Auto merge of #50763 - KyleStach1678:unused-loop-label, r=petrochenkov

Add lint checks for unused loop labels

Previously no warning was generated when a loop label was written out but never used (i.e. in a `break` or `continue` statement):
```rust
fn main() {
    'unused_label: loop {}
}
```
would compile without complaint.

This fix introduces `-W unused_loop_label`, which generates the following warning message for the above snippet:
```
warning: unused loop label
 --> main.rs:2:5
  |
2 |     'unused_label: loop {}
  |     ^^^^^^^^^^^^^
  |
  = note: #[warn(unused_loop_label)] on by default
```

Fixes: #50751.

src/librustc/lint/builtin.rs

@@ -273,6 +273,12 @@ declare_lint! {
     "detects name collision with an existing but unstable method"
 }
 
+declare_lint! {
+    pub UNUSED_LABELS,
+    Allow,
+    "detects labels that are never used"
+}
+
 /// Does nothing as a lint pass, but registers some `Lint`s
 /// which are used by other parts of the compiler.
 #[derive(Copy, Clone)]
@@ -318,6 +324,7 @@ impl LintPass for HardwiredLints {
             UNUSED_MUT,
             SINGLE_USE_LIFETIME,
             UNUSED_LIFETIME,
+            UNUSED_LABELS,
             TYVAR_BEHIND_RAW_POINTER,
             ELIDED_LIFETIME_IN_PATH,
             BARE_TRAIT_OBJECT,

src/librustc_lint/lib.rs

@@ -177,6 +177,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                     UNUSED_DOC_COMMENT,
                     UNUSED_EXTERN_CRATES,
                     UNUSED_FEATURES,
+                    UNUSED_LABELS,
                     UNUSED_PARENS);
 
     add_lint_group!(sess,

src/librustc_resolve/check_unused.rs

@@ -142,6 +142,10 @@ pub fn check_crate(resolver: &mut Resolver, krate: &ast::Crate) {
         }
     }
 
+    for (id, span) in resolver.unused_labels.iter() {
+        resolver.session.buffer_lint(lint::builtin::UNUSED_LABELS, *id, *span, "unused label");
+    }
+
     let mut visitor = UnusedImportCheckVisitor {
         resolver,
         unused_imports: NodeMap(),

src/librustc_resolve/lib.rs

@@ -1468,6 +1468,10 @@ pub struct Resolver<'a> {
     pub maybe_unused_trait_imports: NodeSet,
     pub maybe_unused_extern_crates: Vec<(NodeId, Span)>,
 
+    /// A list of labels as of yet unused. Labels will be removed from this map when
+    /// they are used (in a `break` or `continue` statement)
+    pub unused_labels: FxHashMap<NodeId, Span>,
+
     /// privacy errors are delayed until the end in order to deduplicate them
     privacy_errors: Vec<PrivacyError<'a>>,
     /// ambiguity errors are delayed for deduplication
@@ -1747,6 +1751,8 @@ impl<'a> Resolver<'a> {
             maybe_unused_trait_imports: NodeSet(),
             maybe_unused_extern_crates: Vec::new(),
 
+            unused_labels: FxHashMap(),
+
             privacy_errors: Vec::new(),
             ambiguity_errors: Vec::new(),
             use_injections: Vec::new(),
@@ -3682,6 +3688,7 @@ impl<'a> Resolver<'a> {
         where F: FnOnce(&mut Resolver)
     {
         if let Some(label) = label {
+            self.unused_labels.insert(id, label.ident.span);
             let def = Def::Label(id);
             self.with_label_rib(|this| {
                 this.label_ribs.last_mut().unwrap().bindings.insert(label.ident, def);
@@ -3730,9 +3737,10 @@ impl<'a> Resolver<'a> {
                                       ResolutionError::UndeclaredLabel(&label.ident.name.as_str(),
                                                                        close_match));
                     }
-                    Some(def @ Def::Label(_)) => {
+                    Some(Def::Label(id)) => {
                         // Since this def is a label, it is never read.
-                        self.record_def(expr.id, PathResolution::new(def));
+                        self.record_def(expr.id, PathResolution::new(Def::Label(id)));
+                        self.unused_labels.remove(&id);
                     }
                     Some(_) => {
                         span_bug!(expr.span, "label wasn't mapped to a label def!");

src/test/ui/label_break_value_continue.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(label_break_value)]
+#![allow(unused_labels)]
 
