Correctly create artificial span for formatting closure body

This commit partially reverts rust-lang#3934, opting to create a span that covers
the entire body of a closure when formatting a closure body with a
block-formatting strategy, rather than having the block-formatting code
determine if the visitor pointer should be rewound. The problem with
rewinding the visitor pointer is it may be incorrect for other (i.e.
non-artificial) AST nodes, as in the case of rust-lang#4382.

Closes rust-lang#4382

Author: ayazhafiz

src/closures.rs

@@ -133,6 +133,7 @@ fn veto_block(e: &ast::Expr) -> bool {
 }
 
 // Rewrite closure with a single expression wrapping its body with block.
+// || { #[attr] foo() } -> Block { #[attr] foo() }
 fn rewrite_closure_with_block(
     body: &ast::Expr,
     prefix: &str,
@@ -154,8 +155,12 @@ fn rewrite_closure_with_block(
         }],
         id: ast::NodeId::root(),
         rules: ast::BlockCheckMode::Default,
-        span: body.span,
         tokens: None,
+        span: body
+            .attrs
+            .first()
+            .map(|attr| attr.span.to(body.span))
+            .unwrap_or(body.span),
     };
     let block = crate::expr::rewrite_block_with_visitor(
         context,

src/expr.rs

@@ -528,17 +528,7 @@ pub(crate) fn rewrite_block_with_visitor(
             let open_pos = snippet.find_uncommented("{")?;
             visitor.last_pos = block.span.lo() + BytePos(open_pos as u32)
         }
-        (ast::BlockCheckMode::Default, None) => {
-            visitor.last_pos = block.span.lo();
-            if let Some(attrs) = attrs {
-                if let Some(first) = attrs.first() {
-                    let first_lo_span = first.span.lo();
-                    if first_lo_span < visitor.last_pos {
-                        visitor.last_pos = first_lo_span;
-                    }
-                }
-            }
-        }
+        (ast::BlockCheckMode::Default, None) => visitor.last_pos = block.span.lo(),
     }
 
     let inner_attrs = attrs.map(inner_attributes);