Emit a note to explain unused_pub_items_in_binary

Author: mu001999

compiler/rustc_passes/src/dead.rs

@@ -26,7 +26,8 @@ use rustc_session::lint::{self, Lint, LintExpectationId};
 use rustc_span::{Symbol, kw};
 
 use crate::errors::{
-    ChangeFields, IgnoredDerivedImpls, MultipleDeadCodes, ParentInfo, UselessAssignment,
+    ChangeFields, IgnoredDerivedImpls, MultipleDeadCodes, ParentInfo, UnusedPubItemsInBinaryNote,
+    UselessAssignment,
 };
 
 /// Any local definition that may call something in its body block should be explored. For example,
@@ -983,6 +984,13 @@ impl<'tcx> DeadVisitor<'tcx> {
         (level.level, level.lint_id)
     }
 
+    fn unused_pub_items_in_binary_note(&self) -> Option<UnusedPubItemsInBinaryNote> {
+        self.target_lint
+            .name
+            .eq(UNUSED_PUB_ITEMS_IN_BINARY.name)
+            .then_some(UnusedPubItemsInBinaryNote)
+    }
+
     // # Panics
     // All `dead_codes` must have the same lint level, otherwise we will intentionally ICE.
     // This is because we emit a multi-spanned lint using the lint level of the `dead_codes`'s
@@ -1094,6 +1102,7 @@ impl<'tcx> DeadVisitor<'tcx> {
                     descr,
                     participle,
                     name_list,
+                    unused_pub_items_in_binary_note: self.unused_pub_items_in_binary_note(),
                     change_fields_suggestion: fields_suggestion,
                     parent_info,
                     ignored_derived_impls,
@@ -1130,6 +1139,7 @@ impl<'tcx> DeadVisitor<'tcx> {
                     descr,
                     participle,
                     name_list,
+                    unused_pub_items_in_binary_note: self.unused_pub_items_in_binary_note(),
                     parent_info,
                     ignored_derived_impls,
                     enum_variants_with_same_name,

compiler/rustc_passes/src/errors.rs

@@ -950,6 +950,8 @@ pub(crate) enum MultipleDeadCodes<'tcx> {
         participle: &'tcx str,
         name_list: DiagSymbolList,
         #[subdiagnostic]
+        unused_pub_items_in_binary_note: Option<UnusedPubItemsInBinaryNote>,
+        #[subdiagnostic]
         // only on DeadCodes since it's never a problem for tuple struct fields
         enum_variants_with_same_name: Vec<EnumVariantSameName<'tcx>>,
         #[subdiagnostic]
@@ -973,6 +975,8 @@ pub(crate) enum MultipleDeadCodes<'tcx> {
         participle: &'tcx str,
         name_list: DiagSymbolList,
         #[subdiagnostic]
+        unused_pub_items_in_binary_note: Option<UnusedPubItemsInBinaryNote>,
+        #[subdiagnostic]
         change_fields_suggestion: ChangeFields,
         #[subdiagnostic]
         parent_info: Option<ParentInfo<'tcx>>,
@@ -981,6 +985,12 @@ pub(crate) enum MultipleDeadCodes<'tcx> {
     },
 }
 
+#[derive(Subdiagnostic)]
+#[note(
+    "in libraries, `pub` items can be used by dependent crates; in binaries, they cannot, so this `pub` item is unused"
+)]
+pub(crate) struct UnusedPubItemsInBinaryNote;
+
 #[derive(Subdiagnostic)]
 #[note(
     "it is impossible to refer to the {$dead_descr} `{$dead_name}` because it is shadowed by this enum variant with the same name"

tests/ui/lint/unused-pub-items-in-binary/allow-dead-code-transitive.stderr

@@ -4,6 +4,7 @@ error: function `g` is never used
 LL | pub fn g() {}
    |        ^
    |
+   = note: in libraries, `pub` items can be used by dependent crates; in binaries, they cannot, so this `pub` item is unused
 note: the lint level is defined here
   --> $DIR/allow-dead-code-transitive.rs:2:9
    |

tests/ui/lint/unused-pub-items-in-binary/basic.stderr

@@ -4,6 +4,7 @@ error: function `unused_pub_fn` is never used
 LL | pub fn unused_pub_fn() {}
    |        ^^^^^^^^^^^^^
    |
+   = note: in libraries, `pub` items can be used by dependent crates; in binaries, they cannot, so this `pub` item is unused
 note: the lint level is defined here
   --> $DIR/basic.rs:2:9
    |

tests/ui/lint/unused-pub-items-in-binary/deny-lint.stderr

@@ -4,6 +4,7 @@ error: function `unused_pub_fn` is never used
 LL | pub fn unused_pub_fn() {}
    |        ^^^^^^^^^^^^^
    |
+   = note: in libraries, `pub` items can be used by dependent crates; in binaries, they cannot, so this `pub` item is unused
 note: the lint level is defined here
   --> $DIR/deny-lint.rs:1:9
    |

tests/ui/lint/unused-pub-items-in-binary/interaction-allow-dead-code.stderr

@@ -4,6 +4,7 @@ error: function `unused_pub_fn` is never used
 LL | pub fn unused_pub_fn() {}
    |        ^^^^^^^^^^^^^
    |
+   = note: in libraries, `pub` items can be used by dependent crates; in binaries, they cannot, so this `pub` item is unused
 note: the lint level is defined here
   --> $DIR/interaction-allow-dead-code.rs:2:9
    |