Rollup merge of rust-lang#83004 - sunjay:field-never-read-issue-81658, r=pnkfelix

Improve diagnostic for when field is never read

Related to (but does not close) rust-lang#81658

This completes the first step of ``@pnkfelix's`` [mentoring instructions](rust-lang#81658 (comment)) but does not actually improve the diagnostics (yet!). The two tests are heavily reduced versions of code from the original bug report.

I've confirmed that the reduced `field-used-in-ffi` test [fails on nightly](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=f0862c89ddca028c55c20a5ed05e679a) but [passes on stable](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f0862c89ddca028c55c20a5ed05e679a). This confirms that the regression is reproduced correctly. The `drop-only-field` test is a case that ``@pnkfelix`` mentioned in his mentoring instructions. It is not a regression, but will come in handy when we make the diagnostic smarter by looking at whether the field type implements `Drop`.

Per the [rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/tests/adding.html), each test includes a comment summarizing what it is about.

Author: Dylan-DPC

compiler/rustc_passes/src/dead.rs

@@ -3,6 +3,7 @@
 // from live codes are live, and everything else is dead.
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
@@ -15,7 +16,7 @@ use rustc_middle::middle::privacy;
 use rustc_middle::ty::{self, DefIdTree, TyCtxt};
 use rustc_session::lint;
 
-use rustc_span::symbol::{sym, Symbol};
+use rustc_span::symbol::{sym, Ident, Symbol};
 
 // Any local node that may call something in its body block should be
 // explored. For example, if it's a live Node::Item that is a
@@ -507,6 +508,13 @@ fn find_live<'tcx>(
     symbol_visitor.live_symbols
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+enum ExtraNote {
+    /// Use this to provide some examples in the diagnostic of potential other purposes for a value
+    /// or field that is dead code
+    OtherPurposeExamples,
+}
+
 struct DeadVisitor<'tcx> {
     tcx: TyCtxt<'tcx>,
     live_symbols: FxHashSet<hir::HirId>,
@@ -572,14 +580,42 @@ impl DeadVisitor<'tcx> {
         &mut self,
         id: hir::HirId,
         span: rustc_span::Span,
-        name: Symbol,
+        name: Ident,
         participle: &str,
+        extra_note: Option<ExtraNote>,
     ) {
         if !name.as_str().starts_with('_') {
             self.tcx.struct_span_lint_hir(lint::builtin::DEAD_CODE, id, span, |lint| {
                 let def_id = self.tcx.hir().local_def_id(id);
                 let descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
-                lint.build(&format!("{} is never {}: `{}`", descr, participle, name)).emit()
+
+                let prefixed = vec![(name.span, format!("_{}", name))];
+
+                let mut diag =
+                    lint.build(&format!("{} is never {}: `{}`", descr, participle, name));
+
+                diag.multipart_suggestion(
+                    "if this is intentional, prefix it with an underscore",
+                    prefixed,
+                    Applicability::MachineApplicable,
+                );
+
+                let mut note = format!(
+                    "the leading underscore signals that this {} serves some other \
+                    purpose even if it isn't used in a way that we can detect.",
+                    descr,
+                );
+                if matches!(extra_note, Some(ExtraNote::OtherPurposeExamples)) {
+                    note += " (e.g. for its effect when dropped or in foreign code)";
+                }
+
+                diag.note(&note);
+
+                // Force the note we added to the front, before any other subdiagnostics
+                // added in lint.build(...)
+                diag.children.rotate_right(1);
+
+                diag.emit()
             });
         }
     }
@@ -625,7 +661,7 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
                 hir::ItemKind::Struct(..) => "constructed", // Issue #52325
                 _ => "used",
             };
-            self.warn_dead_code(item.hir_id(), span, item.ident.name, participle);
+            self.warn_dead_code(item.hir_id(), span, item.ident, participle, None);
         } else {
             // Only continue if we didn't warn
             intravisit::walk_item(self, item);
@@ -639,22 +675,28 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
         id: hir::HirId,
     ) {
         if self.should_warn_about_variant(&variant) {
-            self.warn_dead_code(variant.id, variant.span, variant.ident.name, "constructed");
+            self.warn_dead_code(variant.id, variant.span, variant.ident, "constructed", None);
         } else {
             intravisit::walk_variant(self, variant, g, id);
         }
     }
 
     fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem<'tcx>) {
         if self.should_warn_about_foreign_item(fi) {
-            self.warn_dead_code(fi.hir_id(), fi.span, fi.ident.name, "used");
+            self.warn_dead_code(fi.hir_id(), fi.span, fi.ident, "used", None);
         }
         intravisit::walk_foreign_item(self, fi);
     }
 
     fn visit_field_def(&mut self, field: &'tcx hir::FieldDef<'tcx>) {
         if self.should_warn_about_field(&field) {
-            self.warn_dead_code(field.hir_id, field.span, field.ident.name, "read");
+            self.warn_dead_code(
+                field.hir_id,
+                field.span,
+                field.ident,
+                "read",
+                Some(ExtraNote::OtherPurposeExamples),
+            );
         }
         intravisit::walk_field_def(self, field);
     }
@@ -666,8 +708,9 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
                     self.warn_dead_code(
                         impl_item.hir_id(),
                         impl_item.span,
-                        impl_item.ident.name,
+                        impl_item.ident,
                         "used",
+                        None,
                     );
                 }
                 self.visit_nested_body(body_id)
@@ -685,7 +728,7 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
                     } else {
                         impl_item.ident.span
                     };
-                    self.warn_dead_code(impl_item.hir_id(), span, impl_item.ident.name, "used");
+                    self.warn_dead_code(impl_item.hir_id(), span, impl_item.ident, "used", None);
                 }
                 self.visit_nested_body(body_id)
             }

src/test/ui/associated-consts/associated-const-dead-code.stderr

@@ -2,8 +2,11 @@ error: associated constant is never used: `BAR`
   --> $DIR/associated-const-dead-code.rs:6:5
    |
 LL |     const BAR: u32 = 1;
-   |     ^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^---^^^^^^^^^^
+   |           |
+   |           help: if this is intentional, prefix it with an underscore: `_BAR`
    |
+   = note: the leading underscore signals that this associated constant serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/associated-const-dead-code.rs:1:9
    |

src/test/ui/derive-uninhabited-enum-38885.stderr

@@ -2,8 +2,11 @@ warning: variant is never constructed: `Void`
   --> $DIR/derive-uninhabited-enum-38885.rs:13:5
    |
 LL |     Void(Void),
-   |     ^^^^^^^^^^
+   |     ----^^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_Void`
    |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
    = note: `-W dead-code` implied by `-W unused`
 
 warning: 1 warning emitted

src/test/ui/issues/issue-37515.stderr

@@ -2,8 +2,11 @@ warning: type alias is never used: `Z`
   --> $DIR/issue-37515.rs:5:1
    |
 LL | type Z = dyn for<'x> Send;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | ^^^^^-^^^^^^^^^^^^^^^^^^^^
+   |      |
+   |      help: if this is intentional, prefix it with an underscore: `_Z`
    |
+   = note: the leading underscore signals that this type alias serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/issue-37515.rs:3:9
    |

src/test/ui/lint/dead-code/basic.stderr

@@ -2,8 +2,9 @@ error: function is never used: `foo`
   --> $DIR/basic.rs:4:4
    |
 LL | fn foo() {
-   |    ^^^
+   |    ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
    |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/basic.rs:1:9
    |

src/test/ui/lint/dead-code/const-and-self.stderr

@@ -2,8 +2,9 @@ warning: variant is never constructed: `B`
   --> $DIR/const-and-self.rs:33:5
    |
 LL |     B,
-   |     ^
+   |     ^ help: if this is intentional, prefix it with an underscore: `_B`
    |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/const-and-self.rs:3:9
    |
@@ -14,7 +15,9 @@ warning: variant is never constructed: `C`
   --> $DIR/const-and-self.rs:34:5
    |
 LL |     C,
-   |     ^
+   |     ^ help: if this is intentional, prefix it with an underscore: `_C`
+   |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 
 warning: 2 warnings emitted
 

