fix double parens issue (#78)

## Description of changes

closes #76

- added a test
- fixed some testing code
- fixed bug

Author: fdncred

src/formatting.rs

@@ -674,8 +674,16 @@ impl<'a> Formatter<'a> {
 
     /// Format a subexpression
     fn format_subexpression(&mut self, block_id: nu_protocol::BlockId) {
-        self.write("(");
         let block = self.working_set.get_block(block_id);
+        // Special case: subexpressions containing only a string interpolation don't need parentheses
+        if block.pipelines.len() == 1 && block.pipelines[0].elements.len() == 1 {
+            if let Expr::StringInterpolation(_) = &block.pipelines[0].elements[0].expr.expr {
+                self.format_block(block);
+                return;
+            }
+        }
+
+        self.write("(");
         let is_simple = block.pipelines.len() == 1 && block.pipelines[0].elements.len() <= 3;
 
         if is_simple {

tests/fixtures/expected/issue76.nu

@@ -0,0 +1 @@
+def pretty-print-command [] { ($"`(ansi default_dimmed)(ansi default_italic)($in)(ansi reset)`") }

tests/fixtures/expected/subexpression.nu

@@ -8,4 +8,5 @@
 let result = (1 + 2) * 3
 let value = ($x + (($y * 2)))
 print (echo "hello")
+let msg = $"Hello ($name)"
 if (true) { print "yes" }

tests/fixtures/input/issue76.nu

@@ -0,0 +1 @@
+def pretty-print-command [] { ($"`(ansi default_dimmed)(ansi default_italic)($in)(ansi reset)`") }

tests/fixtures/input/subexpression.nu

@@ -8,4 +8,5 @@
 let result  =  (1  +  2)  *  3
 let value  =  ($x  +  (($y  *  2)))
 print  (echo "hello")
+let msg = ($"Hello ($name)")
 if  (true)   {  print "yes"  }