Auto merge of #118297 - shepmaster:warn-dead-tuple-fields, r=WaffleLapkin

Merge `unused_tuple_struct_fields` into `dead_code`

This implicitly upgrades the lint from `allow` to `warn` and places it into the `unused` lint group.

[Discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Moving.20.60unused_tuple_struct_fields.60.20from.20allow.20to.20warn)

3 files changed, 43 insertions, 52 deletions

diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index 93904fb5c56..76c630fc456 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -328,6 +328,7 @@ fn register_builtins(store: &mut LintStore) {
     store.register_renamed("disjoint_capture_migration", "rust_2021_incompatible_closure_captures");
     store.register_renamed("or_patterns_back_compat", "rust_2021_incompatible_or_patterns");
     store.register_renamed("non_fmt_panic", "non_fmt_panics");
+    store.register_renamed("unused_tuple_struct_fields", "dead_code");
 
     // These were moved to tool lints, but rustc still sees them when compiling normally, before
     // tool lints are registered, so `check_tool_name_for_backwards_compat` doesn't work. Use
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index e35d1ee0461..19da51b7dcf 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -125,7 +125,6 @@ declare_lint_pass! {
         UNUSED_MACROS,
         UNUSED_MUT,
         UNUSED_QUALIFICATIONS,
-        UNUSED_TUPLE_STRUCT_FIELDS,
         UNUSED_UNSAFE,
         UNUSED_VARIABLES,
         USELESS_DEPRECATED,
@@ -697,8 +696,13 @@ declare_lint! {
     /// Dead code may signal a mistake or unfinished code. To silence the
     /// warning for individual items, prefix the name with an underscore such
     /// as `_foo`. If it was intended to expose the item outside of the crate,
-    /// consider adding a visibility modifier like `pub`. Otherwise consider
-    /// removing the unused code.
+    /// consider adding a visibility modifier like `pub`.
+    ///
+    /// To preserve the numbering of tuple structs with unused fields,
+    /// change the unused fields to have unit type or use
+    /// `PhantomData`.
+    ///
+    /// Otherwise consider removing the unused code.
     pub DEAD_CODE,
     Warn,
     "detect unused, unexported items"
@@ -733,32 +737,6 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `unused_tuple_struct_fields` lint detects fields of tuple structs
-    /// that are never read.
-    ///
-    /// ### Example
-    ///
-    /// ```rust
-    /// #[warn(unused_tuple_struct_fields)]
-    /// struct S(i32, i32, i32);
-    /// let s = S(1, 2, 3);
-    /// let _ = (s.0, s.2);
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// Tuple struct fields that are never read anywhere may indicate a
-    /// mistake or unfinished code. To silence this warning, consider
-    /// removing the unused field(s) or, to preserve the numbering of the
-    /// remaining fields, change the unused field(s) to have unit type.
-    pub UNUSED_TUPLE_STRUCT_FIELDS,
-    Allow,
-    "detects tuple struct fields that are never read"
-}
-
-declare_lint! {
     /// The `unreachable_code` lint detects unreachable code paths.
     ///
     /// ### Example
diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index 22aac1e775e..ac2ca23ad41 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -15,8 +15,8 @@ use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::middle::privacy::Level;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, TyCtxt};
-use rustc_session::lint::builtin::{DEAD_CODE, UNUSED_TUPLE_STRUCT_FIELDS};
-use rustc_session::lint::{self, Lint, LintId};
+use rustc_session::lint;
+use rustc_session::lint::builtin::DEAD_CODE;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_target::abi::FieldIdx;
 use std::mem;
@@ -766,6 +766,12 @@ enum ShouldWarnAboutField {
     No,
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+enum ReportOn {
+    TupleField,
+    NamedField,
+}
+
 impl<'tcx> DeadVisitor<'tcx> {
     fn should_warn_about_field(&mut self, field: &ty::FieldDef) -> ShouldWarnAboutField {
         if self.live_symbols.contains(&field.did.expect_local()) {
@@ -787,9 +793,9 @@ impl<'tcx> DeadVisitor<'tcx> {
         ShouldWarnAboutField::Yes
     }
 
-    fn def_lint_level(&self, lint: &'static Lint, id: LocalDefId) -> lint::Level {
+    fn def_lint_level(&self, id: LocalDefId) -> lint::Level {
         let hir_id = self.tcx.local_def_id_to_hir_id(id);
-        self.tcx.lint_level_at_node(lint, hir_id).0
+        self.tcx.lint_level_at_node(DEAD_CODE, hir_id).0
     }
 
     // # Panics
@@ -803,7 +809,7 @@ impl<'tcx> DeadVisitor<'tcx> {
         dead_codes: &[&DeadItem],
         participle: &str,
         parent_item: Option<LocalDefId>,
-        lint: &'static Lint,
+        report_on: ReportOn,
     ) {
         let Some(&first_item) = dead_codes.first() else {
             return;
@@ -864,8 +870,8 @@ impl<'tcx> DeadVisitor<'tcx> {
                 None
             };
 
