Added basic lint and tests

Author: pmk21

CHANGELOG.md

@@ -1291,6 +1291,7 @@ Released 2018-09-13
 [`ifs_same_cond`]: https://rust-lang.github.io/rust-clippy/master/index.html#ifs_same_cond
 [`implicit_hasher`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_hasher
 [`implicit_return`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_return
+[`implicit_saturating_sub`]: https://rust-lang.github.io/rust-clippy/master/index.html#implicit_saturating_sub
 [`imprecise_flops`]: https://rust-lang.github.io/rust-clippy/master/index.html#imprecise_flops
 [`inconsistent_digit_grouping`]: https://rust-lang.github.io/rust-clippy/master/index.html#inconsistent_digit_grouping
 [`indexing_slicing`]: https://rust-lang.github.io/rust-clippy/master/index.html#indexing_slicing

clippy_lints/src/implicit_saturating_sub.rs

@@ -0,0 +1,90 @@
+use crate::utils::{higher, in_macro, span_lint_and_sugg};
+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.
+    ///
+    /// **Why is this bad?** Simplicity and readability. Instead we can easily use an inbuilt function.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// let end = 10;
+    /// let start = 5;
+    ///
+    /// let mut i = end - start;
+    ///
+    /// // Bad
+    /// if i != 0 {
+    ///     i -= 1;
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let end = 10;
+    /// let start = 5;
+    ///
+    /// let mut i = end - start;
+    ///
+    /// // Good
+    /// i.saturating_sub(1);
+    /// ```
+    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<'a, 'tcx> LateLintPass<'a, 'tcx> for ImplicitSaturatingSub {
+    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'tcx>) {
+        if in_macro(expr.span) {
+            return;
+        }
+        if_chain! {
+            if let Some((ref cond, ref then, None)) = higher::if_block(&expr);
+            // Check if the conditional expression is a binary operation
+            if let ExprKind::Binary(ref op, ref left, ref right) = cond.kind;
+            // Ensure that the binary operator is > or !=
+            if BinOpKind::Ne == op.node || BinOpKind::Gt == op.node;
+            if let ExprKind::Path(ref cond_path) = left.kind;
+            // Get the literal on the right hand side
+            if let ExprKind::Lit(ref lit) = right.kind;
+            if let LitKind::Int(0, _) = lit.node;
+            // Check if the true condition block has only one statement
+            if let ExprKind::Block(ref block, _) = then.kind;
+            if block.stmts.len() == 1;
+            // Check if assign operation is done
+            if let StmtKind::Semi(ref e) = block.stmts[0].kind;
+            if let ExprKind::AssignOp(ref op1, ref target, ref value) = e.kind;
+            if BinOpKind::Sub == op1.node;
+            if let ExprKind::Path(ref assign_path) = target.kind;
+            // Check if the variable in the condition and assignment statement are the same
+            if let (QPath::Resolved(_, ref cres_path), QPath::Resolved(_, ref ares_path)) = (cond_path, assign_path);
+            if cres_path.res == ares_path.res;
+            if let ExprKind::Lit(ref lit1) = value.kind;
+            if let LitKind::Int(assign_lit, _) = lit1.node;
+            then {
+                // Get the variable name
+                let var_name = ares_path.segments[0].ident.name.as_str();
+                let applicability = Applicability::MaybeIncorrect;
+                span_lint_and_sugg(
+                    cx,
+                    IMPLICIT_SATURATING_SUB,
+                    expr.span,
+                    "Implicitly performing saturating subtraction",
+                    "try",
+                    format!("{}.saturating_sub({});", var_name, assign_lit.to_string()),
+                    applicability
+                );
+            }
+        }
+    }
+}