src/test/ui/lint/dead-code/drop-only-field-issue-81658.rs

@@ -0,0 +1,42 @@
+//! The field `guard` is never used directly, but it is still useful for its side effect when
+//! dropped. Since rustc doesn't consider a `Drop` impl as a use, we want to make sure we at least
+//! produce a helpful diagnostic that points the user to what they can do if they indeed intended to
+//! have a field that is only used for its `Drop` side effect.
+//!
+//! Issue: https://github.com/rust-lang/rust/issues/81658
+
+#![deny(dead_code)]
+
+use std::sync::{Mutex, MutexGuard};
+
+/// Holds a locked value until it is dropped
+pub struct Locked<'a, T> {
+    // Field is kept for its affect when dropped, but otherwise unused
+    guard: MutexGuard<'a, T>, //~ ERROR field is never read
+}
+
+impl<'a, T> Locked<'a, T> {
+    pub fn new(value: &'a Mutex<T>) -> Self {
+        Self {
+            guard: value.lock().unwrap(),
+        }
+    }
+}
+
+fn main() {
+    let items = Mutex::new(vec![1, 2, 3]);
+
+    // Hold a lock on items while doing something else
+    let result = {
+        // The lock will be released at the end of this scope
+        let _lock = Locked::new(&items);
+
+        do_something_else()
+    };
+
+    println!("{}", result);
+}
+
+fn do_something_else() -> i32 {
+    1 + 1
+}

src/test/ui/lint/dead-code/drop-only-field-issue-81658.stderr

@@ -0,0 +1,17 @@
+error: field is never read: `guard`
+  --> $DIR/drop-only-field-issue-81658.rs:15:5
+   |
+LL |     guard: MutexGuard<'a, T>,
+   |     -----^^^^^^^^^^^^^^^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_guard`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
+note: the lint level is defined here
+  --> $DIR/drop-only-field-issue-81658.rs:8:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/lint/dead-code/empty-unused-enum.stderr

@@ -2,8 +2,9 @@ error: enum is never used: `E`
   --> $DIR/empty-unused-enum.rs:3:6
    |
 LL | enum E {}
-   |      ^
+   |      ^ help: if this is intentional, prefix it with an underscore: `_E`
    |
+   = note: the leading underscore signals that this enum serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/empty-unused-enum.rs:1:9
    |

src/test/ui/lint/dead-code/field-used-in-ffi-issue-81658.rs

@@ -0,0 +1,50 @@
+//! The field `items` is being "used" by FFI (implicitly through pointers). However, since rustc
+//! doesn't know how to detect that, we produce a message that says the field is unused. This can
+//! cause some confusion and we want to make sure our diagnostics help as much as they can.
+//!
+//! Issue: https://github.com/rust-lang/rust/issues/81658
+
+#![deny(dead_code)]
+
+/// A struct for holding on to data while it is being used in our FFI code
+pub struct FFIData<T> {
+    /// These values cannot be dropped while the pointers to each item
+    /// are still in use
+    items: Option<Vec<T>>, //~ ERROR field is never read
+}
+
+impl<T> FFIData<T> {
+    pub fn new() -> Self {
+        Self {items: None}
+    }
+
+    /// Load items into this type and return pointers to each item that can
+    /// be passed to FFI
+    pub fn load(&mut self, items: Vec<T>) -> Vec<*const T> {
+        let ptrs = items.iter().map(|item| item as *const _).collect();
+
+        self.items = Some(items);
+
+        ptrs
+    }
+}
+
+extern {
+    /// The FFI code that uses items
+    fn process_item(item: *const i32);
+}
+
+fn main() {
+    // Data cannot be dropped until the end of this scope or else the items
+    // will be dropped before they are processed
+    let mut data = FFIData::new();
+
+    let ptrs = data.load(vec![1, 2, 3, 4, 5]);
+
+    for ptr in ptrs {
+        // Safety: This pointer is valid as long as the arena is in scope
+        unsafe { process_item(ptr); }
+    }
+
+    // Items will be safely freed at the end of this scope
+}

src/test/ui/lint/dead-code/field-used-in-ffi-issue-81658.stderr

@@ -0,0 +1,17 @@
+error: field is never read: `items`
+  --> $DIR/field-used-in-ffi-issue-81658.rs:13:5
+   |
+LL |     items: Option<Vec<T>>,
+   |     -----^^^^^^^^^^^^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_items`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
+note: the lint level is defined here
+  --> $DIR/field-used-in-ffi-issue-81658.rs:7:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/lint/dead-code/impl-trait.stderr

