(#1955): Suggests x > y over x >= y + 1 for ints

This module handles the following cases:
- `... >= ... + 1` and `... >= 1 + ...`
- `... - 1 >= ...` and `-1 + ... >= ...`
- `... + 1 <= ...` and `... + 1 <= ...`
- `... <= ... - 1` and `... <= -1 + ...`

Note: this only goes 1 level deep (i.e., does not constant-fold) and
does not currently simplify expressions. Examples of these
cases include:
```rust
let x = 1;
y >= y + x; // won't catch this case or any permutation

x + 1 >= y + 2; // won't catch this case

x + 1 - 1 >= y - 1 + 1; // WILL catch this case when it likely shouldn't
```

clippy_lints/src/int_plus_one.rs

@@ -0,0 +1,115 @@
+//! lint on blocks unnecessarily using >= with a + 1 or - 1
+
+use rustc::lint::*;
+use syntax::ast::*;
+
+use utils::span_help_and_lint;
+
+/// **What it does:** Checks for usage of `x >= y + 1` or `x - 1 >= y` (and `<=`) in a block
+///
+///
+/// **Why is this bad?** Readability -- better to use `> y` instead of `>= y + 1`.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// x >= y + 1
+/// ```
+///
+/// Could be written:
+///
+/// ```rust
+/// x > y
+/// ```
+declare_lint! {
+    pub INT_PLUS_ONE,
+    Allow,
+    "instead of using x >= y + 1, use x > y"
+}
+
+pub struct IntPlusOne;
+
+impl LintPass for IntPlusOne {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(INT_PLUS_ONE)
+    }
+}
+
+// cases:
+// BinOpKind::Ge
+// x >= y + 1
+// x - 1 >= y
+//
+// BinOpKind::Le
+// x + 1 <= y
+// x <= y - 1
+
+impl IntPlusOne {
+    #[allow(cast_sign_loss)]
+    fn check_lit(&self, lit: &Lit, target_value: i128) -> bool {
+        if let LitKind::Int(value, ..) = lit.node {
+            return value == (target_value as u128)
+        }
+        false
+    }
+
+    fn check_binop(&self, binop: BinOpKind, lhs: &Expr, rhs: &Expr) -> bool {
+        match (binop, &lhs.node, &rhs.node) {
+            // case where `x - 1 >= ...` or `-1 + x >= ...`
+            (BinOpKind::Ge, &ExprKind::Binary(ref lhskind, ref lhslhs, ref lhsrhs), _) => {
+                match (lhskind.node, &lhslhs.node, &lhsrhs.node) {
+                    // `-1 + x`
+                    (BinOpKind::Add, &ExprKind::Lit(ref lit), _) => self.check_lit(lit, -1),
+                    // `x - 1`
+                    (BinOpKind::Sub, _, &ExprKind::Lit(ref lit)) => self.check_lit(lit, 1),
+                    _ => false
+                }
+            },
+            // case where `... >= y + 1` or `... >= 1 + y`
+            (BinOpKind::Ge, _, &ExprKind::Binary(ref rhskind, ref rhslhs, ref rhsrhs)) if rhskind.node == BinOpKind::Add => {
+                match (&rhslhs.node, &rhsrhs.node) {
+                    // `y + 1` and `1 + y`
+                    (&ExprKind::Lit(ref lit), _)|(_, &ExprKind::Lit(ref lit)) => self.check_lit(lit, 1),
+                    _ => false
+                }
+            },
+            // case where `x + 1 <= ...` or `1 + x <= ...`
+            (BinOpKind::Le, &ExprKind::Binary(ref lhskind, ref lhslhs, ref lhsrhs), _) if lhskind.node == BinOpKind::Add => {
+                match (&lhslhs.node, &lhsrhs.node) {
+                    // `1 + x` and `x + 1`
+                    (&ExprKind::Lit(ref lit), _)|(_, &ExprKind::Lit(ref lit)) => self.check_lit(lit, 1),
+                    _ => false
+                }
+            },
+            // case where `... >= y - 1` or `... >= -1 + y`
+            (BinOpKind::Le, _, &ExprKind::Binary(ref rhskind, ref rhslhs, ref rhsrhs)) => {
+                match (rhskind.node, &rhslhs.node, &rhsrhs.node) {
+                    // `-1 + y`
+                    (BinOpKind::Add, &ExprKind::Lit(ref lit), _) => self.check_lit(lit, -1),
+                    // `y - 1`
+                    (BinOpKind::Sub, _, &ExprKind::Lit(ref lit)) => self.check_lit(lit, 1),
+                    _ => false
+                }
+            },
+            _ => false
+        }
+    }
+
+}
+
+impl EarlyLintPass for IntPlusOne {
+    fn check_expr(&mut self, cx: &EarlyContext, item: &Expr) {
+        if let ExprKind::Binary(ref kind, ref lhs, ref rhs) = item.node {
+            if self.check_binop(kind.node, lhs, rhs) {
+                span_help_and_lint(
+                    cx,
+                    INT_PLUS_ONE,
+                    item.span,
+                    "Unnecessary `>= y + 1` or `x - 1 >=`",
+                    "Consider reducing `x >= y + 1` or `x - 1 >= y` to `x > y`",
+                );
+            }
+        }
+    }
+}