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.

Author: shepmaster

compiler/rustc_lint_defs/src/builtin.rs

@@ -569,8 +569,13 @@ 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"
@@ -604,32 +609,6 @@ declare_lint! {
     "detects attributes that were not used by the compiler"
 }
 
-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.
     ///
@@ -3466,7 +3445,6 @@ declare_lint_pass! {
         UNUSED_MACROS,
         UNUSED_MUT,
         UNUSED_QUALIFICATIONS,
-        UNUSED_TUPLE_STRUCT_FIELDS,
         UNUSED_UNSAFE,
         UNUSED_VARIABLES,
         USELESS_DEPRECATED,

compiler/rustc_passes/src/dead.rs

@@ -835,12 +835,6 @@ impl<'tcx> DeadVisitor<'tcx> {
         let multiple = num > 6;
         let name_list = names.into();
 
-        let lint = if is_positional {
-            lint::builtin::UNUSED_TUPLE_STRUCT_FIELDS
-        } else {
-            lint::builtin::DEAD_CODE
-        };
-
         let parent_info = if let Some(parent_item) = parent_item {
             let parent_descr = tcx.def_descr(parent_item.to_def_id());
             let span = if let DefKind::Impl { .. } = tcx.def_kind(parent_item) {
@@ -893,7 +887,12 @@ impl<'tcx> DeadVisitor<'tcx> {
             }
         };
 
-        self.tcx.emit_spanned_lint(lint, first_hir_id, MultiSpan::from_spans(spans), diag);
+        self.tcx.emit_spanned_lint(
+            lint::builtin::DEAD_CODE,
+            first_hir_id,
+            MultiSpan::from_spans(spans),
+            diag,
+        );
     }
 
     fn warn_multiple(
@@ -1013,17 +1012,11 @@ fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalModDefId) {
                         if let ShouldWarnAboutField::Yes(is_pos) =
                             visitor.should_warn_about_field(field)
                         {
-                            let level = tcx
-                                .lint_level_at_node(
-                                    if is_pos {
-                                        is_positional = true;
-                                        lint::builtin::UNUSED_TUPLE_STRUCT_FIELDS
-                                    } else {
-                                        lint::builtin::DEAD_CODE
-                                    },
-                                    hir_id,
-                                )
-                                .0;
+                            if is_pos {
+                                is_positional = true;
+                            }
+
+                            let level = tcx.lint_level_at_node(lint::builtin::DEAD_CODE, hir_id).0;
                             Some(DeadItem { def_id, name: field.name, level })
                         } else {
                             None