Implement let-else rewriting in terms of rewrite_let_else_block

`rewrite_let_else_block` gives us more control over allowing the `else`
block to be formatted on a single line than
`<ast::Block as Rewrite>::rewrite`.

src/expr.rs

@@ -609,6 +609,16 @@ fn rewrite_block_inner(
     result
 }
 
+/// Rewrite the divergent block of a `let-else` statement.
+pub(crate) fn rewrite_let_else_block(
+    block: &ast::Block,
+    allow_single_line: bool,
+    context: &RewriteContext<'_>,
+    shape: Shape,
+) -> Option<String> {
+    rewrite_block_inner(block, None, None, allow_single_line, context, shape)
+}
+
 // Rewrite condition if the given expression has one.
 pub(crate) fn rewrite_cond(
     context: &RewriteContext<'_>,

src/items.rs

@@ -18,7 +18,8 @@
 use crate::config::{BraceStyle, Config, IndentStyle, Version};
 use crate::expr::{
     is_empty_block, is_simple_block_stmt, rewrite_assign_rhs, rewrite_assign_rhs_with,
-    rewrite_assign_rhs_with_comments, rewrite_else_kw_with_comments, RhsAssignKind, RhsTactics,
+    rewrite_assign_rhs_with_comments, rewrite_else_kw_with_comments, rewrite_let_else_block,
+    RhsAssignKind, RhsTactics,
 };
 use crate::lists::{definitive_tactic, itemize_list, write_list, ListFormatting, Separator};
 use crate::macros::{rewrite_macro, MacroPosition};
@@ -132,7 +133,13 @@ fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String>
                     shape,
                 );
                 result.push_str(&else_kw);
-                result.push_str(&block.rewrite(context, shape)?);
+                let allow_single_line = !result.contains('\n');
+                result.push_str(&rewrite_let_else_block(
+                    block,
+                    allow_single_line,
+                    context,
+                    shape,
+                )?);
             };
         }