New shorter diagnostic note that is different for items versus fields

Author: sunjay

compiler/rustc_passes/src/dead.rs

@@ -508,6 +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>,
@@ -575,6 +582,7 @@ impl DeadVisitor<'tcx> {
         span: rustc_span::Span,
         name: Symbol,
         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| {
@@ -585,19 +593,26 @@ impl DeadVisitor<'tcx> {
 
                 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,
-                )
-                .note(&format!(
-                    "The leading underscore signals to the reader that while the {} may not be {}\n\
-                    by any Rust code, it still serves some other purpose that isn't detected by rustc.\n\
-                    (e.g. some values are used for their effect when dropped or used in FFI code\n\
-                    exclusively through raw pointers)",
-                    descr, participle,
-                ));
+                );
+
+                let mut note = format!(
+                    "the leading underscore signals that this {} serves some other \
+                    purpose\neven 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\nwhen 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()
@@ -646,7 +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.name, participle, None);
         } else {
             // Only continue if we didn't warn
             intravisit::walk_item(self, item);
@@ -660,22 +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.name, "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.name, "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.name,
+                "read",
+                Some(ExtraNote::OtherPurposeExamples),
+            );
         }
         intravisit::walk_field_def(self, field);
     }
@@ -689,6 +710,7 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
                         impl_item.span,
                         impl_item.ident.name,
                         "used",
+                        None,
                     );
                 }
                 self.visit_nested_body(body_id)
@@ -706,7 +728,13 @@ 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.name,
+                        "used",
+                        None,
+                    );
                 }
                 self.visit_nested_body(body_id)
             }

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

@@ -4,10 +4,8 @@ error: associated constant is never used: `BAR`
 LL |     const BAR: u32 = 1;
    |     ^^^^^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_BAR`
    |
-   = note: The leading underscore signals to the reader that while the associated constant may not be used
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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

@@ -4,10 +4,8 @@ warning: variant is never constructed: `Void`
 LL |     Void(Void),
    |     ^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_Void`
    |
-   = note: The leading underscore signals to the reader that while the variant may not be constructed
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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

@@ -4,10 +4,8 @@ warning: type alias is never used: `Z`
 LL | type Z = dyn for<'x> Send;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_Z`
    |
-   = note: The leading underscore signals to the reader that while the type alias may not be used
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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

@@ -4,10 +4,8 @@ error: function is never used: `foo`
 LL | fn foo() {
    |    ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
    |
-   = note: The leading underscore signals to the reader that while the function may not be used
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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

@@ -4,10 +4,8 @@ warning: variant is never constructed: `B`
 LL |     B,
    |     ^ help: if this is intentional, prefix it with an underscore: `_B`
    |
-   = note: The leading underscore signals to the reader that while the variant may not be constructed
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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
    |
@@ -20,10 +18,8 @@ warning: variant is never constructed: `C`
 LL |     C,
    |     ^ help: if this is intentional, prefix it with an underscore: `_C`
    |
-   = note: The leading underscore signals to the reader that while the variant may not be constructed
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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.stderr

@@ -4,10 +4,9 @@ error: field is never read: `guard`
 LL |     guard: MutexGuard<'a, T>,
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_guard`
    |
-   = note: The leading underscore signals to the reader that while the field may not be read
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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
    |

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

@@ -4,10 +4,8 @@ error: enum is never used: `E`
 LL | enum E {}
    |      ^ help: if this is intentional, prefix it with an underscore: `_E`
    |
-   = note: The leading underscore signals to the reader that while the enum may not be used
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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.stderr

@@ -4,10 +4,9 @@ error: field is never read: `items`
 LL |     items: Option<Vec<T>>,
    |     ^^^^^^^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_items`
    |
-   = note: The leading underscore signals to the reader that while the field may not be read
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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
    |

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

@@ -4,10 +4,8 @@ error: type alias is never used: `Unused`
 LL | type Unused = ();
    | ^^^^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_Unused`
    |
-   = note: The leading underscore signals to the reader that while the type alias may not be used
-           by any Rust code, it still serves some other purpose that isn't detected by rustc.
-           (e.g. some values are used for their effect when dropped or used in FFI code
-           exclusively through raw pointers)
+   = 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
    |