Rustdoc: Report Layout of enum variants

src/librustdoc/html/render/print_item.rs

@@ -8,10 +8,12 @@ use rustc_hir as hir;
 use rustc_hir::def::CtorKind;
 use rustc_hir::def_id::DefId;
 use rustc_middle::middle::stability;
+use rustc_middle::span_bug;
 use rustc_middle::ty::layout::LayoutError;
-use rustc_middle::ty::TyCtxt;
+use rustc_middle::ty::{Adt, TyCtxt};
 use rustc_span::hygiene::MacroKind;
 use rustc_span::symbol::{kw, sym, Symbol};
+use rustc_target::abi::{Layout, Primitive, TagEncoding, Variants};
 
 use super::{
     collect_paths_for_type, document, ensure_trailing_slash, item_ty_to_strs, notable_traits_decl,
@@ -1604,6 +1606,15 @@ fn document_non_exhaustive(w: &mut Buffer, item: &clean::Item) {
 }
 
 fn document_type_layout(w: &mut Buffer, cx: &Context<'_>, ty_def_id: DefId) {
+    fn write_size_of_layout(w: &mut Buffer, layout: &Layout, tag_size: u64) {
+        if layout.abi.is_unsized() {
+            write!(w, "(unsized)");
+        } else {
+            let bytes = layout.size.bytes() - tag_size;
+            write!(w, "{size} byte{pl}", size = bytes, pl = if bytes == 1 { "" } else { "s" },);
+        }
+    }
+
     if !cx.shared.show_type_layout {
         return;
     }
@@ -1625,16 +1636,40 @@ fn document_type_layout(w: &mut Buffer, cx: &Context<'_>, ty_def_id: DefId) {
                  <a href=\"https://doc.rust-lang.org/reference/type-layout.html\">“Type Layout”</a> \
                  chapter for details on type layout guarantees.</p></div>"
             );
-            if ty_layout.layout.abi.is_unsized() {
-                writeln!(w, "<p><strong>Size:</strong> (unsized)</p>");
-            } else {
-                let bytes = ty_layout.layout.size.bytes();
-                writeln!(
-                    w,
-                    "<p><strong>Size:</strong> {size} byte{pl}</p>",
-                    size = bytes,
-                    pl = if bytes == 1 { "" } else { "s" },
-                );
+            w.write_str("<p><strong>Size:</strong> ");
+            write_size_of_layout(w, ty_layout.layout, 0);
+            writeln!(w, "</p>");
+            if let Variants::Multiple { variants, tag, tag_encoding, .. } =
+                &ty_layout.layout.variants
+            {
+                if !variants.is_empty() {
+                    w.write_str(
+                        "<p><strong>Size for each variant:</strong></p>\
+                            <ul>",
+                    );
+
+                    let adt = if let Adt(adt, _) = ty_layout.ty.kind() {
+                        adt
+                    } else {
+                        span_bug!(tcx.def_span(ty_def_id), "not an adt")
+                    };
+
+                    let tag_size = if let TagEncoding::Niche { .. } = tag_encoding {
+                        0
+                    } else if let Primitive::Int(i, _) = tag.value {
+                        i.size().bytes()
+                    } else {
+                        span_bug!(tcx.def_span(ty_def_id), "tag is neither niche nor int")
+                    };
+
+                    for (index, layout) in variants.iter_enumerated() {
+                        let ident = adt.variants[index].ident;
+                        write!(w, "<li><code>{name}</code>: ", name = ident);
+                        write_size_of_layout(w, layout, tag_size);
+                        writeln!(w, "</li>");
+                    }
+                    w.write_str("</ul>");
+                }
             }
         }
         // This kind of layout error can occur with valid code, e.g. if you try to

src/test/rustdoc/type-layout.rs

@@ -52,3 +52,21 @@ pub struct Unsized([u8]);
 
 // @!has type_layout/trait.MyTrait.html 'Size: '
 pub trait MyTrait {}
+
+// @has type_layout/enum.Variants.html 'Size: '
+// @has - '2 bytes'
+// @has - '<code>A</code>: 0 bytes'
+// @has - '<code>B</code>: 1 byte'
+pub enum Variants {
+    A,
+    B(u8),
+}
+
+// @has type_layout/enum.WithNiche.html 'Size: '
+// @has - //p '4 bytes'
+// @has - '<code>None</code>: 0 bytes'
+// @has - '<code>Some</code>: 4 bytes'
+pub enum WithNiche {
+    None,
+    Some(std::num::NonZeroU32),
+}