Add urls to rust lang reference

Author: sancho20021

compiler/rustc_hir/src/hir.rs

@@ -1632,6 +1632,9 @@ pub struct ConstBlock {
 }
 
 /// An expression.
+/// For more details, see the [rust lang reference]
+///
+/// [rust lang reference]: https://doc.rust-lang.org/reference/expressions.html
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub struct Expr<'hir> {
     pub hir_id: HirId,
@@ -3146,6 +3149,9 @@ impl ItemId {
 /// An item
 ///
 /// The name might be a dummy name in case of anonymous items
+/// For more details, see the [rust lang reference]
+///
+/// [rust lang reference]: https://doc.rust-lang.org/reference/items.html
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub struct Item<'hir> {
     pub ident: Ident,