@@ -2,8 +2,11 @@ error: type alias is never used: `Unused`
   --> $DIR/impl-trait.rs:12:1
    |
 LL | type Unused = ();
-   | ^^^^^^^^^^^^^^^^^
+   | ^^^^^------^^^^^^
+   |      |
+   |      help: if this is intentional, prefix it with an underscore: `_Unused`
    |
+   = note: the leading underscore signals that this type alias serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/impl-trait.rs:1:9
    |

src/test/ui/lint/dead-code/lint-dead-code-1.stderr

@@ -2,8 +2,9 @@ error: struct is never constructed: `Bar`
   --> $DIR/lint-dead-code-1.rs:12:16
    |
 LL |     pub struct Bar;
-   |                ^^^
+   |                ^^^ help: if this is intentional, prefix it with an underscore: `_Bar`
    |
+   = note: the leading underscore signals that this struct serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/lint-dead-code-1.rs:5:9
    |
@@ -14,55 +15,77 @@ error: static is never used: `priv_static`
   --> $DIR/lint-dead-code-1.rs:20:1
    |
 LL | static priv_static: isize = 0;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | ^^^^^^^-----------^^^^^^^^^^^^
+   |        |
+   |        help: if this is intentional, prefix it with an underscore: `_priv_static`
+   |
+   = note: the leading underscore signals that this static serves some other purpose even if it isn't used in a way that we can detect.
 
 error: constant is never used: `priv_const`
   --> $DIR/lint-dead-code-1.rs:27:1
    |
 LL | const priv_const: isize = 0;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | ^^^^^^----------^^^^^^^^^^^^
+   |       |
+   |       help: if this is intentional, prefix it with an underscore: `_priv_const`
+   |
+   = note: the leading underscore signals that this constant serves some other purpose even if it isn't used in a way that we can detect.
 
 error: struct is never constructed: `PrivStruct`
   --> $DIR/lint-dead-code-1.rs:35:8
    |
 LL | struct PrivStruct;
-   |        ^^^^^^^^^^
+   |        ^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_PrivStruct`
+   |
+   = note: the leading underscore signals that this struct serves some other purpose even if it isn't used in a way that we can detect.
 
 error: enum is never used: `priv_enum`
   --> $DIR/lint-dead-code-1.rs:64:6
    |
 LL | enum priv_enum { foo2, bar2 }
-   |      ^^^^^^^^^
+   |      ^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_priv_enum`
+   |
+   = note: the leading underscore signals that this enum serves some other purpose even if it isn't used in a way that we can detect.
 
 error: variant is never constructed: `bar3`
   --> $DIR/lint-dead-code-1.rs:67:5
    |
 LL |     bar3
-   |     ^^^^
+   |     ^^^^ help: if this is intentional, prefix it with an underscore: `_bar3`
+   |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 
 error: function is never used: `priv_fn`
   --> $DIR/lint-dead-code-1.rs:88:4
    |
 LL | fn priv_fn() {
-   |    ^^^^^^^
+   |    ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_priv_fn`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: function is never used: `foo`
   --> $DIR/lint-dead-code-1.rs:93:4
    |
 LL | fn foo() {
-   |    ^^^
+   |    ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: function is never used: `bar`
   --> $DIR/lint-dead-code-1.rs:98:4
    |
 LL | fn bar() {
-   |    ^^^
+   |    ^^^ help: if this is intentional, prefix it with an underscore: `_bar`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: function is never used: `baz`
   --> $DIR/lint-dead-code-1.rs:102:4
    |
 LL | fn baz() -> impl Copy {
-   |    ^^^
+   |    ^^^ help: if this is intentional, prefix it with an underscore: `_baz`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: aborting due to 10 previous errors
 

src/test/ui/lint/dead-code/lint-dead-code-2.stderr

@@ -2,8 +2,9 @@ error: function is never used: `dead_fn`
   --> $DIR/lint-dead-code-2.rs:22:4
    |
 LL | fn dead_fn() {}
-   |    ^^^^^^^
+   |    ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_dead_fn`
    |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/lint-dead-code-2.rs:2:9
    |
@@ -14,13 +15,17 @@ error: function is never used: `dead_fn2`
   --> $DIR/lint-dead-code-2.rs:25:4
    |
 LL | fn dead_fn2() {}
-   |    ^^^^^^^^
+   |    ^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_dead_fn2`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: function is never used: `main`
   --> $DIR/lint-dead-code-2.rs:38:4
    |
 LL | fn main() {
-   |    ^^^^
+   |    ^^^^ help: if this is intentional, prefix it with an underscore: `_main`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: aborting due to 3 previous errors
 

