Add internal attribute and tests.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -603,6 +603,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(TEST, rustc_dump_program_clauses, AssumedUsed, template!(Word)),
     rustc_attr!(TEST, rustc_dump_env_program_clauses, AssumedUsed, template!(Word)),
     rustc_attr!(TEST, rustc_object_lifetime_default, AssumedUsed, template!(Word)),
+    rustc_attr!(TEST, rustc_dump_vtable, AssumedUsed, template!(Word)),
     rustc_attr!(TEST, rustc_dummy, Normal, template!(Word /* doesn't matter*/)),
     gated!(
         omit_gdb_pretty_printer_section, AssumedUsed, template!(Word),

compiler/rustc_span/src/symbol.rs

@@ -1047,6 +1047,7 @@ symbols! {
         rustc_dump_env_program_clauses,
         rustc_dump_program_clauses,
         rustc_dump_user_substs,
+        rustc_dump_vtable,
         rustc_error,
         rustc_evaluate_where_clauses,
         rustc_expected_cgu_reuse,

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -34,7 +34,7 @@ use rustc_middle::ty::{
     self, GenericParamDefKind, ToPredicate, Ty, TyCtxt, VtblEntry, WithConstness,
     COMMON_VTABLE_ENTRIES,
 };
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 use smallvec::SmallVec;
 
 use std::fmt::Debug;
@@ -614,6 +614,11 @@ fn prepare_vtable_segments<'tcx, T>(
     }
 }
 
+fn dump_vtable_entries<'tcx>(tcx: TyCtxt<'tcx>, entries: &[VtblEntry<'tcx>]) {
+    let msg = format!("Vtable Entries: {:#?}", entries);
+    tcx.sess.struct_span_err(rustc_span::DUMMY_SP, &msg).emit();
+}
+
 /// Given a trait `trait_ref`, iterates the vtable entries
 /// that come from `trait_ref`, including its supertraits.
 fn vtable_entries<'tcx>(
@@ -691,6 +696,11 @@ fn vtable_entries<'tcx>(
     };
 
     let _ = prepare_vtable_segments(tcx, trait_ref, vtable_segment_callback);
+
+    if tcx.has_attr(trait_ref.def_id(), sym::rustc_dump_vtable) {
+        dump_vtable_entries(tcx, &entries);
+    }
+
     tcx.arena.alloc_from_iter(entries.into_iter())
 }
 

src/test/ui/traits/vtable/vtable-diamond.rs

@@ -0,0 +1,39 @@
+// build-fail
+//~^ error Vtable
+//~^^ error Vtable
+#![feature(rustc_attrs)]
+
+#[rustc_dump_vtable]
+trait A {
+    fn foo_a(&self) {}
+}
+
+#[rustc_dump_vtable]
+trait B: A {
+    fn foo_b(&self) {}
+}
+
+#[rustc_dump_vtable]
+trait C: A {
+    fn foo_c(&self) {}
+}
+
+#[rustc_dump_vtable]
+trait D: B + C {
+    fn foo_d(&self) {}
+}
+
+struct S;
+
+impl A for S {}
+impl B for S {}
+impl C for S {}
+impl D for S {}
+
+fn foo(d: &dyn D) {
+    d.foo_d();
+}
+
+fn main() {
+    foo(&S);
+}

src/test/ui/traits/vtable/vtable-diamond.stderr

@@ -0,0 +1,56 @@
+error: Vtable Entries: [
+    MetadataDropInPlace,
+    MetadataSize,
+    MetadataAlign,
+    Method(
+        DefId(0:4 ~ vtable_diamond[4564]::A::foo_a),
+        [
+            S,
+        ],
+    ),
+    Method(
+        DefId(0:6 ~ vtable_diamond[4564]::B::foo_b),
+        [
+            S,
+        ],
+    ),
+    Method(
+        DefId(0:8 ~ vtable_diamond[4564]::C::foo_c),
+        [
+            S,
+        ],
+    ),
+    TraitVPtr(
+        Binder(
+            C,
+            [],
+        ),
+    ),
+    Method(
+        DefId(0:10 ~ vtable_diamond[4564]::D::foo_d),
+        [
+            S,
+        ],
+    ),
+]
+
+error: Vtable Entries: [
+    MetadataDropInPlace,
+    MetadataSize,
+    MetadataAlign,
+    Method(
+        DefId(0:4 ~ vtable_diamond[4564]::A::foo_a),
+        [
+            S,
+        ],
+    ),
+    Method(
+        DefId(0:8 ~ vtable_diamond[4564]::C::foo_c),
+        [
+            S,
+        ],
+    ),
+]
+
+error: aborting due to 2 previous errors
+

src/test/ui/traits/vtable/vtable-multiple.rs

@@ -0,0 +1,31 @@
+// build-fail
+//~^ error Vtable
+//~^^ error Vtable
+#![feature(rustc_attrs)]
+
+#[rustc_dump_vtable]
+trait A {
+    fn foo_a(&self) {}
+}
+
+#[rustc_dump_vtable]
+trait B {
+    fn foo_b(&self) {}
+}
+
+#[rustc_dump_vtable]
+trait C: A + B {
+    fn foo_c(&self) {}
+}
+
+struct S;
+
+impl A for S {}
+impl B for S {}
+impl C for S {}
+
+fn foo(c: &dyn C) {}
+
+fn main() {
+    foo(&S);
+}

src/test/ui/traits/vtable/vtable-multiple.stderr

@@ -0,0 +1,44 @@
+error: Vtable Entries: [
+    MetadataDropInPlace,
+    MetadataSize,
+    MetadataAlign,
+    Method(
+        DefId(0:4 ~ vtable_multiple[5246]::A::foo_a),
+        [
+            S,
+        ],
+    ),
+    Method(
+        DefId(0:6 ~ vtable_multiple[5246]::B::foo_b),
+        [
+            S,
+        ],
+    ),
+    TraitVPtr(
+        Binder(
+            B,
+            [],
+        ),
+    ),
+    Method(
+        DefId(0:8 ~ vtable_multiple[5246]::C::foo_c),
+        [
+            S,
+        ],
+    ),
+]
+
+error: Vtable Entries: [
+    MetadataDropInPlace,
+    MetadataSize,
+    MetadataAlign,
+    Method(
+        DefId(0:6 ~ vtable_multiple[5246]::B::foo_b),
+        [
+            S,
+        ],
+    ),
+]
+
+error: aborting due to 2 previous errors
+