Add slice_iter_nth lint

CHANGELOG.md

@@ -234,6 +234,7 @@ All notable changes to this project will be documented in this file.
 [`single_char_pattern`]: https://github.com/Manishearth/rust-clippy/wiki#single_char_pattern
 [`single_match`]: https://github.com/Manishearth/rust-clippy/wiki#single_match
 [`single_match_else`]: https://github.com/Manishearth/rust-clippy/wiki#single_match_else
+[`slice_iter_nth`]: https://github.com/Manishearth/rust-clippy/wiki#slice_iter_nth
 [`str_to_string`]: https://github.com/Manishearth/rust-clippy/wiki#str_to_string
 [`string_add`]: https://github.com/Manishearth/rust-clippy/wiki#string_add
 [`string_add_assign`]: https://github.com/Manishearth/rust-clippy/wiki#string_add_assign

README.md

@@ -17,7 +17,7 @@ Table of contents:
 
 ## Lints
 
-There are 152 lints included in this crate:
+There are 153 lints included in this crate:
 
 name                                                                                                                 | default | meaning
 ---------------------------------------------------------------------------------------------------------------------|---------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
@@ -140,6 +140,7 @@ name
 [single_char_pattern](https://github.com/Manishearth/rust-clippy/wiki#single_char_pattern)                           | warn    | using a single-character str where a char could be used, e.g. `_.split("x")`
 [single_match](https://github.com/Manishearth/rust-clippy/wiki#single_match)                                         | warn    | a match statement with a single nontrivial arm (i.e, where the other arm is `_ => {}`) is used; recommends `if let` instead
 [single_match_else](https://github.com/Manishearth/rust-clippy/wiki#single_match_else)                               | allow   | a match statement with a two arms where the second arm's pattern is a wildcard; recommends `if let` instead
+[slice_iter_nth](https://github.com/Manishearth/rust-clippy/wiki#slice_iter_nth)                                     | warn    | using `.iter().nth()` on a slice
 [string_add](https://github.com/Manishearth/rust-clippy/wiki#string_add)                                             | allow   | using `x + ..` where x is a `String`; suggests using `push_str()` instead
 [string_add_assign](https://github.com/Manishearth/rust-clippy/wiki#string_add_assign)                               | allow   | using `x = x + ..` where x is a `String`; suggests using `push_str()` instead
 [string_lit_as_bytes](https://github.com/Manishearth/rust-clippy/wiki#string_lit_as_bytes)                           | warn    | calling `as_bytes` on a string literal; suggests using a byte string literal instead

clippy_lints/src/lib.rs

@@ -357,6 +357,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         methods::SEARCH_IS_SOME,
         methods::SHOULD_IMPLEMENT_TRAIT,
         methods::SINGLE_CHAR_PATTERN,
+        methods::SLICE_ITER_NTH,
         methods::TEMPORARY_CSTRING_AS_PTR,
         methods::WRONG_SELF_CONVENTION,
         minmax::MIN_MAX,

clippy_lints/src/methods.rs

@@ -312,6 +312,28 @@ declare_lint! {
     "getting the inner pointer of a temporary `CString`"
 }
 
+/// **What it does:** This lint checks for use of `.iter().nth()` on a slice.
+///
+/// **Why is this bad?** `.get()` is more efficient and more readable.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// let some_slice = &[0, 1, 2, 3][..];
+/// let third_elem = some_slice.iter().nth(3);
+/// ```
+/// The correct use would be:
+/// ```rust
+/// let some_slice = &[0, 1, 2, 3][..];
+/// let third_elem = some_slice.get(3);
+/// ```
+declare_lint! {
+    pub SLICE_ITER_NTH,
+    Warn,
+    "using `.iter().nth()` on a slice"
+}
+
 impl LintPass for MethodsPass {
     fn get_lints(&self) -> LintArray {
         lint_array!(EXTEND_FROM_SLICE,
@@ -330,7 +352,8 @@ impl LintPass for MethodsPass {
                     NEW_RET_NO_SELF,
                     SINGLE_CHAR_PATTERN,
                     SEARCH_IS_SOME,
-                    TEMPORARY_CSTRING_AS_PTR)
+                    TEMPORARY_CSTRING_AS_PTR,
+                    SLICE_ITER_NTH)
     }
 }
 
@@ -363,6 +386,8 @@ impl LateLintPass for MethodsPass {
                     lint_extend(cx, expr, arglists[0]);
                 } else if let Some(arglists) = method_chain_args(expr, &["unwrap", "as_ptr"]) {
                     lint_cstring_as_ptr(cx, expr, &arglists[0][0], &arglists[1][0]);
+                } else if let Some(arglists) = method_chain_args(expr, &["iter", "nth"]) {
+                    lint_slice_iter_nth(cx, expr, arglists[0]);
                 }
 
                 lint_or_fun_call(cx, expr, &name.node.as_str(), args);
@@ -616,6 +641,18 @@ fn lint_cstring_as_ptr(cx: &LateContext, expr: &hir::Expr, new: &hir::Expr, unwr
     }}
 }
 
+#[allow(ptr_arg)]
+// Type of MethodArgs is potentially a Vec
+fn lint_slice_iter_nth(cx: &LateContext, expr: &hir::Expr, iter_args: &MethodArgs){
+    // lint if the caller of `.iter().nth` is a `slice`
+    if let Some(_) = derefs_to_slice(cx, &iter_args[0], &cx.tcx.expr_ty(&iter_args[0])) {
+        span_lint(cx,
+                  SLICE_ITER_NTH,
+                  expr.span,
+                  "called `.iter().nth()` on a slice. Calling `.get()` is both faster and more readable");
+    }
+}
+
 fn derefs_to_slice(cx: &LateContext, expr: &hir::Expr, ty: &ty::Ty) -> Option<(Span, &'static str)> {
     fn may_slice(cx: &LateContext, ty: &ty::Ty) -> bool {
         match ty.sty {