Lint unused labels

Author: mcarton

README.md

@@ -130,6 +130,7 @@ name
 [unstable_as_mut_slice](https://github.com/Manishearth/rust-clippy/wiki#unstable_as_mut_slice)                       | warn    | as_mut_slice is not stable and can be replaced by &mut v[..]see https://github.com/rust-lang/rust/issues/27729
 [unstable_as_slice](https://github.com/Manishearth/rust-clippy/wiki#unstable_as_slice)                               | warn    | as_slice is not stable and can be replaced by & v[..]see https://github.com/rust-lang/rust/issues/27729
 [unused_collect](https://github.com/Manishearth/rust-clippy/wiki#unused_collect)                                     | warn    | `collect()`ing an iterator without using the result; this is usually better written as a for loop
+[unused_label](https://github.com/Manishearth/rust-clippy/wiki#unused_label)                                         | warn    | unused label
 [unused_lifetimes](https://github.com/Manishearth/rust-clippy/wiki#unused_lifetimes)                                 | warn    | unused lifetimes in function definitions
 [use_debug](https://github.com/Manishearth/rust-clippy/wiki#use_debug)                                               | allow   | use `Debug`-based formatting
 [used_underscore_binding](https://github.com/Manishearth/rust-clippy/wiki#used_underscore_binding)                   | warn    | using a binding which is prefixed with an underscore

src/lib.rs

@@ -93,6 +93,7 @@ pub mod temporary_assignment;
 pub mod transmute;
 pub mod types;
 pub mod unicode;
+pub mod unused_label;
 pub mod vec;
 pub mod zero_div_zero;
 // end lints modules, do not remove this comment, it’s used in `update_lints`
@@ -173,6 +174,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_early_lint_pass(box formatting::Formatting);
     reg.register_late_lint_pass(box swap::Swap);
     reg.register_early_lint_pass(box if_not_else::IfNotElse);
+    reg.register_late_lint_pass(box unused_label::UnusedLabel);
 
     reg.register_lint_group("clippy_pedantic", vec![
         enum_glob_use::ENUM_GLOB_USE,
@@ -306,6 +308,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         types::TYPE_COMPLEXITY,
         types::UNIT_CMP,
         unicode::ZERO_WIDTH_SPACE,
+        unused_label::UNUSED_LABEL,
         vec::USELESS_VEC,
         zero_div_zero::ZERO_DIVIDED_BY_ZERO,
     ]);

src/unused_label.rs

@@ -0,0 +1,78 @@
+use rustc::lint::*;
+use rustc_front::hir;
+use rustc_front::intravisit::{FnKind, Visitor, walk_expr, walk_fn};
+use std::collections::HashMap;
+use syntax::ast;
+use syntax::codemap::Span;
+use syntax::parse::token::InternedString;
+use utils::{in_macro, span_lint};
+
+/// **What it does:** This lint checks for unused labels.
+///
+/// **Why is this bad?** Maybe the label should be used in which case there is an error in the
+/// code or it should be removed.
+///
+/// **Known problems:** Hopefully none.
+///
+/// **Example:**
+/// ```rust,ignore
+/// fn unused_label() {
+///     'label: for i in 1..2 {
+///         if i > 4 { continue }
+///     }
+/// ```
+declare_lint! {
+    pub UNUSED_LABEL,
+    Warn,
+    "unused label"
+}
+
+pub struct UnusedLabel;
+
+#[derive(Default)]
+struct UnusedLabelVisitor {
+    labels: HashMap<InternedString, Span>,
+}
+
+impl UnusedLabelVisitor {
+    pub fn new() -> UnusedLabelVisitor {
+        ::std::default::Default::default()
+    }
+}
+
+impl LintPass for UnusedLabel {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(UNUSED_LABEL)
+    }
+}
+
+impl LateLintPass for UnusedLabel {
+    fn check_fn(&mut self, cx: &LateContext, kind: FnKind, decl: &hir::FnDecl, body: &hir::Block, span: Span, _: ast::NodeId) {
+        if in_macro(cx, span) {
+            return;
+        }
+
+        let mut v = UnusedLabelVisitor::new();
+        walk_fn(&mut v, kind, decl, body, span);
+
+        for (label, span) in v.labels {
+            span_lint(cx, UNUSED_LABEL, span, &format!("unused label `{}`", label));
+        }
+    }
+}
+
+impl<'v> Visitor<'v> for UnusedLabelVisitor {
+    fn visit_expr(&mut self, expr: &hir::Expr) {
+        match expr.node {
+            hir::ExprBreak(Some(label)) | hir::ExprAgain(Some(label)) => {
+                self.labels.remove(&label.node.name.as_str());
+            }
+            hir::ExprLoop(_, Some(label)) | hir::ExprWhile(_, _, Some(label)) => {
+                self.labels.insert(label.name.as_str(), expr.span);
+            }
+            _ => (),
+        }
+
+        walk_expr(self, expr);
+    }
+}

tests/compile-fail/unused_labels.rs

@@ -0,0 +1,35 @@
+#![plugin(clippy)]
+#![feature(plugin)]
+
+#![allow(dead_code, items_after_statements)]
+#![deny(unused_label)]
+
+fn unused_label() {
+    'label: for i in 1..2 { //~ERROR: unused label `'label`
+        if i > 4 { continue }
+    }
+}
+
+fn foo() {
+    'same_label_in_two_fns: loop {
+        break 'same_label_in_two_fns;
+    }
+}
+
+
+fn bla() {
+    'a: loop { break } //~ERROR: unused label `'a`
+    fn blub() {}
+}
+
+fn main() {
+    'a: for _ in 0..10 {
+        while let Some(42) = None {
+            continue 'a;
+        }
+    }
+
+    'same_label_in_two_fns: loop { //~ERROR: unused label `'same_label_in_two_fns`
+        let _ = 1;
+    }
+}