src/test/ui/lint/dead-code/lint-dead-code-3.stderr

@@ -2,8 +2,9 @@ error: struct is never constructed: `Foo`
   --> $DIR/lint-dead-code-3.rs:14:8
    |
 LL | struct Foo;
-   |        ^^^
+   |        ^^^ help: if this is intentional, prefix it with an underscore: `_Foo`
    |
+   = note: the leading underscore signals that this struct serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/lint-dead-code-3.rs:4:9
    |
@@ -14,25 +15,35 @@ error: associated function is never used: `foo`
   --> $DIR/lint-dead-code-3.rs:16:8
    |
 LL |     fn foo(&self) {
-   |        ^^^
+   |        ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
+   |
+   = note: the leading underscore signals that this associated function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: function is never used: `bar`
   --> $DIR/lint-dead-code-3.rs:21:4
    |
 LL | fn bar() {
-   |    ^^^
+   |    ^^^ help: if this is intentional, prefix it with an underscore: `_bar`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: enum is never used: `c_void`
   --> $DIR/lint-dead-code-3.rs:60:6
    |
 LL | enum c_void {}
-   |      ^^^^^^
+   |      ^^^^^^ help: if this is intentional, prefix it with an underscore: `_c_void`
+   |
+   = note: the leading underscore signals that this enum serves some other purpose even if it isn't used in a way that we can detect.
 
 error: function is never used: `free`
   --> $DIR/lint-dead-code-3.rs:62:5
    |
 LL |     fn free(p: *const c_void);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^----^^^^^^^^^^^^^^^^^^^
+   |        |
+   |        help: if this is intentional, prefix it with an underscore: `_free`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: aborting due to 5 previous errors
 

src/test/ui/lint/dead-code/lint-dead-code-4.stderr

@@ -2,8 +2,11 @@ error: field is never read: `b`
   --> $DIR/lint-dead-code-4.rs:7:5
    |
 LL |     b: bool,
-   |     ^^^^^^^
+   |     -^^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_b`
    |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 note: the lint level is defined here
   --> $DIR/lint-dead-code-4.rs:3:9
    |
@@ -14,59 +17,88 @@ error: variant is never constructed: `X`
   --> $DIR/lint-dead-code-4.rs:15:5
    |
 LL |     X,
-   |     ^
+   |     ^ help: if this is intentional, prefix it with an underscore: `_X`
+   |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 
 error: variant is never constructed: `Y`
   --> $DIR/lint-dead-code-4.rs:16:5
    |
-LL | /     Y {
+LL |       Y {
+   |       ^ help: if this is intentional, prefix it with an underscore: `_Y`
+   |  _____|
+   | |
 LL | |         a: String,
 LL | |         b: i32,
 LL | |         c: i32,
 LL | |     },
    | |_____^
+   |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 
 error: enum is never used: `ABC`
   --> $DIR/lint-dead-code-4.rs:24:6
    |
 LL | enum ABC {
-   |      ^^^
+   |      ^^^ help: if this is intentional, prefix it with an underscore: `_ABC`
+   |
+   = note: the leading underscore signals that this enum serves some other purpose even if it isn't used in a way that we can detect.
 
 error: variant is never constructed: `I`
   --> $DIR/lint-dead-code-4.rs:36:5
    |
 LL |     I,
-   |     ^
+   |     ^ help: if this is intentional, prefix it with an underscore: `_I`
+   |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 
 error: field is never read: `b`
   --> $DIR/lint-dead-code-4.rs:39:9
    |
 LL |         b: i32,
-   |         ^^^^^^
+   |         -^^^^^
+   |         |
+   |         help: if this is intentional, prefix it with an underscore: `_b`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: field is never read: `c`
   --> $DIR/lint-dead-code-4.rs:40:9
    |
 LL |         c: i32,
-   |         ^^^^^^
+   |         -^^^^^
+   |         |
+   |         help: if this is intentional, prefix it with an underscore: `_c`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: variant is never constructed: `K`
   --> $DIR/lint-dead-code-4.rs:42:5
    |
 LL |     K
-   |     ^
+   |     ^ help: if this is intentional, prefix it with an underscore: `_K`
+   |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 
 error: field is never read: `x`
   --> $DIR/lint-dead-code-4.rs:61:5
    |
 LL |     x: usize,
-   |     ^^^^^^^^
+   |     -^^^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_x`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: field is never read: `c`
   --> $DIR/lint-dead-code-4.rs:63:5
    |
 LL |     c: bool,
-   |     ^^^^^^^
+   |     -^^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_c`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: aborting due to 10 previous errors
 

