From 423a9666ca5c688dfd333cc7e6a93944b23a9f6f Mon Sep 17 00:00:00 2001
From: Matthew Hall <matthew@quickbeam.me.uk>
Date: Sun, 31 May 2015 13:17:31 +0100
Subject: [PATCH] Implements #45 - any number mod 1 will be 0

---
 src/lib.rs                       |  2 ++
 src/misc.rs                      | 31 +++++++++++++++++++++++++++++++
 tests/compile-fail/modulo_one.rs |  8 ++++++++
 3 files changed, 41 insertions(+)
 create mode 100644 tests/compile-fail/modulo_one.rs

diff --git a/src/lib.rs b/src/lib.rs
index 00d28deee..1d760bde9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -47,6 +47,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_lint_pass(box len_zero::LenZero as LintPassObject);
     reg.register_lint_pass(box misc::CmpOwned as LintPassObject);
     reg.register_lint_pass(box collapsible_if::CollapsibleIf as LintPassObject);
+    reg.register_lint_pass(box misc::ModuloOne as LintPassObject);
     
     reg.register_lint_group("clippy", vec![types::BOX_VEC, types::LINKEDLIST,
                                            misc::SINGLE_MATCH, misc::STR_TO_STRING,
@@ -64,5 +65,6 @@ pub fn plugin_registrar(reg: &mut Registry) {
                                            len_zero::LEN_ZERO,
                                            len_zero::LEN_WITHOUT_IS_EMPTY,
                                            collapsible_if::COLLAPSIBLE_IF,
+                                           misc::MODULO_ONE,
                                            ]);
 }
diff --git a/src/misc.rs b/src/misc.rs
index b8ce6d725..590ebbc3f 100644
--- a/src/misc.rs
+++ b/src/misc.rs
@@ -266,3 +266,34 @@ fn is_str_arg(cx: &Context, args: &[P<Expr>]) -> bool {
 	args.len() == 1 && if let ty_str = 
 		walk_ty(expr_ty(cx.tcx, &*args[0])).sty { true } else { false }
 }
+
+
+declare_lint!(pub MODULO_ONE, Warn, "Warn on expressions that include % 1, which is always 0");
+
+#[derive(Copy,Clone)]
+pub struct ModuloOne;
+
+impl LintPass for ModuloOne {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(MODULO_ONE)
+    }
+
+    fn check_expr(&mut self, cx: &Context, expr: &Expr) {
+        if let ExprBinary(ref cmp, _, ref right) = expr.node {
+            if let &Spanned {node: BinOp_::BiRem, ..} = cmp {
+                if is_lit_one(right) {
+                    cx.span_lint(MODULO_ONE, expr.span, "Any number modulo 1 will be 0");
+                }
+            }
+        }
+    }
+}
+
+fn is_lit_one(expr: &Expr) -> bool {
+    if let ExprLit(ref spanned) = expr.node {
+        if let LitInt(1, _) = spanned.node {
+            return true;
+        }
+    }
+    false
+}
diff --git a/tests/compile-fail/modulo_one.rs b/tests/compile-fail/modulo_one.rs
new file mode 100644
index 000000000..26c7de855
--- /dev/null
+++ b/tests/compile-fail/modulo_one.rs
@@ -0,0 +1,8 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+#![deny(modulo_one)]
+
+fn main() {
+    10 % 1; //~ERROR Any number modulo 1 will be 0
+    10 % 2;
+}