Add linter for a single element for loop

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

CHANGELOG.md

@@ -1815,6 +1815,7 @@ Released 2018-09-13
 [`single_char_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern
 [`single_char_push_str`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_char_push_str
 [`single_component_path_imports`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_component_path_imports
+[`single_element_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_element_loop
 [`single_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match
 [`single_match_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
 [`skip_while_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#skip_while_next

clippy_lints/src/lib.rs

@@ -627,6 +627,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &loops::NEEDLESS_RANGE_LOOP,
         &loops::NEVER_LOOP,
         &loops::SAME_ITEM_PUSH,
+        &loops::SINGLE_ELEMENT_LOOP,
         &loops::WHILE_IMMUTABLE_CONDITION,
         &loops::WHILE_LET_LOOP,
         &loops::WHILE_LET_ON_ITERATOR,
@@ -1353,6 +1354,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&loops::NEEDLESS_RANGE_LOOP),
         LintId::of(&loops::NEVER_LOOP),
         LintId::of(&loops::SAME_ITEM_PUSH),
+        LintId::of(&loops::SINGLE_ELEMENT_LOOP),
         LintId::of(&loops::WHILE_IMMUTABLE_CONDITION),
         LintId::of(&loops::WHILE_LET_LOOP),
         LintId::of(&loops::WHILE_LET_ON_ITERATOR),
@@ -1563,6 +1565,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&loops::FOR_KV_MAP),
         LintId::of(&loops::NEEDLESS_RANGE_LOOP),
         LintId::of(&loops::SAME_ITEM_PUSH),
+        LintId::of(&loops::SINGLE_ELEMENT_LOOP),
         LintId::of(&loops::WHILE_LET_ON_ITERATOR),
         LintId::of(&main_recursion::MAIN_RECURSION),
         LintId::of(&manual_async_fn::MANUAL_ASYNC_FN),

clippy_lints/src/loops.rs

@@ -453,6 +453,31 @@ declare_clippy_lint! {
     "the same item is pushed inside of a for loop"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks whether a for loop has a single element.
+    ///
+    /// **Why is this bad?** There is no reason to have a loop of a
+    /// single element.
+    /// **Known problems:** None
+    ///
+    /// **Example:**
+    /// ```rust
+    /// let item1 = 2;
+    /// for item in &[item1] {
+    ///     println!("{}", item);
+    /// }
+    /// ```
+    /// could be written as
+    /// ```rust
+    /// let item1 = 2;
+    /// let item = &item1;
+    /// println!("{}", item);
+    /// ```
+    pub SINGLE_ELEMENT_LOOP,
+    style,
+    "there is no reason to have a single element loop"
+}
+
 declare_lint_pass!(Loops => [
     MANUAL_MEMCPY,
     NEEDLESS_RANGE_LOOP,
@@ -470,6 +495,7 @@ declare_lint_pass!(Loops => [
     MUT_RANGE_BOUND,
     WHILE_IMMUTABLE_CONDITION,
     SAME_ITEM_PUSH,
+    SINGLE_ELEMENT_LOOP,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Loops {
@@ -774,6 +800,7 @@ fn check_for_loop<'tcx>(
     check_for_loop_explicit_counter(cx, pat, arg, body, expr);
     check_for_loop_over_map_kv(cx, pat, arg, body, expr);
     check_for_mut_range_bound(cx, arg, body);
+    check_for_single_element_loop(cx, pat, arg, body, expr);
     detect_manual_memcpy(cx, pat, arg, body, expr);
     detect_same_item_push(cx, pat, arg, body, expr);
 }
@@ -1684,6 +1711,27 @@ fn check_for_loop_over_map_kv<'tcx>(
     }
 }
 
+fn check_for_single_element_loop<'tcx>(
+    cx: &LateContext<'tcx>,
+    _: &'tcx Pat<'_>,
+    arg: &'tcx Expr<'_>,
+    _: &'tcx Expr<'_>,
+    _: &'tcx Expr<'_>,
+) {
+    if let ExprKind::AddrOf(BorrowKind::Ref, _, ref expr) = arg.kind {
+        if let ExprKind::Array(ref expr_list) = expr.kind {
+            if expr_list.len() == 1 {
+                span_lint(
+                    cx,
+                    SINGLE_ELEMENT_LOOP,
+                    expr.span,
+                    "do not iterate over a single element array",
+                );
+            }
+        }
+    }
+}
+
 struct MutatePairDelegate<'a, 'tcx> {
     cx: &'a LateContext<'tcx>,
     hir_id_low: Option<HirId>,

src/lintlist/mod.rs

@@ -2110,6 +2110,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "single_component_path_imports",
     },
+    Lint {
+        name: "single_element_loop",
+        group: "style",
+        desc: "there is no reason to have a single element loop",
+        deprecation: None,
+        module: "loops",
+    },
     Lint {
         name: "single_match",
         group: "style",

tests/ui/single_element_loop.rs

@@ -0,0 +1,9 @@
+// Tests from for_loop.rs that don't have suggestions
+
+#[warn(clippy::single_element_loop)]
+fn main() {
+    let item1 = 2;
+    for item in &[item1] {
+        println!("{}", item);
+    }
+}

tests/ui/single_element_loop.stderr

@@ -0,0 +1,10 @@
+error: do not iterate over a single element array.
+  --> $DIR/single_element_loop.rs:6:18
+   |
+LL |     for item in &[item1] {
+   |                  ^^^^^^^
+   |
+   = note: `-D clippy::single-element-loop` implied by `-D warnings`
+
+error: aborting due to previous error
+