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::{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                 match cond_num_val.kind {
  86                     ExprKind::Lit(ref cond_lit) => {
  87                         // Check if the constant is zero
  88                         if let LitKind::Int(0, _) = cond_lit.node {
  89                             if cx.typeck_results().expr_ty(cond_left).is_signed() {
  90                             } else {
  91                                 print_lint_and_sugg(cx, var_name, expr);
  92                             };
  93                         }
  94                     },
  95                     ExprKind::Path(QPath::TypeRelative(_, name)) => {
  96                         if_chain! {
  97                             if name.ident.as_str() == "MIN";
  98                             if let Some(const_id) = cx.typeck_results().type_dependent_def_id(cond_num_val.hir_id);
  99                             if let Some(impl_id) = cx.tcx.impl_of_method(const_id);
 100                             if let None = cx.tcx.impl_trait_ref(impl_id); // An inherent impl
 101                             if cx.tcx.type_of(impl_id).is_integral();
 102                             then {
 103                                 print_lint_and_sugg(cx, var_name, expr)
 104                             }
 105                         }
 106                     },
 107                     ExprKind::Call(func, []) => {
 108                         if_chain! {
 109                             if let ExprKind::Path(QPath::TypeRelative(_, name)) = func.kind;
 110                             if name.ident.as_str() == "min_value";
 111                             if let Some(func_id) = cx.typeck_results().type_dependent_def_id(func.hir_id);
 112                             if let Some(impl_id) = cx.tcx.impl_of_method(func_id);
 113                             if let None = cx.tcx.impl_trait_ref(impl_id); // An inherent impl
 114                             if cx.tcx.type_of(impl_id).is_integral();
 115                             then {
 116                                 print_lint_and_sugg(cx, var_name, expr)
 117                             }
 118                         }
 119                     },
 120                     _ => (),
 121                 }
 122             }
 123         }
 124     }
 125 }
 126
 127 fn subtracts_one<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<&'a Expr<'a>> {
 128     match peel_blocks_with_stmt(expr).kind {
 129         ExprKind::AssignOp(ref op1, target, value) => {
 130             if_chain! {
 131                 if BinOpKind::Sub == op1.node;
 132                 // Check if literal being subtracted is one
 133                 if let ExprKind::Lit(ref lit1) = value.kind;
 134                 if let LitKind::Int(1, _) = lit1.node;
 135                 then {
 136                     Some(target)
 137                 } else {
 138                     None
 139                 }
 140             }
 141         },
 142         ExprKind::Assign(target, value, _) => {
 143             if_chain! {
 144                 if let ExprKind::Binary(ref op1, left1, right1) = value.kind;
 145                 if BinOpKind::Sub == op1.node;
 146
 147                 if SpanlessEq::new(cx).eq_expr(left1, target);
 148
 149                 if let ExprKind::Lit(ref lit1) = right1.kind;
 150                 if let LitKind::Int(1, _) = lit1.node;
 151                 then {
 152                     Some(target)
 153                 } else {
 154                     None
 155                 }
 156             }
 157         },
 158         _ => None,
 159     }
 160 }
 161
 162 fn print_lint_and_sugg(cx: &LateContext<'_>, var_name: &str, expr: &Expr<'_>) {
 163     span_lint_and_sugg(
 164         cx,
 165         IMPLICIT_SATURATING_SUB,
 166         expr.span,
 167         "implicitly performing saturating subtraction",
 168         "try",
 169         format!("{} = {}.saturating_sub({});", var_name, var_name, '1'),
 170         Applicability::MachineApplicable,
 171     );
 172 }