Add lint for iter.nth(0)

- Encourage iter.next() rather than iter.nth(0),
  which is less readable

CHANGELOG.md

@@ -1141,6 +1141,7 @@ Released 2018-09-13
 [`iter_cloned_collect`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_cloned_collect
 [`iter_next_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_next_loop
 [`iter_nth`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_nth
+[`iter_nth_zero`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_nth_zero
 [`iter_skip_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_skip_next
 [`iterator_step_by_zero`]: https://rust-lang.github.io/rust-clippy/master/index.html#iterator_step_by_zero
 [`just_underscores_and_digits`]: https://rust-lang.github.io/rust-clippy/master/index.html#just_underscores_and_digits

clippy_lints/src/iter_nth_zero.rs

@@ -0,0 +1,57 @@
+use crate::utils::span_lint_and_sugg;
+
+use rustc::declare_lint_pass;
+use rustc::lint::{EarlyContext, EarlyLintPass, LintArray, LintPass};
+use rustc_errors::Applicability;
+use rustc_session::declare_tool_lint;
+use syntax::ast::{Expr, ExprKind, PathSegment};
+
+declare_clippy_lint! {
+  /// **What it does:** Checks for the use of `iter.nth(0)`.
+  ///
+  /// **Why is this bad?** `iter.nth(0)` is unnecessary, and `iter.next()`
+  /// is more readable.
+  ///
+  /// **Known problems:** None.
+  ///
+  /// **Example:**
+  ///
+  /// ```rust,ignore
+  /// // Bad
+  ///  let mut s = HashSet::new();
+  ///  s.insert(1);
+  ///  let x = s.iter().nth(0);
+  ///
+  /// // Good
+  ///  let mut s = HashSet::new();
+  ///  s.insert(1);
+  ///  let x = s.iter().next();
+  /// ```
+  pub ITER_NTH_ZERO,
+  style,
+  "replace `iter.nth(0)` with `iter.next()`"
+}
+
+declare_lint_pass!(IterNthZero => [ITER_NTH_ZERO]);
+
+impl EarlyLintPass for IterNthZero {
+    fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &Expr) {
+        if let ExprKind::MethodCall(PathSegment { ident, .. }, args) = &expr.kind {
+            if ident.name.as_str() == "nth" {
+                if let ExprKind::Lit(literal) = &args[1].kind {
+                    if literal.token.symbol.as_str() == "0" {
+                        span_lint_and_sugg(
+                            cx,
+                            ITER_NTH_ZERO,
+                            expr.span,
+                            "calling `iter.nth(0)` is redundant",
+                            "try",
+                            "using `iter.next()` instead".to_string(),
+                            Applicability::MachineApplicable,
+                        );
+                    }
+                }
+            }
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -217,6 +217,7 @@ pub mod inline_fn_without_body;
 pub mod int_plus_one;
 pub mod integer_division;
 pub mod items_after_statements;
+pub mod iter_nth_zero;
 pub mod large_enum_variant;
 pub mod large_stack_arrays;
 pub mod len_zero;
@@ -554,6 +555,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         &int_plus_one::INT_PLUS_ONE,
         &integer_division::INTEGER_DIVISION,
         &items_after_statements::ITEMS_AFTER_STATEMENTS,
+        &iter_nth_zero::ITER_NTH_ZERO,
         &large_enum_variant::LARGE_ENUM_VARIANT,
         &large_stack_arrays::LARGE_STACK_ARRAYS,
         &len_zero::LEN_WITHOUT_IS_EMPTY,
@@ -980,6 +982,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
     store.register_early_pass(|| box as_conversions::AsConversions);
     store.register_early_pass(|| box utils::internal_lints::ProduceIce);
     store.register_late_pass(|| box let_underscore::LetUnderscore);
+    store.register_early_pass(|| box iter_nth_zero::IterNthZero);
 
     store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
         LintId::of(&arithmetic::FLOAT_ARITHMETIC),
@@ -1146,6 +1149,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         LintId::of(&inherent_to_string::INHERENT_TO_STRING_SHADOW_DISPLAY),
         LintId::of(&inline_fn_without_body::INLINE_FN_WITHOUT_BODY),
         LintId::of(&int_plus_one::INT_PLUS_ONE),
+        LintId::of(&iter_nth_zero::ITER_NTH_ZERO),
         LintId::of(&large_enum_variant::LARGE_ENUM_VARIANT),
         LintId::of(&len_zero::LEN_WITHOUT_IS_EMPTY),
         LintId::of(&len_zero::LEN_ZERO),
@@ -1352,6 +1356,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         LintId::of(&functions::MUST_USE_UNIT),
         LintId::of(&infallible_destructuring_match::INFALLIBLE_DESTRUCTURING_MATCH),
         LintId::of(&inherent_to_string::INHERENT_TO_STRING),
+        LintId::of(&iter_nth_zero::ITER_NTH_ZERO),
         LintId::of(&len_zero::LEN_WITHOUT_IS_EMPTY),
         LintId::of(&len_zero::LEN_ZERO),
         LintId::of(&let_if_seq::USELESS_LET_IF_SEQ),

src/lintlist/mod.rs

@@ -875,6 +875,13 @@ pub const ALL_LINTS: [Lint; 342] = [
         deprecation: None,
         module: "methods",
     },
+    Lint {
+        name: "iter_nth_zero",
+        group: "style",
+        desc: "replace `iter.nth(0)` with `iter.next()`",
+        deprecation: None,
+        module: "iter_nth_zero",
+    },
     Lint {
         name: "iter_skip_next",
         group: "style",

tests/ui/iter_nth_zero.rs

@@ -0,0 +1,8 @@
+#![warn(clippy::iter_nth_zero)]
+use std::collections::HashSet;
+
+fn main() {
+    let mut s = HashSet::new();
+    s.insert(1);
+    let x = s.iter().nth(0);
+}

tests/ui/iter_nth_zero.stderr

@@ -0,0 +1,10 @@
+error: calling `iter.nth(0)` is redundant
+  --> $DIR/iter_nth_zero.rs:7:13
+   |
+LL |     let x = s.iter().nth(0);
+   |             ^^^^^^^^^^^^^^^ help: try: `using `iter.next()` instead`
+   |
+   = note: `-D clippy::iter-nth-zero` implied by `-D warnings`
+
+error: aborting due to previous error
+