-        let diag = if LintId::of(lint) == LintId::of(UNUSED_TUPLE_STRUCT_FIELDS) {
-            MultipleDeadCodes::UnusedTupleStructFields {
+        let diag = match report_on {
+            ReportOn::TupleField => MultipleDeadCodes::UnusedTupleStructFields {
                 multiple,
                 num,
                 descr,
@@ -874,9 +880,9 @@ impl<'tcx> DeadVisitor<'tcx> {
                 change_fields_suggestion: ChangeFieldsToBeOfUnitType { num, spans: spans.clone() },
                 parent_info,
                 ignored_derived_impls,
-            }
-        } else {
-            MultipleDeadCodes::DeadCodes {
+            },
+
+            ReportOn::NamedField => MultipleDeadCodes::DeadCodes {
                 multiple,
                 num,
                 descr,
@@ -884,11 +890,11 @@ impl<'tcx> DeadVisitor<'tcx> {
                 name_list,
                 parent_info,
                 ignored_derived_impls,
-            }
+            },
         };
 
         let hir_id = tcx.local_def_id_to_hir_id(first_item.def_id);
-        self.tcx.emit_spanned_lint(lint, hir_id, MultiSpan::from_spans(spans), diag);
+        self.tcx.emit_spanned_lint(DEAD_CODE, hir_id, MultiSpan::from_spans(spans), diag);
     }
 
     fn warn_multiple(
@@ -896,7 +902,7 @@ impl<'tcx> DeadVisitor<'tcx> {
         def_id: LocalDefId,
         participle: &str,
         dead_codes: Vec<DeadItem>,
-        lint: &'static Lint,
+        report_on: ReportOn,
     ) {
         let mut dead_codes = dead_codes
             .iter()
@@ -907,7 +913,7 @@ impl<'tcx> DeadVisitor<'tcx> {
         }
         dead_codes.sort_by_key(|v| v.level);
         for group in dead_codes[..].group_by(|a, b| a.level == b.level) {
-            self.lint_at_single_level(&group, participle, Some(def_id), lint);
+            self.lint_at_single_level(&group, participle, Some(def_id), report_on);
         }
     }
 
@@ -915,9 +921,9 @@ impl<'tcx> DeadVisitor<'tcx> {
         let item = DeadItem {
             def_id: id,
             name: self.tcx.item_name(id.to_def_id()),
-            level: self.def_lint_level(DEAD_CODE, id),
+            level: self.def_lint_level(id),
         };
-        self.lint_at_single_level(&[&item], participle, None, DEAD_CODE);
+        self.lint_at_single_level(&[&item], participle, None, ReportOn::NamedField);
     }
 
     fn check_definition(&mut self, def_id: LocalDefId) {
@@ -964,12 +970,12 @@ fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalModDefId) {
                 let def_id = item.id.owner_id.def_id;
                 if !visitor.is_live_code(def_id) {
                     let name = tcx.item_name(def_id.to_def_id());
-                    let level = visitor.def_lint_level(DEAD_CODE, def_id);
+                    let level = visitor.def_lint_level(def_id);
 
                     dead_items.push(DeadItem { def_id, name, level })
                 }
             }
-            visitor.warn_multiple(item.owner_id.def_id, "used", dead_items, DEAD_CODE);
+            visitor.warn_multiple(item.owner_id.def_id, "used", dead_items, ReportOn::NamedField);
         }
 
         if !live_symbols.contains(&item.owner_id.def_id) {
@@ -991,7 +997,7 @@ fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalModDefId) {
                 let def_id = variant.def_id.expect_local();
                 if !live_symbols.contains(&def_id) {
                     // Record to group diagnostics.
-                    let level = visitor.def_lint_level(DEAD_CODE, def_id);
+                    let level = visitor.def_lint_level(def_id);
                     dead_variants.push(DeadItem { def_id, name: variant.name, level });
                     continue;
                 }
@@ -999,24 +1005,30 @@ fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalModDefId) {
                 let is_positional = variant.fields.raw.first().map_or(false, |field| {
                     field.name.as_str().starts_with(|c: char| c.is_ascii_digit())
                 });
-                let lint = if is_positional { UNUSED_TUPLE_STRUCT_FIELDS } else { DEAD_CODE };
+                let report_on =
+                    if is_positional { ReportOn::TupleField } else { ReportOn::NamedField };
                 let dead_fields = variant
                     .fields
                     .iter()
                     .filter_map(|field| {
                         let def_id = field.did.expect_local();
                         if let ShouldWarnAboutField::Yes = visitor.should_warn_about_field(field) {
-                            let level = visitor.def_lint_level(lint, def_id);
+                            let level = visitor.def_lint_level(def_id);
                             Some(DeadItem { def_id, name: field.name, level })
                         } else {
                             None
                         }
                     })
                     .collect();
-                visitor.warn_multiple(def_id, "read", dead_fields, lint);
+                visitor.warn_multiple(def_id, "read", dead_fields, report_on);
             }
 
-            visitor.warn_multiple(item.owner_id.def_id, "constructed", dead_variants, DEAD_CODE);
+            visitor.warn_multiple(
+                item.owner_id.def_id,
+                "constructed",
+                dead_variants,
+                ReportOn::NamedField,
+            );
         }
     }