clippy_lints/src/implicit_saturating_sub.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::{in_macro, SpanlessEq};
   3 use if_chain::if_chain;
   4 use rustc_ast::ast::LitKind;
   5 use rustc_errors::Applicability;
   6 use rustc_hir::{lang_items::LangItem, 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
  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     ///
  22     /// let mut i: u32 = end - start;
  23     ///
  24     /// // Bad
  25     /// if i != 0 {
  26     ///     i -= 1;
  27     /// }
  28     ///
  29     /// // Good
  30     /// i = i.saturating_sub(1);
  31     /// ```
  32     pub IMPLICIT_SATURATING_SUB,
  33     pedantic,
  34     "Perform saturating subtraction instead of implicitly checking lower bound of data type"
  35 }
  36
  37 declare_lint_pass!(ImplicitSaturatingSub => [IMPLICIT_SATURATING_SUB]);
  38
  39 impl<'tcx> LateLintPass<'tcx> for ImplicitSaturatingSub {
  40     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
  41         if in_macro(expr.span) {
  42             return;
  43         }
  44         if_chain! {
  45             if let ExprKind::If(cond, then, None) = &expr.kind;
  46
  47             // Check if the conditional expression is a binary operation
  48             if let ExprKind::Binary(ref cond_op, cond_left, cond_right) = cond.kind;
  49
  50             // Ensure that the binary operator is >, != and <
  51             if BinOpKind::Ne == cond_op.node || BinOpKind::Gt == cond_op.node || BinOpKind::Lt == cond_op.node;
  52
  53             // Check if the true condition block has only one statement
  54             if let ExprKind::Block(block, _) = then.kind;
  55             if block.stmts.len() == 1 && block.expr.is_none();
  56
  57             // Check if assign operation is done
  58             if let StmtKind::Semi(e) = block.stmts[0].kind;
  59             if let Some(target) = subtracts_one(cx, e);
  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                 const INT_TYPES: [LangItem; 5] = [
  90                     LangItem::I8,
  91                     LangItem::I16,
  92                     LangItem::I32,
  93                     LangItem::I64,
  94                     LangItem::Isize
  95                 ];
  96
  97                 match cond_num_val.kind {
  98                     ExprKind::Lit(ref cond_lit) => {
  99                         // Check if the constant is zero
 100                         if let LitKind::Int(0, _) = cond_lit.node {
 101                             if cx.typeck_results().expr_ty(cond_left).is_signed() {
 102                             } else {
 103                                 print_lint_and_sugg(cx, &var_name, expr);
 104                             };
 105                         }
 106                     },
 107                     ExprKind::Path(QPath::TypeRelative(_, name)) => {
 108                         if_chain! {
 109                             if name.ident.as_str() == "MIN";
 110                             if let Some(const_id) = cx.typeck_results().type_dependent_def_id(cond_num_val.hir_id);
 111                             if let Some(impl_id) = cx.tcx.impl_of_method(const_id);
 112                             let mut int_ids = INT_TYPES.iter().filter_map(|&ty| cx.tcx.lang_items().require(ty).ok());
 113                             if int_ids.any(|int_id| int_id == impl_id);
 114                             then {
 115                                 print_lint_and_sugg(cx, &var_name, expr)
 116                             }
 117                         }
 118                     },
 119                     ExprKind::Call(func, []) => {
 120                         if_chain! {
 121                             if let ExprKind::Path(QPath::TypeRelative(_, name)) = func.kind;
 122                             if name.ident.as_str() == "min_value";
 123                             if let Some(func_id) = cx.typeck_results().type_dependent_def_id(func.hir_id);
 124                             if let Some(impl_id) = cx.tcx.impl_of_method(func_id);
 125                             let mut int_ids = INT_TYPES.iter().filter_map(|&ty| cx.tcx.lang_items().require(ty).ok());
 126                             if int_ids.any(|int_id| int_id == impl_id);
 127                             then {
 128                                 print_lint_and_sugg(cx, &var_name, expr)
 129                             }
 130                         }
 131                     },
 132                     _ => (),
 133                 }
 134             }
 135         }
 136     }
 137 }
 138
 139 fn subtracts_one<'a>(cx: &LateContext<'_>, expr: &Expr<'a>) -> Option<&'a Expr<'a>> {
 140     match expr.kind {
 141         ExprKind::AssignOp(ref op1, target, value) => {
 142             if_chain! {
 143                 if BinOpKind::Sub == op1.node;
 144                 // Check if literal being subtracted is one
 145                 if let ExprKind::Lit(ref lit1) = value.kind;
 146                 if let LitKind::Int(1, _) = lit1.node;
 147                 then {
 148                     Some(target)
 149                 } else {
 150                     None
 151                 }
 152             }
 153         },
 154         ExprKind::Assign(target, value, _) => {
 155             if_chain! {
 156                 if let ExprKind::Binary(ref op1, left1, right1) = value.kind;
 157                 if BinOpKind::Sub == op1.node;
 158
 159                 if SpanlessEq::new(cx).eq_expr(left1, target);
 160
 161                 if let ExprKind::Lit(ref lit1) = right1.kind;
 162                 if let LitKind::Int(1, _) = lit1.node;
 163                 then {
 164                     Some(target)
 165                 } else {
 166                     None
 167                 }
 168             }
 169         },
 170         _ => None,
 171     }
 172 }
 173
 174 fn print_lint_and_sugg(cx: &LateContext<'_>, var_name: &str, expr: &Expr<'_>) {
 175     span_lint_and_sugg(
 176         cx,
 177         IMPLICIT_SATURATING_SUB,
 178         expr.span,
 179         "implicitly performing saturating subtraction",
 180         "try",
 181         format!("{} = {}.saturating_sub({});", var_name, var_name, '1'),
 182         Applicability::MachineApplicable,
 183     );
 184 }