 // Simple continue pointing to an unlabeled break should yield in an error
 fn continue_simple() {

src/test/ui/label_break_value_continue.stderr

@@ -1,25 +1,25 @@
 error[E0695]: unlabeled `continue` inside of a labeled block
-  --> $DIR/label_break_value_continue.rs:16:9
+  --> $DIR/label_break_value_continue.rs:17:9
    |
 LL |         continue; //~ ERROR unlabeled `continue` inside of a labeled block
    |         ^^^^^^^^ `continue` statements that would diverge to or through a labeled block need to bear a label
 
 error[E0696]: `continue` pointing to a labeled block
-  --> $DIR/label_break_value_continue.rs:23:9
+  --> $DIR/label_break_value_continue.rs:24:9
    |
 LL |         continue 'b; //~ ERROR `continue` pointing to a labeled block
    |         ^^^^^^^^^^^ labeled blocks cannot be `continue`'d
    |
 note: labeled block the continue points to
-  --> $DIR/label_break_value_continue.rs:22:5
+  --> $DIR/label_break_value_continue.rs:23:5
    |
 LL | /     'b: {
 LL | |         continue 'b; //~ ERROR `continue` pointing to a labeled block
 LL | |     }
    | |_____^
 
 error[E0695]: unlabeled `continue` inside of a labeled block
-  --> $DIR/label_break_value_continue.rs:31:13
+  --> $DIR/label_break_value_continue.rs:32:13
    |
 LL |             continue; //~ ERROR unlabeled `continue` inside of a labeled block
    |             ^^^^^^^^ `continue` statements that would diverge to or through a labeled block need to bear a label

src/test/ui/label_break_value_unlabeled_break.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(label_break_value)]
+#![allow(unused_labels)]
 
 // Simple unlabeled break should yield in an error
 fn unlabeled_break_simple() {

src/test/ui/label_break_value_unlabeled_break.stderr

@@ -1,11 +1,11 @@
 error[E0695]: unlabeled `break` inside of a labeled block
-  --> $DIR/label_break_value_unlabeled_break.rs:16:9
+  --> $DIR/label_break_value_unlabeled_break.rs:17:9
    |
 LL |         break; //~ ERROR unlabeled `break` inside of a labeled block
    |         ^^^^^ `break` statements that would diverge to or through a labeled block need to bear a label
 
 error[E0695]: unlabeled `break` inside of a labeled block
-  --> $DIR/label_break_value_unlabeled_break.rs:24:13
+  --> $DIR/label_break_value_unlabeled_break.rs:25:13
    |
 LL |             break; //~ ERROR unlabeled `break` inside of a labeled block
    |             ^^^^^ `break` statements that would diverge to or through a labeled block need to bear a label

src/test/ui/lint/unused_labels.rs

@@ -0,0 +1,96 @@
+// 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
+
+#![feature(label_break_value)]
+#![warn(unused_labels)]
+
+fn main() {
+    'unused_while_label: while 0 == 0 {
+        //~^ WARN unused label
+    }
+
+    let opt = Some(0);
+    'unused_while_let_label: while let Some(_) = opt {
+        //~^ WARN unused label
+    }
+
+    'unused_for_label: for _ in 0..10 {
+        //~^ WARN unused label
+    }
+
+    'used_loop_label: loop {
+        break 'used_loop_label;
+    }
+
+    'used_loop_label_outer_1: for _ in 0..10 {
+        'used_loop_label_inner_1: for _ in 0..10 {
+            break 'used_loop_label_inner_1;
+        }
+        break 'used_loop_label_outer_1;
+    }
+
+    'used_loop_label_outer_2: for _ in 0..10 {
+        'unused_loop_label_inner_2: for _ in 0..10 {
+            //~^ WARN unused label
+            break 'used_loop_label_outer_2;
+        }
+    }
+
+    'unused_loop_label_outer_3: for _ in 0..10 {
+        //~^ WARN unused label
+        'used_loop_label_inner_3: for _ in 0..10 {
+            break 'used_loop_label_inner_3;
+        }
+    }
+
+    // You should be able to break the same label many times
+    'many_used: loop {
+        if true {
+            break 'many_used;
+        } else {
+            break 'many_used;
+        }
+    }
+
+    // Test breaking many times with the same inner label doesn't break the
+    // warning on the outer label
+    'many_used_shadowed: for _ in 0..10 {
+        //~^ WARN unused label
+        'many_used_shadowed: for _ in 0..10 {
+            //~^ WARN label name `'many_used_shadowed` shadows a label name that is already in scope
+            if 1 % 2 == 0 {
+                break 'many_used_shadowed;
+            } else {
+                break 'many_used_shadowed;
+            }
+        }
+    }
+
+    'unused_loop_label: loop {
+        //~^ WARN unused label
+        break;
+    }
+
+    // Make sure unused block labels give warnings...
+    'unused_block_label: {
+        //~^ WARN unused label
+    }
+
+    // ...and that used ones don't:
+    'used_block_label: {
+        break 'used_block_label;
+    }
+}

