Add lint checks for unused loop labels

src/librustc_lint/lib.rs

@@ -110,6 +110,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
 
     add_early_builtin_with_new!(sess,
                                 DeprecatedAttr,
+                                UnusedLoopLabel,
                                 );
 
     add_builtin!(sess,
@@ -177,7 +178,8 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                     UNUSED_DOC_COMMENT,
                     UNUSED_EXTERN_CRATES,
                     UNUSED_FEATURES,
-                    UNUSED_PARENS);
+                    UNUSED_PARENS,
+                    UNUSED_LOOP_LABEL);
 
     add_lint_group!(sess,
                     "rust_2018_idioms",

src/librustc_lint/unused.rs

@@ -25,6 +25,8 @@ use syntax_pos::Span;
 
 use rustc::hir;
 
+use std::vec;
+
 declare_lint! {
     pub UNUSED_MUST_USE,
     Warn,
@@ -464,3 +466,64 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedAllocation {
         }
     }
 }
+
+declare_lint! {
+    pub(super) UNUSED_LOOP_LABEL,
+    Warn,
+    "warns on unused labels for loops"
+}
+
+#[derive(Clone)]
+pub struct UnusedLoopLabel(pub vec::Vec<ast::Label>);
+
+impl UnusedLoopLabel {
+    pub fn new() -> Self {
+        UnusedLoopLabel(vec![])
+    }
+}
+
+impl LintPass for UnusedLoopLabel {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(UNUSED_LOOP_LABEL)
+    }
+}
+
+impl EarlyLintPass for UnusedLoopLabel {
+    fn check_expr(&mut self, _: &EarlyContext, expr: &ast::Expr) {
+        match expr.node {
+            ast::ExprKind::While(_, _, Some(ref label))
+            | ast::ExprKind::WhileLet(_, _, _, Some(ref label))
+            | ast::ExprKind::ForLoop(_, _, _, Some(ref label))
+            | ast::ExprKind::Loop(_, Some(ref label)) => {
+                self.0.push(*label);
+            }
+            ast::ExprKind::Break(Some(ref label), _) | ast::ExprKind::Continue(Some(ref label)) => {
+                'remove_used_label: for i in (0..self.0.len()).rev() {
+                    if self.0.get(i).unwrap().ident.name == label.ident.name {
+                        self.0.remove(i);
+                        break 'remove_used_label;
+                    }
+                }
+            }
+            _ => {}
+        }
+    }
+
+    fn check_expr_post(&mut self, ctxt: &EarlyContext, expr: &ast::Expr) {
+        match expr.node {
+            ast::ExprKind::While(_, _, Some(ref label))
+            | ast::ExprKind::WhileLet(_, _, _, Some(ref label))
+            | ast::ExprKind::ForLoop(_, _, _, Some(ref label))
+            | ast::ExprKind::Loop(_, Some(ref label)) => if !self.0.is_empty() {
+                {
+                    let unused_label = self.0.last().unwrap();
+                    if label.ident.name == unused_label.ident.name {
+                        ctxt.span_lint(UNUSED_LOOP_LABEL, label.ident.span, "unused loop label");
+                    }
+                }
+                self.0.pop();
+            },
+            _ => {}
+        }
+    }
+}

src/librustc_mir/interpret/eval_context.rs

@@ -1705,7 +1705,7 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M
             let mut trace_text = "\n\nAn error occurred in miri:\n".to_string();
             backtrace.resolve();
             write!(trace_text, "backtrace frames: {}\n", backtrace.frames().len()).unwrap();
-            'frames: for (i, frame) in backtrace.frames().iter().enumerate() {
+            for (i, frame) in backtrace.frames().iter().enumerate() {
                 if frame.symbols().is_empty() {
                     write!(trace_text, "{}: no symbols\n", i).unwrap();
                 }

src/test/ui/lint/unused_loop_label.rs

@@ -0,0 +1,55 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// The output should warn when a loop label is not used. However, it
+// should also deal with the edge cases where a label is shadowed,
+// within nested loops
+
+// compile-pass
+// compile-flags: -W unused_loop_label
+
+fn main() {
+    'unused_while_label: while 0 == 0 {
+        //~^ WARN unused loop label
+    }
+
+    let opt = Some(0);
+    'unused_while_let_label: while let Some(_) = opt {
+        //~^ WARN unused loop label
+    }
+
+    'unused_for_label: for _ in 0..10 {
+        //~^ WARN unused loop label
+    }
+
+    'used_loop_label: loop {
+        break 'used_loop_label;
+    }
+
+    'used_loop_label_outer: loop {
+        'used_loop_label_inner: loop {
+            break 'used_loop_label_inner;
+        }
+        break 'used_loop_label_outer;
+    }
+
+    'unused_loop_label_outer: loop {
+        'unused_loop_label_inner: loop {
+            //~^ WARN unused loop label
+            break 'unused_loop_label_outer;
+        }
+    }
+
+    // This is diverging, so put it at the end so we don't get unreachable_code errors everywhere
+    // else
+    'unused_loop_label: loop {
+        //~^ WARN unused loop label
+    }
+}

src/test/ui/lint/unused_loop_label.stderr

@@ -0,0 +1,32 @@
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:19:5
+   |
+LL |     'unused_while_label: while 0 == 0 {
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: requested on the command line with `-W unused-loop-label`
+
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:24:5
+   |
+LL |     'unused_while_let_label: while let Some(_) = opt {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:28:5
+   |
+LL |     'unused_for_label: for _ in 0..10 {
+   |     ^^^^^^^^^^^^^^^^^
+
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:44:9
+   |
+LL |         'unused_loop_label_inner: loop {
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:52:5
+   |
+LL |     'unused_loop_label: loop {
+   |     ^^^^^^^^^^^^^^^^^^
+