clippy_lints/src/implicit_saturating_sub.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::{higher, peel_blocks_with_stmt, 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};
   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     #[clippy::version = "1.44.0"]
  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 expr.span.from_expansion() {
  43             return;
  44         }
  45         if_chain! {
  46             if let Some(higher::If { cond, then, r#else: None }) = 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 assign operation is done
  55             if let Some(target) = subtracts_one(cx, then);
  56
  57             // Extracting out the variable name
  58             if let ExprKind::Path(QPath::Resolved(_, ares_path)) = target.kind;
  59
  60             then {
  61                 // Handle symmetric conditions in the if statement
  62                 let (cond_var, cond_num_val) = if SpanlessEq::new(cx).eq_expr(cond_left, target) {
  63                     if BinOpKind::Gt == cond_op.node || BinOpKind::Ne == cond_op.node {
  64                         (cond_left, cond_right)
  65                     } else {
  66                         return;
  67                     }
  68                 } else if SpanlessEq::new(cx).eq_expr(cond_right, target) {
  69                     if BinOpKind::Lt == cond_op.node || BinOpKind::Ne == cond_op.node {
  70                         (cond_right, cond_left)
  71                     } else {
  72                         return;
  73                     }
  74                 } else {
  75                     return;
  76                 };
  77
  78                 // Check if the variable in the condition statement is an integer
  79                 if !cx.typeck_results().expr_ty(cond_var).is_integral() {
  80                     return;
  81                 }
  82
  83                 // Get the variable name
  84                 let var_name = ares_path.segments[0].ident.name.as_str();
  85                 const INT_TYPES: [LangItem; 5] = [
  86                     LangItem::I8,
  87                     LangItem::I16,
  88                     LangItem::I32,
  89                     LangItem::I64,
  90                     LangItem::Isize
  91                 ];
  92
  93                 match cond_num_val.kind {
  94                     ExprKind::Lit(ref cond_lit) => {
  95                         // Check if the constant is zero
  96                         if let LitKind::Int(0, _) = cond_lit.node {
  97                             if cx.typeck_results().expr_ty(cond_left).is_signed() {
  98                             } else {
  99                                 print_lint_and_sugg(cx, var_name, expr);
 100                             };
 101                         }
 102                     },
 103                     ExprKind::Path(QPath::TypeRelative(_, name)) => {
 104                         if_chain! {
 105                             if name.ident.as_str() == "MIN";
 106                             if let Some(const_id) = cx.typeck_results().type_dependent_def_id(cond_num_val.hir_id);
 107                             if let Some(impl_id) = cx.tcx.impl_of_method(const_id);
 108                             let mut int_ids = INT_TYPES.iter().filter_map(|&ty| cx.tcx.lang_items().require(ty).ok());
 109                             if int_ids.any(|int_id| int_id == impl_id);
 110                             then {
 111                                 print_lint_and_sugg(cx, var_name, expr)
 112                             }
 113                         }
 114                     },
 115                     ExprKind::Call(func, []) => {
 116                         if_chain! {
 117                             if let ExprKind::Path(QPath::TypeRelative(_, name)) = func.kind;
 118                             if name.ident.as_str() == "min_value";
 119                             if let Some(func_id) = cx.typeck_results().type_dependent_def_id(func.hir_id);
 120                             if let Some(impl_id) = cx.tcx.impl_of_method(func_id);
 121                             let mut int_ids = INT_TYPES.iter().filter_map(|&ty| cx.tcx.lang_items().require(ty).ok());
 122                             if int_ids.any(|int_id| int_id == impl_id);
 123                             then {
 124                                 print_lint_and_sugg(cx, var_name, expr)
 125                             }
 126                         }
 127                     },
 128                     _ => (),
 129                 }
 130             }
 131         }
 132     }
 133 }
 134
 135 fn subtracts_one<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<&'a Expr<'a>> {
 136     match peel_blocks_with_stmt(expr).kind {
 137         ExprKind::AssignOp(ref op1, target, value) => {
 138             if_chain! {
 139                 if BinOpKind::Sub == op1.node;
 140                 // Check if literal being subtracted is one
 141                 if let ExprKind::Lit(ref lit1) = value.kind;
 142                 if let LitKind::Int(1, _) = lit1.node;
 143                 then {
 144                     Some(target)
 145                 } else {
 146                     None
 147                 }
 148             }
 149         },
 150         ExprKind::Assign(target, value, _) => {
 151             if_chain! {
 152                 if let ExprKind::Binary(ref op1, left1, right1) = value.kind;
 153                 if BinOpKind::Sub == op1.node;
 154
 155                 if SpanlessEq::new(cx).eq_expr(left1, target);
 156
 157                 if let ExprKind::Lit(ref lit1) = right1.kind;
 158                 if let LitKind::Int(1, _) = lit1.node;
 159                 then {
 160                     Some(target)
 161                 } else {
 162                     None
 163                 }
 164             }
 165         },
 166         _ => None,
 167     }
 168 }
 169
 170 fn print_lint_and_sugg(cx: &LateContext<'_>, var_name: &str, expr: &Expr<'_>) {
 171     span_lint_and_sugg(
 172         cx,
 173         IMPLICIT_SATURATING_SUB,
 174         expr.span,
 175         "implicitly performing saturating subtraction",
 176         "try",
 177         format!("{} = {}.saturating_sub({});", var_name, var_name, '1'),
 178         Applicability::MachineApplicable,
 179     );
 180 }