src/test/ui/lint/unused_labels.stderr

@@ -0,0 +1,63 @@
+warning: unused label
+  --> $DIR/unused_labels.rs:21:5
+   |
+LL |     'unused_while_label: while 0 == 0 {
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/unused_labels.rs:18:9
+   |
+LL | #![warn(unused_labels)]
+   |         ^^^^^^^^^^^^^
+
+warning: unused label
+  --> $DIR/unused_labels.rs:26:5
+   |
+LL |     'unused_while_let_label: while let Some(_) = opt {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unused label
+  --> $DIR/unused_labels.rs:30:5
+   |
+LL |     'unused_for_label: for _ in 0..10 {
+   |     ^^^^^^^^^^^^^^^^^
+
+warning: unused label
+  --> $DIR/unused_labels.rs:46:9
+   |
+LL |         'unused_loop_label_inner_2: for _ in 0..10 {
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unused label
+  --> $DIR/unused_labels.rs:52:5
+   |
+LL |     'unused_loop_label_outer_3: for _ in 0..10 {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unused label
+  --> $DIR/unused_labels.rs:70:5
+   |
+LL |     'many_used_shadowed: for _ in 0..10 {
+   |     ^^^^^^^^^^^^^^^^^^^
+
+warning: unused label
+  --> $DIR/unused_labels.rs:82:5
+   |
+LL |     'unused_loop_label: loop {
+   |     ^^^^^^^^^^^^^^^^^^
+
+warning: unused label
+  --> $DIR/unused_labels.rs:88:5
+   |
+LL |     'unused_block_label: {
+   |     ^^^^^^^^^^^^^^^^^^^
+
+warning: label name `'many_used_shadowed` shadows a label name that is already in scope
+  --> $DIR/unused_labels.rs:72:9
+   |
+LL |     'many_used_shadowed: for _ in 0..10 {
+   |     ------------------- first declared here
+LL |         //~^ WARN unused label
+LL |         'many_used_shadowed: for _ in 0..10 {
+   |         ^^^^^^^^^^^^^^^^^^^ lifetime 'many_used_shadowed already in scope
+

src/test/ui/loops-reject-duplicate-labels-2.rs

@@ -18,6 +18,7 @@
 // discussed here:
 // https://internals.rust-lang.org/t/psa-rejecting-duplicate-loop-labels/1833
 
+#[allow(unused_labels)]
 pub fn foo() {
     { 'fl: for _ in 0..10 { break; } }
     { 'fl: loop { break; } }             //~ WARN label name `'fl` shadows a label name that is already in scope

src/test/ui/loops-reject-duplicate-labels-2.stderr

@@ -1,69 +1,69 @@
 warning: label name `'fl` shadows a label name that is already in scope
-  --> $DIR/loops-reject-duplicate-labels-2.rs:23:7
+  --> $DIR/loops-reject-duplicate-labels-2.rs:24:7
    |
 LL |     { 'fl: for _ in 0..10 { break; } }
    |       --- first declared here
 LL |     { 'fl: loop { break; } }             //~ WARN label name `'fl` shadows a label name that is already in scope
    |       ^^^ lifetime 'fl already in scope
 
