src/tools/clippy/clippy_lints/src/implicit_saturating_sub.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::higher;
   3 use clippy_utils::{in_macro, SpanlessEq};
   4 use if_chain::if_chain;
   5 use rustc_ast::ast::LitKind;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{lang_items::LangItem, BinOpKind, Expr, ExprKind, QPath, StmtKind};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10
  11 declare_clippy_lint! {
  12     /// ### What it does
  13     /// Checks for implicit saturating subtraction.
  14     ///
  15     /// ### Why is this bad?
  16     /// Simplicity and readability. Instead we can easily use an builtin function.
  17     ///
  18     /// ### Example
  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 Some(higher::If { cond, then, .. }) = higher::If::hir(expr);
  47
  48             // Check if the conditional expression is a binary operation
  49             if let ExprKind::Binary(ref cond_op, cond_left, 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(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(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(_, 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: [LangItem; 5] = [
  91                     LangItem::I8,
  92                     LangItem::I16,
  93                     LangItem::I32,
  94                     LangItem::I64,
  95                     LangItem::Isize
  96                 ];
  97
  98                 match cond_num_val.kind {
  99                     ExprKind::Lit(ref cond_lit) => {
 100                         // Check if the constant is zero
 101                         if let LitKind::Int(0, _) = cond_lit.node {
 102                             if cx.typeck_results().expr_ty(cond_left).is_signed() {
 103                             } else {
 104                                 print_lint_and_sugg(cx, &var_name, expr);
 105                             };
 106                         }
 107                     },
 108                     ExprKind::Path(QPath::TypeRelative(_, name)) => {
 109                         if_chain! {
 110                             if name.ident.as_str() == "MIN";
 111                             if let Some(const_id) = cx.typeck_results().type_dependent_def_id(cond_num_val.hir_id);
 112                             if let Some(impl_id) = cx.tcx.impl_of_method(const_id);
 113                             let mut int_ids = INT_TYPES.iter().filter_map(|&ty| cx.tcx.lang_items().require(ty).ok());
 114                             if int_ids.any(|int_id| int_id == impl_id);
 115                             then {
 116                                 print_lint_and_sugg(cx, &var_name, expr)
 117                             }
 118                         }
 119                     },
 120                     ExprKind::Call(func, []) => {
 121                         if_chain! {
 122                             if let ExprKind::Path(QPath::TypeRelative(_, name)) = func.kind;
 123                             if name.ident.as_str() == "min_value";
 124                             if let Some(func_id) = cx.typeck_results().type_dependent_def_id(func.hir_id);
 125                             if let Some(impl_id) = cx.tcx.impl_of_method(func_id);
 126                             let mut int_ids = INT_TYPES.iter().filter_map(|&ty| cx.tcx.lang_items().require(ty).ok());
 127                             if int_ids.any(|int_id| int_id == impl_id);
 128                             then {
 129                                 print_lint_and_sugg(cx, &var_name, expr)
 130                             }
 131                         }
 132                     },
 133                     _ => (),
 134                 }
 135             }
 136         }
 137     }
 138 }
 139
 140 fn subtracts_one<'a>(cx: &LateContext<'_>, expr: &Expr<'a>) -> Option<&'a Expr<'a>> {
 141     match expr.kind {
 142         ExprKind::AssignOp(ref op1, target, value) => {
 143             if_chain! {
 144                 if BinOpKind::Sub == op1.node;
 145                 // Check if literal being subtracted is one
 146                 if let ExprKind::Lit(ref lit1) = value.kind;
 147                 if let LitKind::Int(1, _) = lit1.node;
 148                 then {
 149                     Some(target)
 150                 } else {
 151                     None
 152                 }
 153             }
 154         },
 155         ExprKind::Assign(target, value, _) => {
 156             if_chain! {
 157                 if let ExprKind::Binary(ref op1, left1, right1) = value.kind;
 158                 if BinOpKind::Sub == op1.node;
 159
 160                 if SpanlessEq::new(cx).eq_expr(left1, target);
 161
 162                 if let ExprKind::Lit(ref lit1) = right1.kind;
 163                 if let LitKind::Int(1, _) = lit1.node;
 164                 then {
 165                     Some(target)
 166                 } else {
 167                     None
 168                 }
 169             }
 170         },
 171         _ => None,
 172     }
 173 }
 174
 175 fn print_lint_and_sugg(cx: &LateContext<'_>, var_name: &str, expr: &Expr<'_>) {
 176     span_lint_and_sugg(
 177         cx,
 178         IMPLICIT_SATURATING_SUB,
 179         expr.span,
 180         "implicitly performing saturating subtraction",
 181         "try",
 182         format!("{} = {}.saturating_sub({});", var_name, var_name, '1'),
 183         Applicability::MachineApplicable,
 184     );
 185 }