Add ECR.render for rendering without setting up an IO (#6371)

spec/std/ecr/ecr_spec.cr

@@ -64,4 +64,8 @@ describe "ECR" do
     ECR.embed "#{__DIR__}/../data/test_template6.ecr", io
     io.to_s.should eq("string with -%")
   end
+
+  it ".render" do
+    ECR.render("#{__DIR__}/../data/test_template2.ecr").should eq("123")
+  end
 end

src/ecr/macros.cr

@@ -38,11 +38,11 @@ module ECR
     end
   end
 
-  # Embeds an ECR file contained in *filename* into the program.
+  # Embeds an ECR file *filename* into the program and appends the content to
+  # an IO in the variable *io_name*.
   #
   # The generated code is the result of translating the contents of
-  # the ECR file to Crystal, a program that appends to the IO
-  # with the given *io_name*.
+  # the ECR file to Crystal, a program that appends to an IO.
   #
   # ```text
   # # greeting.ecr
@@ -59,7 +59,7 @@ module ECR
   # io.to_s # => "Hello World!"
   # ```
   #
-  # The `ECR.embed` line basically generates this:
+  # The `ECR.embed` line basically generates this Crystal code:
   #
   # ```
   # io << "Hello "
@@ -69,4 +69,38 @@ module ECR
   macro embed(filename, io_name)
     \{{ run("ecr/process", {{filename}}, {{io_name.id.stringify}}) }}
   end
+
+  # Embeds an ECR file *filename* into the program and renders it to a string.
+  #
+  # The generated code is the result of translating the contents of
+  # the ECR file to Crystal, a program that appends to an IO and returns a string.
+  #
+  # ```text
+  # # greeting.ecr
+  # Hello <%= name %>!
+  # ```
+  #
+  # ```
+  # require "ecr/macros"
+  #
+  # name = "World"
+  #
+  # rendered = ECR.render "greeting.ecr"
+  # rendered # => "Hello World!"
+  # ```
+  #
+  # The `ECR.render` basically generates this Crystal code:
+  #
+  # ```
+  # String.build do |io|
+  #   io << "Hello "
+  #   io << name
+  #   io << '!'
+  # end
+  # ```
+  macro render(filename)
+    ::String.build do |%io|
+      ::ECR.embed({{filename}}, %io)
+    end
+  end
 end