 warning: label name `'lf` shadows a label name that is already in scope
-  --> $DIR/loops-reject-duplicate-labels-2.rs:25:7
+  --> $DIR/loops-reject-duplicate-labels-2.rs:26:7
    |
 LL |     { 'lf: loop { break; } }
    |       --- first declared here
 LL |     { 'lf: for _ in 0..10 { break; } }   //~ WARN label name `'lf` shadows a label name that is already in scope
    |       ^^^ lifetime 'lf already in scope
 
 warning: label name `'wl` shadows a label name that is already in scope
-  --> $DIR/loops-reject-duplicate-labels-2.rs:27:7
+  --> $DIR/loops-reject-duplicate-labels-2.rs:28:7
    |
 LL |     { 'wl: while 2 > 1 { break; } }
    |       --- first declared here
 LL |     { 'wl: loop { break; } }             //~ WARN label name `'wl` shadows a label name that is already in scope
    |       ^^^ lifetime 'wl already in scope
 
 warning: label name `'lw` shadows a label name that is already in scope
-  --> $DIR/loops-reject-duplicate-labels-2.rs:29:7
+  --> $DIR/loops-reject-duplicate-labels-2.rs:30:7
    |
 LL |     { 'lw: loop { break; } }
    |       --- first declared here
 LL |     { 'lw: while 2 > 1 { break; } }      //~ WARN label name `'lw` shadows a label name that is already in scope
    |       ^^^ lifetime 'lw already in scope
 
 warning: label name `'fw` shadows a label name that is already in scope
-  --> $DIR/loops-reject-duplicate-labels-2.rs:31:7
+  --> $DIR/loops-reject-duplicate-labels-2.rs:32:7
    |
 LL |     { 'fw: for _ in 0..10 { break; } }
    |       --- first declared here
 LL |     { 'fw: while 2 > 1 { break; } }      //~ WARN label name `'fw` shadows a label name that is already in scope
    |       ^^^ lifetime 'fw already in scope
 
 warning: label name `'wf` shadows a label name that is already in scope
-  --> $DIR/loops-reject-duplicate-labels-2.rs:33:7
+  --> $DIR/loops-reject-duplicate-labels-2.rs:34:7
    |
 LL |     { 'wf: while 2 > 1 { break; } }
    |       --- first declared here
 LL |     { 'wf: for _ in 0..10 { break; } }   //~ WARN label name `'wf` shadows a label name that is already in scope
    |       ^^^ lifetime 'wf already in scope
 
 warning: label name `'tl` shadows a label name that is already in scope
-  --> $DIR/loops-reject-duplicate-labels-2.rs:35:7
+  --> $DIR/loops-reject-duplicate-labels-2.rs:36:7
    |
 LL |     { 'tl: while let Some(_) = None::<i32> { break; } }
    |       --- first declared here
 LL |     { 'tl: loop { break; } }             //~ WARN label name `'tl` shadows a label name that is already in scope
    |       ^^^ lifetime 'tl already in scope
 
 warning: label name `'lt` shadows a label name that is already in scope
-  --> $DIR/loops-reject-duplicate-labels-2.rs:37:7
+  --> $DIR/loops-reject-duplicate-labels-2.rs:38:7
    |
 LL |     { 'lt: loop { break; } }
    |       --- first declared here
 LL |     { 'lt: while let Some(_) = None::<i32> { break; } }
    |       ^^^ lifetime 'lt already in scope
 
 error: compilation successful
-  --> $DIR/loops-reject-duplicate-labels-2.rs:42:1
+  --> $DIR/loops-reject-duplicate-labels-2.rs:43:1
    |
 LL | / pub fn main() { //~ ERROR compilation successful
 LL | |     foo();

src/test/ui/loops-reject-duplicate-labels.rs

@@ -15,6 +15,7 @@
 // Issue #21633: reject duplicate loop labels in function bodies.
 // This is testing the exact cases that are in the issue description.
 
+#[allow(unused_labels)]
 fn foo() {
     'fl: for _ in 0..10 { break; }
     'fl: loop { break; }           //~ WARN label name `'fl` shadows a label name that is already in scope