New lint for zip with array length instead of enumerate()

Fixes #11.

src/lib.rs

@@ -166,6 +166,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         precedence::PRECEDENCE,
         ptr_arg::PTR_ARG,
         ranges::RANGE_STEP_BY_ZERO,
+        ranges::RANGE_ZIP_WITH_LEN,
         returns::LET_AND_RETURN,
         returns::NEEDLESS_RETURN,
         types::BOX_VEC,

src/ranges.rs

@@ -1,19 +1,23 @@
 use rustc::lint::*;
 use rustc_front::hir::*;
 use syntax::codemap::Spanned;
-use utils::{match_type, is_integer_literal};
+use utils::{is_integer_literal, match_type, snippet};
 
 declare_lint! {
     pub RANGE_STEP_BY_ZERO, Warn,
     "using Range::step_by(0), which produces an infinite iterator"
 }
+declare_lint! {
+    pub RANGE_ZIP_WITH_LEN, Warn,
+    "zipping iterator with a range when enumerate() would do"
+}
 
 #[derive(Copy,Clone)]
 pub struct StepByZero;
 
 impl LintPass for StepByZero {
     fn get_lints(&self) -> LintArray {
-        lint_array!(RANGE_STEP_BY_ZERO)
+        lint_array!(RANGE_STEP_BY_ZERO, RANGE_ZIP_WITH_LEN)
     }
 }
 
@@ -21,13 +25,40 @@ impl LateLintPass for StepByZero {
     fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
         if let ExprMethodCall(Spanned { node: ref name, .. }, _,
                               ref args) = expr.node {
-            // Only warn on literal ranges.
+            // Range with step_by(0).
             if name.as_str() == "step_by" && args.len() == 2 &&
                 is_range(cx, &args[0]) && is_integer_literal(&args[1], 0) {
                 cx.span_lint(RANGE_STEP_BY_ZERO, expr.span,
                              "Range::step_by(0) produces an infinite iterator. \
                               Consider using `std::iter::repeat()` instead")
             }
+
+            // x.iter().zip(0..x.len())
+            else if name.as_str() == "zip" && args.len() == 2 {
+                let iter = &args[0].node;
+                let zip_arg = &args[1].node;
+                if_let_chain! {
+                    [
+                        // .iter() call
+                        let &ExprMethodCall( Spanned { node: ref iter_name, .. }, _, ref iter_args ) = iter,
+                        iter_name.as_str() == "iter",
+                        // range expression in .zip() call: 0..x.len()
+                        let &ExprRange(Some(ref from), Some(ref to)) = zip_arg,
+                        is_integer_literal(from, 0),
+                        // .len() call
+                        let ExprMethodCall(Spanned { node: ref len_name, .. }, _, ref len_args) = to.node,
+                        len_name.as_str() == "len" && len_args.len() == 1,
+                        // .iter() and .len() called on same Path
+                        let ExprPath(_, Path { segments: ref iter_path, .. }) = iter_args[0].node,
+                        let ExprPath(_, Path { segments: ref len_path, .. }) = len_args[0].node,
+                        iter_path == len_path
+                     ], {
+                        cx.span_lint(RANGE_ZIP_WITH_LEN, expr.span,
+                                     &format!("It is more idiomatic to use {}.iter().enumerate()",
+                                              snippet(cx, iter_args[0].span, "_")));
+                    }
+                }
+            }
         }
     }
 }