src/test/ui/lint/dead-code/lint-dead-code-5.stderr

@@ -2,8 +2,9 @@ error: variant is never constructed: `Variant2`
   --> $DIR/lint-dead-code-5.rs:6:5
    |
 LL |     Variant2
-   |     ^^^^^^^^
+   |     ^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_Variant2`
    |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/lint-dead-code-5.rs:2:9
    |
@@ -14,19 +15,29 @@ error: variant is never constructed: `Variant5`
   --> $DIR/lint-dead-code-5.rs:13:5
    |
 LL |     Variant5 { _x: isize },
-   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |     --------^^^^^^^^^^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_Variant5`
+   |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 
 error: variant is never constructed: `Variant6`
   --> $DIR/lint-dead-code-5.rs:14:5
    |
 LL |     Variant6(isize),
-   |     ^^^^^^^^^^^^^^^
+   |     --------^^^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_Variant6`
+   |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 
 error: enum is never used: `Enum3`
   --> $DIR/lint-dead-code-5.rs:35:6
    |
 LL | enum Enum3 {
-   |      ^^^^^
+   |      ^^^^^ help: if this is intentional, prefix it with an underscore: `_Enum3`
+   |
+   = note: the leading underscore signals that this enum serves some other purpose even if it isn't used in a way that we can detect.
 
 error: aborting due to 4 previous errors
 

src/test/ui/lint/dead-code/lint-dead-code-6.stderr

@@ -2,8 +2,9 @@ error: struct is never constructed: `UnusedStruct`
   --> $DIR/lint-dead-code-6.rs:3:8
    |
 LL | struct UnusedStruct;
-   |        ^^^^^^^^^^^^
+   |        ^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_UnusedStruct`
    |
+   = note: the leading underscore signals that this struct serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/lint-dead-code-6.rs:1:9
    |
@@ -14,19 +15,25 @@ error: associated function is never used: `unused_impl_fn_1`
   --> $DIR/lint-dead-code-6.rs:5:8
    |
 LL |     fn unused_impl_fn_1() {
-   |        ^^^^^^^^^^^^^^^^
+   |        ^^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused_impl_fn_1`
+   |
+   = note: the leading underscore signals that this associated function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: associated function is never used: `unused_impl_fn_2`
   --> $DIR/lint-dead-code-6.rs:9:8
    |
 LL |     fn unused_impl_fn_2(var: i32) {
-   |        ^^^^^^^^^^^^^^^^
+   |        ^^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused_impl_fn_2`
+   |
+   = note: the leading underscore signals that this associated function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: associated function is never used: `unused_impl_fn_3`
   --> $DIR/lint-dead-code-6.rs:13:8
    |
 LL |     fn unused_impl_fn_3(
-   |        ^^^^^^^^^^^^^^^^
+   |        ^^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused_impl_fn_3`
+   |
+   = note: the leading underscore signals that this associated function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: aborting due to 4 previous errors
 

src/test/ui/lint/dead-code/newline-span.stderr

@@ -2,8 +2,9 @@ error: function is never used: `unused`
   --> $DIR/newline-span.rs:3:4
    |
 LL | fn unused() {
-   |    ^^^^^^
+   |    ^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused`
    |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/newline-span.rs:1:9
    |
@@ -14,13 +15,17 @@ error: function is never used: `unused2`
   --> $DIR/newline-span.rs:7:4
    |
 LL | fn unused2(var: i32) {
-   |    ^^^^^^^
+   |    ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused2`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: function is never used: `unused3`
   --> $DIR/newline-span.rs:11:4
    |
 LL | fn unused3(
-   |    ^^^^^^^
+   |    ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused3`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 error: aborting due to 3 previous errors
 

src/test/ui/lint/dead-code/type-alias.stderr

@@ -2,8 +2,11 @@ error: type alias is never used: `Unused`
   --> $DIR/type-alias.rs:4:1
    |
 LL | type Unused = u8;
-   | ^^^^^^^^^^^^^^^^^
+   | ^^^^^------^^^^^^
+   |      |
+   |      help: if this is intentional, prefix it with an underscore: `_Unused`
    |
+   = note: the leading underscore signals that this type alias serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/type-alias.rs:1:9
    |

