Add note to docs when layout cannot be computed

This should prevent confusion about why generic types don't have layout
docs.

src/librustdoc/html/render/print_item.rs

@@ -7,6 +7,7 @@ use rustc_hir as hir;
 use rustc_hir::def::CtorKind;
 use rustc_hir::def_id::DefId;
 use rustc_middle::middle::stability;
+use rustc_middle::ty::layout::LayoutError;
 use rustc_middle::ty::TyCtxt;
 use rustc_span::hygiene::MacroKind;
 use rustc_span::symbol::{kw, sym, Symbol};
@@ -1540,13 +1541,14 @@ fn document_type_layout(w: &mut Buffer, cx: &Context<'_>, ty_def_id: DefId) {
         return;
     }
 
+    writeln!(w, "<h2 class=\"small-section-header\">Layout</h2>");
+    writeln!(w, "<div class=\"docblock\">");
+
     let tcx = cx.tcx();
     let param_env = tcx.param_env(ty_def_id);
     let ty = tcx.type_of(ty_def_id);
     match tcx.layout_of(param_env.and(ty)) {
         Ok(ty_layout) => {
-            writeln!(w, "<h2 class=\"small-section-header\">Layout</h2>");
-            writeln!(w, "<div class=\"docblock\">");
             writeln!(
                 w,
                 "<div class=\"warning\"><p><strong>Note:</strong> Most layout information is \
@@ -1567,11 +1569,28 @@ fn document_type_layout(w: &mut Buffer, cx: &Context<'_>, ty_def_id: DefId) {
                     pl = if bytes == 1 { "" } else { "s" },
                 );
             }
-            writeln!(w, "</div>");
         }
-        // Layout errors can occur with valid code, e.g. if you try to get the layout
-        // of a generic type such as `Vec<T>`. In case of a layout error, we just
-        // don't show any layout information.
-        Err(_) => {}
+        // This kind of layout error can occur with valid code, e.g. if you try to
+        // get the layout of a generic type such as `Vec<T>`.
+        Err(LayoutError::Unknown(_)) => {
+            writeln!(
+                w,
+                "<p><strong>Note:</strong> Unable to compute type layout, \
+                 possibly due to this type having generic parameters. \
+                 Layout can only be computed for concrete, fully-instantiated types.</p>"
+            );
+        }
+        // This kind of error probably can't happen with valid code, but we don't
+        // want to panic and prevent the docs from building, so we just let the
+        // user know that we couldn't compute the layout.
+        Err(LayoutError::SizeOverflow(_)) => {
+            writeln!(
+                w,
+                "<p><strong>Note:</strong> Encountered an error during type layout; \
+                 the type was too big.</p>"
+            );
+        }
     }
+
+    writeln!(w, "</div>");
 }

src/test/rustdoc/type-layout.rs

@@ -35,9 +35,17 @@ pub struct Y(u8);
 // @has - '0 bytes'
 pub struct Z;
 
-// @!has type_layout/struct.Generic.html 'Size: '
+// We can't compute layout for generic types.
+// @has type_layout/struct.Generic.html 'Unable to compute type layout, possibly due to this type having generic parameters'
+// @!has - 'Size: '
 pub struct Generic<T>(T);
 
+// We *can*, however, compute layout for types that are only generic over lifetimes,
+// because lifetimes are a type-system construct.
+// @has type_layout/struct.GenericLifetimes.html 'Size: '
+// @has - ' bytes'
+pub struct GenericLifetimes<'a>(&'a str);
+
 // @has type_layout/struct.Unsized.html 'Size: '
 // @has - '(unsized)'
 pub struct Unsized([u8]);