Support {% elsif %} when stringifying MacroIf nodes (#15928)

Author: HertzDevil

spec/compiler/parser/to_s_spec.cr

@@ -284,6 +284,44 @@ describe "ASTNode#to_s" do
     end
     CRYSTAL
 
+  expect_to_s <<-'CRYSTAL'
+    {% if 1 %}
+      2
+    {% elsif 3 %}
+      4
+    {% elsif 5 %}
+    {% elsif 6 %}
+    {% else %}
+      7
+    {% end %}
+    CRYSTAL
+
+  expect_to_s <<-'CRYSTAL', <<-'CRYSTAL'
+    {% if 1 %}
+      2
+    {% else %}{% if 3 %}
+    {% end %}{% end %}
+    CRYSTAL
+    {% if 1 %}
+      2
+    {% elsif 3 %}
+    {% end %}
+    CRYSTAL
+
+  expect_to_s <<-'CRYSTAL'
+    {% if 1 %}
+      2
+    {% else %}{% unless 3 %}
+    {% end %}{% end %}
+    CRYSTAL
+
+  expect_to_s <<-'CRYSTAL'
+    {% unless 1 %}
+      2
+    {% else %}{% if 3 %}
+    {% end %}{% end %}
+    CRYSTAL
+
   expect_to_s %(foo do\n  begin\n    bar\n  end\nend)
   expect_to_s %q("\e\0\""), %q("\e\u0000\"")
   expect_to_s %q("#{1}\0"), %q("#{1}\u0000")

src/compiler/crystal/syntax/to_s.cr

@@ -932,20 +932,33 @@ module Crystal
     end
 
     def visit(node : MacroIf)
-      if node.is_unless?
-        @str << "{% unless "
-        then_node = node.else
-        else_node = node.then
-      else
-        @str << "{% if "
-        then_node = node.then
-        else_node = node.else
-      end
-      node.cond.accept self
-      @str << " %}"
+      else_node = nil
 
-      inside_macro do
-        then_node.accept self
+      while true
+        if node.is_unless?
+          @str << "{% unless "
+          then_node = node.else
+          else_node = node.then
+        else
+          @str << (else_node ? "{% elsif " : "{% if ")
+          then_node = node.then
+          else_node = node.else
+        end
+        node.cond.accept self
+        @str << " %}"
+
+        inside_macro do
+          then_node.accept self
+        end
+
+        # combine `{% else %}{% if %}` into `{% elsif %}` (does not apply to
+        # `{% unless %}`, nor when there is whitespace inbetween, as that would
+        # show up as a `MacroLiteral`)
+        if !node.is_unless? && else_node.is_a?(MacroIf) && !else_node.is_unless?
+          node = else_node
+        else
+          break
+        end
       end
 
       unless else_node.nop?