src/test/ui/lint/dead-code/unused-enum.stderr

@@ -2,8 +2,9 @@ error: struct is never constructed: `F`
   --> $DIR/unused-enum.rs:3:8
    |
 LL | struct F;
-   |        ^
+   |        ^ help: if this is intentional, prefix it with an underscore: `_F`
    |
+   = note: the leading underscore signals that this struct serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/unused-enum.rs:1:9
    |
@@ -15,13 +16,17 @@ error: struct is never constructed: `B`
   --> $DIR/unused-enum.rs:4:8
    |
 LL | struct B;
-   |        ^
+   |        ^ help: if this is intentional, prefix it with an underscore: `_B`
+   |
+   = note: the leading underscore signals that this struct serves some other purpose even if it isn't used in a way that we can detect.
 
 error: enum is never used: `E`
   --> $DIR/unused-enum.rs:6:6
    |
 LL | enum E {
-   |      ^
+   |      ^ help: if this is intentional, prefix it with an underscore: `_E`
+   |
+   = note: the leading underscore signals that this enum serves some other purpose even if it isn't used in a way that we can detect.
 
 error: aborting due to 3 previous errors
 

src/test/ui/lint/dead-code/unused-struct-variant.stderr

@@ -2,8 +2,11 @@ error: variant is never constructed: `Bar`
   --> $DIR/unused-struct-variant.rs:8:5
    |
 LL |     Bar(B),
-   |     ^^^^^^
+   |     ---^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_Bar`
    |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/unused-struct-variant.rs:1:9
    |

src/test/ui/lint/dead-code/unused-variant.stderr

@@ -2,8 +2,9 @@ error: variant is never constructed: `Variant1`
   --> $DIR/unused-variant.rs:5:5
    |
 LL |     Variant1,
-   |     ^^^^^^^^
+   |     ^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_Variant1`
    |
+   = note: the leading underscore signals that this variant serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/unused-variant.rs:1:9
    |

src/test/ui/lint/dead-code/with-core-crate.stderr

@@ -2,8 +2,9 @@ error: function is never used: `foo`
   --> $DIR/with-core-crate.rs:7:4
    |
 LL | fn foo() {
-   |    ^^^
+   |    ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
    |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/with-core-crate.rs:1:9
    |

src/test/ui/lint/dead-code/write-only-field.stderr

@@ -2,8 +2,11 @@ error: field is never read: `f`
   --> $DIR/write-only-field.rs:4:5
    |
 LL |     f: i32,
-   |     ^^^^^^
+   |     -^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_f`
    |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 note: the lint level is defined here
   --> $DIR/write-only-field.rs:1:9
    |
@@ -14,31 +17,51 @@ error: field is never read: `sub`
   --> $DIR/write-only-field.rs:5:5
    |
 LL |     sub: Sub,
-   |     ^^^^^^^^
+   |     ---^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_sub`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: field is never read: `f`
   --> $DIR/write-only-field.rs:9:5
    |
 LL |     f: i32,
-   |     ^^^^^^
+   |     -^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_f`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: field is never read: `y`
   --> $DIR/write-only-field.rs:28:9
    |
 LL |         y: bool,
-   |         ^^^^^^^
+   |         -^^^^^^
+   |         |
+   |         help: if this is intentional, prefix it with an underscore: `_y`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: field is never read: `u`
   --> $DIR/write-only-field.rs:58:9
    |
 LL |         u: u32,
-   |         ^^^^^^
+   |         -^^^^^
+   |         |
+   |         help: if this is intentional, prefix it with an underscore: `_u`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: field is never read: `v`
   --> $DIR/write-only-field.rs:59:9
    |
 LL |         v: u32,
-   |         ^^^^^^
+   |         -^^^^^
+   |         |
+   |         help: if this is intentional, prefix it with an underscore: `_v`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: aborting due to 6 previous errors
 

src/test/ui/lint/issue-17718-const-naming.stderr

@@ -2,8 +2,11 @@ error: constant is never used: `foo`
   --> $DIR/issue-17718-const-naming.rs:4:1
    |
 LL | const foo: isize = 3;
-   | ^^^^^^^^^^^^^^^^^^^^^
+   | ^^^^^^---^^^^^^^^^^^^
+   |       |
+   |       help: if this is intentional, prefix it with an underscore: `_foo`
    |
+   = note: the leading underscore signals that this constant serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/issue-17718-const-naming.rs:2:9
    |

