Rollup merge of rust-lang#69901 - RalfJung:rustc_layout, r=eddyb

add #[rustc_layout(debug)]

@eddyb recently told me about the `#[rustc_layout]` attribute, and I think it would be very useful if it could be used to print all the layout information Rust has about a type. When working with layouts (e.g. in Miri), it is often not clear how certain surface language features get represented internally. I have some awful hacks locally to be able to dump this debug information; with this attribute I could get it on the playground which is so much better. :)

Author: Dylan-DPC

src/librustc_passes/layout_test.rs

@@ -17,32 +17,38 @@ use rustc_span::symbol::sym;
 pub fn test_layout(tcx: TyCtxt<'_>) {
     if tcx.features().rustc_attrs {
         // if the `rustc_attrs` feature is not enabled, don't bother testing layout
-        tcx.hir().krate().visit_all_item_likes(&mut VarianceTest { tcx });
+        tcx.hir().krate().visit_all_item_likes(&mut LayoutTest { tcx });
     }
 }
 
-struct VarianceTest<'tcx> {
+struct LayoutTest<'tcx> {
     tcx: TyCtxt<'tcx>,
 }
 
-impl ItemLikeVisitor<'tcx> for VarianceTest<'tcx> {
+impl ItemLikeVisitor<'tcx> for LayoutTest<'tcx> {
     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
         let item_def_id = self.tcx.hir().local_def_id(item.hir_id);
 
-        if let ItemKind::TyAlias(..) = item.kind {
-            for attr in self.tcx.get_attrs(item_def_id).iter() {
-                if attr.check_name(sym::rustc_layout) {
-                    self.dump_layout_of(item_def_id, item, attr);
+        match item.kind {
+            ItemKind::TyAlias(..)
+            | ItemKind::Enum(..)
+            | ItemKind::Struct(..)
+            | ItemKind::Union(..) => {
+                for attr in self.tcx.get_attrs(item_def_id).iter() {
+                    if attr.check_name(sym::rustc_layout) {
+                        self.dump_layout_of(item_def_id, item, attr);
+                    }
                 }
             }
+            _ => {}
         }
     }
 
     fn visit_trait_item(&mut self, _: &'tcx hir::TraitItem<'tcx>) {}
     fn visit_impl_item(&mut self, _: &'tcx hir::ImplItem<'tcx>) {}
 }
 
-impl VarianceTest<'tcx> {
+impl LayoutTest<'tcx> {
     fn dump_layout_of(&self, item_def_id: DefId, item: &hir::Item<'tcx>, attr: &Attribute) {
         let tcx = self.tcx;
         let param_env = self.tcx.param_env(item_def_id);
@@ -81,6 +87,13 @@ impl VarianceTest<'tcx> {
                             );
                         }
 
+                        sym::debug => {
+                            self.tcx.sess.span_err(
+                                item.span,
+                                &format!("layout debugging: {:#?}", *ty_layout),
+                            );
+                        }
+
                         name => {
                             self.tcx.sess.span_err(
                                 meta_item.span(),

src/librustc_span/symbol.rs

@@ -253,6 +253,7 @@ symbols! {
         debug_trait,
         declare_lint_pass,
         decl_macro,
+        debug,
         Debug,
         Decodable,
         Default,

src/test/ui/layout/debug.rs

@@ -0,0 +1,15 @@
+// normalize-stderr-test "pref: Align \{\n *pow2: [1-3],\n *\}" -> "pref: $$PREF_ALIGN"
+#![feature(never_type, rustc_attrs)]
+#![crate_type = "lib"]
+
+#[rustc_layout(debug)]
+enum E { Foo, Bar(!, i32, i32) } //~ ERROR: layout debugging
+
+#[rustc_layout(debug)]
+struct S { f1: i32, f2: (), f3: i32 } //~ ERROR: layout debugging
+
+#[rustc_layout(debug)]
+union U { f1: (i32, i32), f3: i32 } //~ ERROR: layout debugging
+
+#[rustc_layout(debug)]
+type Test = Result<i32, i32>; //~ ERROR: layout debugging