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::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     #[clippy::version = "1.44.0"]
  34     pub IMPLICIT_SATURATING_SUB,
  35     pedantic,
  36     "Perform saturating subtraction instead of implicitly checking lower bound of data type"
  37 }
  38
  39 declare_lint_pass!(ImplicitSaturatingSub => [IMPLICIT_SATURATING_SUB]);
  40
  41 impl<'tcx> LateLintPass<'tcx> for ImplicitSaturatingSub {
  42     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
  43         if expr.span.from_expansion() {
  44             return;
  45         }
  46         if_chain! {
  47             if let Some(higher::If { cond, then, r#else: None }) = higher::If::hir(expr);
  48
  49             // Check if the conditional expression is a binary operation
  50             if let ExprKind::Binary(ref cond_op, cond_left, cond_right) = cond.kind;
  51
  52             // Ensure that the binary operator is >, != and <
  53             if BinOpKind::Ne == cond_op.node || BinOpKind::Gt == cond_op.node || BinOpKind::Lt == cond_op.node;
  54
  55             // Check if the true condition block has only one statement
  56             if let ExprKind::Block(block, _) = then.kind;
  57             if block.stmts.len() == 1 && block.expr.is_none();
  58
  59             // Check if assign operation is done
  60             if let StmtKind::Semi(e) = block.stmts[0].kind;
  61             if let Some(target) = subtracts_one(cx, e);
  62
  63             // Extracting out the variable name
  64             if let ExprKind::Path(QPath::Resolved(_, ares_path)) = target.kind;
  65
  66             then {
  67                 // Handle symmetric conditions in the if statement
  68                 let (cond_var, cond_num_val) = if SpanlessEq::new(cx).eq_expr(cond_left, target) {
  69                     if BinOpKind::Gt == cond_op.node || BinOpKind::Ne == cond_op.node {
  70                         (cond_left, cond_right)
  71                     } else {
  72                         return;
  73                     }
  74                 } else if SpanlessEq::new(cx).eq_expr(cond_right, target) {
  75                     if BinOpKind::Lt == cond_op.node || BinOpKind::Ne == cond_op.node {
  76                         (cond_right, cond_left)
  77                     } else {
  78                         return;
  79                     }
  80                 } else {
  81                     return;
  82                 };
  83
  84                 // Check if the variable in the condition statement is an integer
  85                 if !cx.typeck_results().expr_ty(cond_var).is_integral() {
  86                     return;
  87                 }
  88
  89                 // Get the variable name
  90                 let var_name = ares_path.segments[0].ident.name.as_str();
  91                 const INT_TYPES: [LangItem; 5] = [
  92                     LangItem::I8,
  93                     LangItem::I16,
  94                     LangItem::I32,
  95                     LangItem::I64,
  96                     LangItem::Isize
  97                 ];
  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.typeck_results().expr_ty(cond_left).is_signed() {
 104                             } else {
 105                                 print_lint_and_sugg(cx, &var_name, expr);
 106                             };
 107                         }
 108                     },
 109                     ExprKind::Path(QPath::TypeRelative(_, name)) => {
 110                         if_chain! {
 111                             if name.ident.as_str() == "MIN";
 112                             if let Some(const_id) = cx.typeck_results().type_dependent_def_id(cond_num_val.hir_id);
 113                             if let Some(impl_id) = cx.tcx.impl_of_method(const_id);
 114                             let mut int_ids = INT_TYPES.iter().filter_map(|&ty| cx.tcx.lang_items().require(ty).ok());
 115                             if int_ids.any(|int_id| int_id == impl_id);
 116                             then {
 117                                 print_lint_and_sugg(cx, &var_name, expr)
 118                             }
 119                         }
 120                     },
 121                     ExprKind::Call(func, []) => {
 122                         if_chain! {
 123                             if let ExprKind::Path(QPath::TypeRelative(_, name)) = func.kind;
 124                             if name.ident.as_str() == "min_value";
 125                             if let Some(func_id) = cx.typeck_results().type_dependent_def_id(func.hir_id);
 126                             if let Some(impl_id) = cx.tcx.impl_of_method(func_id);
 127                             let mut int_ids = INT_TYPES.iter().filter_map(|&ty| cx.tcx.lang_items().require(ty).ok());
 128                             if int_ids.any(|int_id| int_id == impl_id);
 129                             then {
 130                                 print_lint_and_sugg(cx, &var_name, expr)
 131                             }
 132                         }
 133                     },
 134                     _ => (),
 135                 }
 136             }
 137         }
 138     }
 139 }
 140
 141 fn subtracts_one<'a>(cx: &LateContext<'_>, expr: &Expr<'a>) -> Option<&'a Expr<'a>> {
 142     match expr.kind {
 143         ExprKind::AssignOp(ref op1, target, value) => {
 144             if_chain! {
 145                 if BinOpKind::Sub == op1.node;
 146                 // Check if literal being subtracted is one
 147                 if let ExprKind::Lit(ref lit1) = value.kind;
 148                 if let LitKind::Int(1, _) = lit1.node;
 149                 then {
 150                     Some(target)
 151                 } else {
 152                     None
 153                 }
 154             }
 155         },
 156         ExprKind::Assign(target, value, _) => {
 157             if_chain! {
 158                 if let ExprKind::Binary(ref op1, left1, right1) = value.kind;
 159                 if BinOpKind::Sub == op1.node;
 160
 161                 if SpanlessEq::new(cx).eq_expr(left1, target);
 162
 163                 if let ExprKind::Lit(ref lit1) = right1.kind;
 164                 if let LitKind::Int(1, _) = lit1.node;
 165                 then {
 166                     Some(target)
 167                 } else {
 168                     None
 169                 }
 170             }
 171         },
 172         _ => None,
 173     }
 174 }
 175
 176 fn print_lint_and_sugg(cx: &LateContext<'_>, var_name: &str, expr: &Expr<'_>) {
 177     span_lint_and_sugg(
 178         cx,
 179         IMPLICIT_SATURATING_SUB,
 180         expr.span,
 181         "implicitly performing saturating subtraction",
 182         "try",
 183         format!("{} = {}.saturating_sub({});", var_name, var_name, '1'),
 184         Applicability::MachineApplicable,
 185     );
 186 }