lit) => {
  94                         // Check if the constant is zero
  95                         if let LitKind::Int(0, _) = cond_lit.node {
  96                             if cx.typeck_results().expr_ty(cond_left).is_signed() {
  97                             } else {
  98                                 print_lint_and_sugg(cx, &var_name, expr);
  99                             };
 100                         }
 101                     },
 102                     ExprKind::Path(ref cond_num_path) => {
 103                         if INT_TYPES.iter().any(|int_type| match_qpath(cond_num_path, &[int_type, "MIN"])) {
 104                             print_lint_and_sugg(cx, &var_name, expr);
 105                         };
 106                     },
 107                     ExprKind::Call(ref func, _) => {
 108                         if let ExprKind::Path(ref cond_num_path) = func.kind {
 109                             if INT_TYPES.iter().any(|int_type| match_qpath(cond_num_path, &[int_type, "min_value"])) {
 110                                 print_lint_and_sugg(cx, &var_name, expr);
 111                             }
 112                         };
 113                     },
 114                     _ => (),
 115                 }
 116             }
 117         }
 118     }
 119 }
 120
 121 fn subtracts_one<'a>(cx: &LateContext<'_>, expr: &Expr<'a>) -> Option<&'a Expr<'a>> {
 122     match expr.kind {
 123         ExprKind::AssignOp(ref op1, ref target, ref value) => {
 124             if_chain! {
 125                 if BinOpKind::Sub == op1.node;
 126                 // Check if literal being subtracted is one
 127                 if let ExprKind::Lit(ref lit1) = value.kind;
 128                 if let LitKind::Int(1, _) = lit1.node;
 129                 then {
 130                     Some(target)
 131                 } else {
 132                     None
 133                 }
 134             }
 135         },
 136         ExprKind::Assign(ref target, ref value, _) => {
 137             if_chain! {
 138                 if let ExprKind::Binary(ref op1, ref left1, ref right1) = value.kind;
 139                 if BinOpKind::Sub == op1.node;
 140
 141                 if SpanlessEq::new(cx).eq_expr(left1, target);
 142
 143                 if let ExprKind::Lit(ref lit1) = right1.kind;
 144                 if let LitKind::Int(1, _) = lit1.node;
 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!("{} = {}.saturating_sub({});", var_name, var_name, '1'),
 164         Applicability::MachineApplicable,
 165     );
 166 }