Add ASTNode#single_expression and refactor with using it

Fixed crystal-lang#5482
Fixed crystal-lang#5511

But this commit contains no spec for crystal-lang#5511 because I don't know where to
place such a spec.

spec/compiler/normalize/case_spec.cr

@@ -53,6 +53,10 @@ describe "Normalize: case" do
     assert_expand "case x = 1; when 2; 3; end", "x = 1\nif 2 === x\n  3\nend"
   end
 
+  it "normalizes case with assignment wrapped by paren" do
+    assert_expand "case (x = 1); when 2; 3; end", "x = 1\nif 2 === x\n  3\nend"
+  end
+
   it "normalizes case without value" do
     assert_expand "case when 2; 3; when 4; 5; end", "if 2\n  3\nelse\n  if 4\n    5\n  end\nend"
   end

src/compiler/crystal/semantic/cleanup_transformer.cr

@@ -128,6 +128,10 @@ module Crystal
     end
 
     def transform(node : Expressions)
+      if exp = node.single_expression?
+        return exp.transform(self)
+      end
+
       exps = [] of ASTNode
 
       node.expressions.each_with_index do |exp, i|
@@ -144,6 +148,7 @@ module Crystal
     end
 
     def flatten_collect(exp, exps)
+      exp = exp.single_expression
       if exp.is_a?(Expressions)
         exp.expressions.each do |subexp|
           return true if flatten_collect(subexp, exps)

src/compiler/crystal/semantic/literal_expander.cr

@@ -208,11 +208,7 @@ module Crystal
     #       temp
     #     end
     def expand(node : And)
-      left = node.left
-
-      if left.is_a?(Expressions) && left.expressions.size == 1
-        left = left.expressions.first
-      end
+      left = node.left.single_expression
 
       new_node = if left.is_a?(Var) || (left.is_a?(IsA) && left.obj.is_a?(Var))
                    If.new(left, node.right, left.clone)
@@ -245,11 +241,7 @@ module Crystal
     #       b
     #     end
     def expand(node : Or)
-      left = node.left
-
-      if left.is_a?(Expressions) && left.expressions.size == 1
-        left = left.expressions.first
-      end
+      left = node.left.single_expression
 
       new_node = if left.is_a?(Var) || (left.is_a?(IsA) && left.obj.is_a?(Var))
                    If.new(left, left.clone, node.right)
@@ -390,7 +382,7 @@ module Crystal
 
         assigns = [] of ASTNode
         temp_vars = conds.map do |cond|
-          case cond
+          case cond = cond.single_expression
           when Var, InstanceVar
             temp_var = cond
           when Assign

src/compiler/crystal/semantic/main_visitor.cr

@@ -1742,7 +1742,7 @@ module Crystal
         target = exp.target
         return target if target.is_a?(Var)
       when Expressions
-        return unless exp = single_expression(exp)
+        return unless exp = exp.single_expression?
         return get_expression_var(exp)
       end
       nil
@@ -1850,13 +1850,13 @@ module Crystal
       # block is when the condition is a Var (in the else it must be
       # nil), IsA (in the else it's not that type), RespondsTo
       # (in the else it doesn't respond to that message) or Not.
-      case cond = single_expression(node.cond) || node.cond
+      case cond = node.cond.single_expression
       when Var, IsA, RespondsTo, Not
         filter_vars cond_type_filters, &.not
       when Or
         # Try to apply boolean logic: `!(a || b)` is `!a && !b`
-        cond_left = single_expression(cond.left) || cond.left
-        cond_right = single_expression(cond.right) || cond.right
+        cond_left = cond.left.single_expression
+        cond_right = cond.right.single_expression
 
         #  We can't deduce anything for sub && or || expressions
         or_left_type_filters = nil if cond_left.is_a?(And) || cond_left.is_a?(Or)
@@ -2026,7 +2026,7 @@ module Crystal
         node.body.accept self
       end
 
-      cond = single_expression(node.cond) || node.cond
+      cond = node.cond.single_expression
 
       endless_while = cond.true_literal?
       merge_while_vars cond, endless_while, before_cond_vars_copy, before_cond_vars, after_cond_vars, @vars, node.break_vars
@@ -2158,7 +2158,7 @@ module Crystal
       when Call
         return get_while_cond_assign_target(node.obj)
       when Expressions
-        return unless node = single_expression(node)
+        return unless node = node.single_expression?
         return get_while_cond_assign_target(node)
       end
 
@@ -2192,16 +2192,6 @@ module Crystal
       end
     end
 
-    def single_expression(node)
-      result = nil
-
-      while node.is_a?(Expressions) && node.expressions.size == 1
-        result = node = node[0]
-      end
-
-      result
-    end
-
     def end_visit(node : Break)
       if last_block_kind == :ensure
         node.raise "can't use break inside ensure"

src/compiler/crystal/syntax/ast.cr

@@ -89,6 +89,18 @@ module Crystal
     def pretty_print(pp)
       pp.text to_s
     end
+
+    # It yields itself for any node, but `Expressions` yields first node
+    # if it holds only a node.
+    def single_expression
+      single_expression? || self
+    end
+
+    # It yields `nil` always.
+    # (It is overrided by `Expressions` to implement `#single_expression`.)
+    def single_expression?
+      nil
+    end
   end
 
   class Nop < ASTNode
@@ -146,6 +158,13 @@ module Crystal
       @end_location || @expressions.last?.try &.end_location
     end
 
+    # It yields first node if this holds only one node, or yields `nil`.
+    def single_expression?
+      return @expressions.first.single_expression if @expressions.size == 1
+
+      nil
+    end
+
     def accept_children(visitor)
       @expressions.each &.accept visitor
     end