clippy_lints/src/implicit_saturating_sub.rs

   1 use crate::utils::{higher, in_macro, match_qpath, span_lint_and_sugg, SpanlessEq};
   2 use if_chain::if_chain;
   3 use rustc_ast::ast::LitKind;
   4 use rustc_errors::Applicability;
   5 use rustc_hir::{BinOpKind, Expr, ExprKind, QPath, StmtKind};
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_session::{declare_lint_pass, declare_tool_lint};
   8
   9 declare_clippy_lint! {
  10     /// **What it does:** Checks for implicit saturating subtraction.
  11     ///
  12     /// **Why is this bad?** Simplicity and readability. Instead we can easily use an builtin function.
  13     ///
  14     /// **Known problems:** None.
  15     ///
  16     /// **Example:**
  17     ///
  18     /// ```rust
  19     /// let end: u32 = 10;
  20     /// let start: u32 = 5;
  21     ///
  22     /// let mut i: u32 = end - start;
  23     ///
  24     /// // Bad
  25     /// if i != 0 {
  26     ///     i -= 1;
  27     /// }
  28     /// ```
  29     /// Use instead:
  30     /// ```rust
  31     /// let end: u32 = 10;
  32     /// let start: u32 = 5;
  33     ///
  34     /// let mut i: u32 = end - start;
  35     ///
  36     /// // Good
  37     /// i = i.saturating_sub(1);
  38     /// ```
  39     pub IMPLICIT_SATURATING_SUB,
  40     pedantic,
  41     "Perform saturating subtraction instead of implicitly checking lower bound of data type"
  42 }
  43
  44 declare_lint_pass!(ImplicitSaturatingSub => [IMPLICIT_SATURATING_SUB]);
  45
  46 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ImplicitSaturatingSub {
  47     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'tcx>) {
  48         if in_macro(expr.span) {
  49             return;
  50         }
  51         if_chain! {
  52             if let Some((ref cond, ref then, None)) = higher::if_block(&expr);
  53
  54             // Check if the conditional expression is a binary operation
  55             if let ExprKind::Binary(ref cond_op, ref cond_left, ref cond_right) = cond.kind;
  56
  57             // Ensure that the binary operator is >, != and <
  58             if BinOpKind::Ne == cond_op.node || BinOpKind::Gt == cond_op.node || BinOpKind::Lt == cond_op.node;
  59
  60             // Check if the true condition block has only one statement
  61             if let ExprKind::Block(ref block, _) = then.kind;
  62             if block.stmts.len() == 1 && block.expr.is_none();
  63
  64             // Check if assign operation is done
  65             if let StmtKind::Semi(ref e) = block.stmts[0].kind;
  66             if let Some(target) = subtracts_one(cx, e);
  67
  68             // Extracting out the variable name
  69             if let ExprKind::Path(ref assign_path) = target.kind;
  70             if let QPath::Resolved(_, ref ares_path) = assign_path;
  71
  72             then {
  73                 // Handle symmetric conditions in the if statement
  74                 let (cond_var, cond_num_val) = if SpanlessEq::new(cx).eq_expr(cond_left, target) {
  75                     if BinOpKind::Gt == cond_op.node || BinOpKind::Ne == cond_op.node {
  76                         (cond_left, cond_right)
  77                     } else {
  78                         return;
  79                     }
  80                 } else if SpanlessEq::new(cx).eq_expr(cond_right, target) {
  81                     if BinOpKind::Lt == cond_op.node || BinOpKind::Ne == cond_op.node {
  82                         (cond_right, cond_left)
  83                     } else {
  84                         return;
  85                     }
  86                 } else {
  87                     return;
  88                 };
  89
  90                 // Check if the variable in the condition statement is an integer
  91                 if !cx.tables.expr_ty(cond_var).is_integral() {
  92                     return;
  93                 }
  94
  95                 // Get the variable name
  96                 let var_name = ares_path.segments[0].ident.name.as_str();
  97                 const INT_TYPES: [&str; 5] = ["i8", "i16", "i32", "i64", "i128"];
  98
  99                 match cond_num_val.kind {
 100                     ExprKind::Lit(ref cond_lit) => {
 101                         // Check if the constant is zero
 102                         if let LitKind::Int(0, _) = cond_lit.node {
 103                             if cx.tables.expr_ty(cond_left).is_signed() {
 104                             } else {
 105                                 print_lint_and_sugg(cx, &var_name, expr);
 106                             };
 107                         }
 108                     },
 109                     ExprKind::Path(ref cond_num_path) => {
 110                         if INT_TYPES.iter().any(|int_type| match_qpath(cond_num_path, &[int_type, "MIN"])) {
 111                             print_lint_and_sugg(cx, &var_name, expr);
 112                         };
 113                     },
 114                     ExprKind::Call(ref func, _) => {
 115                         if let ExprKind::Path(ref cond_num_path) = func.kind {
 116                             if INT_TYPES.iter().any(|int_type| match_qpath(cond_num_path, &[int_type, "min_value"])) {
 117                                 print_lint_and_sugg(cx, &var_name, expr);
 118                             }
 119                         };
 120                     },
 121                     _ => (),
 122                 }
 123             }
 124         }
 125     }
 126 }
 127
 128 fn subtracts_one<'a>(cx: &LateContext<'_, '_>, expr: &Expr<'a>) -> Option<&'a Expr<'a>> {
 129     match expr.kind {
 130         ExprKind::AssignOp(ref op1, ref target, ref value) => {
 131             if_chain! {
 132                 if BinOpKind::Sub == op1.node;
 133                 // Check if literal being subtracted is one
 134                 if let ExprKind::Lit(ref lit1) = value.kind;
 135                 if let LitKind::Int(1, _) = lit1.node;
 136                 then {
 137                     Some(target)
 138                 } else {
 139                     None
 140                 }
 141             }
 142         },
 143         ExprKind::Assign(ref target, ref value, _) => {
 144             if_chain! {
 145                 if let ExprKind::Binary(ref op1, ref left1, ref right1) = value.kind;
 146                 if BinOpKind::Sub == op1.node;
 147
 148                 if SpanlessEq::new(cx).eq_expr(left1, target);
 149
 150                 if let ExprKind::Lit(ref lit1) = right1.kind;
 151                 if let LitKind::Int(1, _) = lit1.node;
 152                 then {
 153                     Some(target)
 154                 } else {
 155                     None
 156                 }
 157             }
 158         },
 159         _ => None,
 160     }
 161 }
 162
 163 fn print_lint_and_sugg(cx: &LateContext<'_, '_>, var_name: &str, expr: &Expr<'_>) {
 164     span_lint_and_sugg(
 165         cx,
 166         IMPLICIT_SATURATING_SUB,
 167         expr.span,
 168         "Implicitly performing saturating subtraction",
 169         "try",
 170         format!("{} = {}.saturating_sub({});", var_name, var_name, 1.to_string()),
 171         Applicability::MachineApplicable,
 172     );
 173 }