src/test/ui/span/macro-span-replacement.stderr

@@ -2,11 +2,14 @@ warning: struct is never constructed: `S`
   --> $DIR/macro-span-replacement.rs:7:14
    |
 LL |         $b $a;
-   |              ^
+   |            --^
+   |            |
+   |            help: if this is intentional, prefix it with an underscore: `_S`
 ...
 LL |     m!(S struct);
    |     ------------- in this macro invocation
    |
+   = note: the leading underscore signals that this struct serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/macro-span-replacement.rs:3:9
    |

src/test/ui/span/unused-warning-point-at-identifier.stderr

@@ -2,8 +2,9 @@ warning: enum is never used: `Enum`
   --> $DIR/unused-warning-point-at-identifier.rs:5:6
    |
 LL | enum Enum {
-   |      ^^^^
+   |      ^^^^ help: if this is intentional, prefix it with an underscore: `_Enum`
    |
+   = note: the leading underscore signals that this enum serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/unused-warning-point-at-identifier.rs:3:9
    |
@@ -15,19 +16,25 @@ warning: struct is never constructed: `Struct`
   --> $DIR/unused-warning-point-at-identifier.rs:12:8
    |
 LL | struct Struct {
-   |        ^^^^^^
+   |        ^^^^^^ help: if this is intentional, prefix it with an underscore: `_Struct`
+   |
+   = note: the leading underscore signals that this struct serves some other purpose even if it isn't used in a way that we can detect.
 
 warning: function is never used: `func`
   --> $DIR/unused-warning-point-at-identifier.rs:19:4
    |
 LL | fn func() -> usize {
-   |    ^^^^
+   |    ^^^^ help: if this is intentional, prefix it with an underscore: `_func`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 warning: function is never used: `func_complete_span`
   --> $DIR/unused-warning-point-at-identifier.rs:24:1
    |
 LL | func_complete_span()
-   | ^^^^^^^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_func_complete_span`
+   |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 
 warning: 4 warnings emitted
 

src/test/ui/test-attrs/test-warns-dead-code.stderr

@@ -2,8 +2,9 @@ error: function is never used: `dead`
   --> $DIR/test-warns-dead-code.rs:5:4
    |
 LL | fn dead() {}
-   |    ^^^^
+   |    ^^^^ help: if this is intentional, prefix it with an underscore: `_dead`
    |
+   = note: the leading underscore signals that this function serves some other purpose even if it isn't used in a way that we can detect.
 note: the lint level is defined here
   --> $DIR/test-warns-dead-code.rs:3:9
    |

src/test/ui/union/union-fields-1.stderr

@@ -2,8 +2,11 @@ error: field is never read: `c`
   --> $DIR/union-fields-1.rs:6:5
    |
 LL |     c: u8,
-   |     ^^^^^
+   |     -^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_c`
    |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 note: the lint level is defined here
   --> $DIR/union-fields-1.rs:1:9
    |
@@ -14,19 +17,31 @@ error: field is never read: `a`
   --> $DIR/union-fields-1.rs:9:5
    |
 LL |     a: u8,
-   |     ^^^^^
+   |     -^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_a`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: field is never read: `a`
   --> $DIR/union-fields-1.rs:13:20
    |
 LL | union NoDropLike { a: u8 }
-   |                    ^^^^^
+   |                    -^^^^
+   |                    |
+   |                    help: if this is intentional, prefix it with an underscore: `_a`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: field is never read: `c`
   --> $DIR/union-fields-1.rs:18:5
    |
 LL |     c: u8,
-   |     ^^^^^
+   |     -^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_c`
+   |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 
 error: aborting due to 4 previous errors
 

src/test/ui/union/union-lint-dead-code.stderr

@@ -2,8 +2,11 @@ error: field is never read: `b`
   --> $DIR/union-lint-dead-code.rs:5:5
    |
 LL |     b: bool,
-   |     ^^^^^^^
+   |     -^^^^^^
+   |     |
+   |     help: if this is intentional, prefix it with an underscore: `_b`
    |
+   = note: the leading underscore signals that this field serves some other purpose even if it isn't used in a way that we can detect. (e.g. for its effect when dropped or in foreign code)
 note: the lint level is defined here
   --> $DIR/union-lint-dead-code.rs:1:9
    |