rust-clippy/clippy_lints/src/implicit_saturating_sub.rs

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use crate::utils::{in_macro, match_qpath, SpanlessEq};
use clippy_utils::diagnostics::span_lint_and_sugg;
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use if_chain::if_chain;
use rustc_ast::ast::LitKind;
use rustc_errors::Applicability;
use rustc_hir::{BinOpKind, Expr, ExprKind, QPath, StmtKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
declare_clippy_lint! {
/// **What it does:** Checks for implicit saturating subtraction.
///
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/// **Why is this bad?** Simplicity and readability. Instead we can easily use an builtin function.
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///
/// **Known problems:** None.
///
/// **Example:**
///
/// ```rust
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/// let end: u32 = 10;
/// let start: u32 = 5;
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///
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/// let mut i: u32 = end - start;
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///
/// // Bad
/// if i != 0 {
/// i -= 1;
/// }
///
/// // Good
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/// i = i.saturating_sub(1);
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/// ```
pub IMPLICIT_SATURATING_SUB,
pedantic,
"Perform saturating subtraction instead of implicitly checking lower bound of data type"
}
declare_lint_pass!(ImplicitSaturatingSub => [IMPLICIT_SATURATING_SUB]);
impl<'tcx> LateLintPass<'tcx> for ImplicitSaturatingSub {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
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if in_macro(expr.span) {
return;
}
if_chain! {
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if let ExprKind::If(cond, then, None) = &expr.kind;
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// Check if the conditional expression is a binary operation
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if let ExprKind::Binary(ref cond_op, ref cond_left, ref cond_right) = cond.kind;
// Ensure that the binary operator is >, != and <
if BinOpKind::Ne == cond_op.node || BinOpKind::Gt == cond_op.node || BinOpKind::Lt == cond_op.node;
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// Check if the true condition block has only one statement
if let ExprKind::Block(ref block, _) = then.kind;
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if block.stmts.len() == 1 && block.expr.is_none();
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// Check if assign operation is done
if let StmtKind::Semi(ref e) = block.stmts[0].kind;
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if let Some(target) = subtracts_one(cx, e);
// Extracting out the variable name
if let ExprKind::Path(QPath::Resolved(_, ref ares_path)) = target.kind;
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then {
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// Handle symmetric conditions in the if statement
let (cond_var, cond_num_val) = if SpanlessEq::new(cx).eq_expr(cond_left, target) {
if BinOpKind::Gt == cond_op.node || BinOpKind::Ne == cond_op.node {
(cond_left, cond_right)
} else {
return;
}
} else if SpanlessEq::new(cx).eq_expr(cond_right, target) {
if BinOpKind::Lt == cond_op.node || BinOpKind::Ne == cond_op.node {
(cond_right, cond_left)
} else {
return;
}
} else {
return;
};
// Check if the variable in the condition statement is an integer
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if !cx.typeck_results().expr_ty(cond_var).is_integral() {
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return;
}
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// Get the variable name
let var_name = ares_path.segments[0].ident.name.as_str();
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const INT_TYPES: [&str; 5] = ["i8", "i16", "i32", "i64", "i128"];
match cond_num_val.kind {
ExprKind::Lit(ref cond_lit) => {
// Check if the constant is zero
if let LitKind::Int(0, _) = cond_lit.node {
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if cx.typeck_results().expr_ty(cond_left).is_signed() {
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} else {
print_lint_and_sugg(cx, &var_name, expr);
};
}
},
ExprKind::Path(ref cond_num_path) => {
if INT_TYPES.iter().any(|int_type| match_qpath(cond_num_path, &[int_type, "MIN"])) {
print_lint_and_sugg(cx, &var_name, expr);
};
},
ExprKind::Call(ref func, _) => {
if let ExprKind::Path(ref cond_num_path) = func.kind {
if INT_TYPES.iter().any(|int_type| match_qpath(cond_num_path, &[int_type, "min_value"])) {
print_lint_and_sugg(cx, &var_name, expr);
}
};
},
_ => (),
}
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}
}
}
}
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fn subtracts_one<'a>(cx: &LateContext<'_>, expr: &Expr<'a>) -> Option<&'a Expr<'a>> {
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match expr.kind {
ExprKind::AssignOp(ref op1, ref target, ref value) => {
if_chain! {
if BinOpKind::Sub == op1.node;
// Check if literal being subtracted is one
if let ExprKind::Lit(ref lit1) = value.kind;
if let LitKind::Int(1, _) = lit1.node;
then {
Some(target)
} else {
None
}
}
},
ExprKind::Assign(ref target, ref value, _) => {
if_chain! {
if let ExprKind::Binary(ref op1, ref left1, ref right1) = value.kind;
if BinOpKind::Sub == op1.node;
if SpanlessEq::new(cx).eq_expr(left1, target);
if let ExprKind::Lit(ref lit1) = right1.kind;
if let LitKind::Int(1, _) = lit1.node;
then {
Some(target)
} else {
None
}
}
},
_ => None,
}
}
fn print_lint_and_sugg(cx: &LateContext<'_>, var_name: &str, expr: &Expr<'_>) {
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span_lint_and_sugg(
cx,
IMPLICIT_SATURATING_SUB,
expr.span,
"implicitly performing saturating subtraction",
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"try",
format!("{} = {}.saturating_sub({});", var_name, var_name, '1'),
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Applicability::MachineApplicable,
);
}