Avoid some unnecessary blocks in derive output.

Author: nnethercote

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -339,11 +339,19 @@ impl BlockOrExpr {
     // Converts it into an expression.
     fn into_expr(self, cx: &ExtCtxt<'_>, span: Span) -> P<Expr> {
         if self.0.is_empty() {
+            // No statements.
             match self.1 {
                 None => cx.expr_block(cx.block(span, vec![])),
                 Some(expr) => expr,
             }
+        } else if self.0.len() == 1
+            && let ast::StmtKind::Expr(expr) = &self.0[0].kind
+            && self.1.is_none()
+        {
+            // There's only a single statement expression. Pull it out.
+            expr.clone()
         } else {
+            // Multiple statements and/or expressions.
             cx.expr_block(self.into_block(cx, span))
         }
     }

src/test/ui/deriving/deriving-all-codegen.stdout

@@ -697,9 +697,8 @@ impl ::core::fmt::Debug for Enum1 {
 impl ::core::hash::Hash for Enum1 {
     fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
         match self {
-            Enum1::Single { x: __self_0 } => {
-                ::core::hash::Hash::hash(__self_0, state)
-            }
+            Enum1::Single { x: __self_0 } =>
+                ::core::hash::Hash::hash(__self_0, state),
         }
     }
 }
@@ -870,10 +869,9 @@ impl ::core::default::Default for Fieldless {
 impl ::core::hash::Hash for Fieldless {
     fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
         match self {
-            _ => {
+            _ =>
                 ::core::hash::Hash::hash(&::core::intrinsics::discriminant_value(self),
-                    state)
-            }
+                    state),
         }
     }
 }
@@ -992,10 +990,9 @@ impl ::core::hash::Hash for Mixed {
                 ::core::hash::Hash::hash(__self_0, state);
                 ::core::hash::Hash::hash(__self_1, state)
             }
-            _ => {
+            _ =>
                 ::core::hash::Hash::hash(&::core::intrinsics::discriminant_value(self),
-                    state)
-            }
+                    state),
         }
     }
 }