Initial runtime test compilation

Allows for compiling single examples at runtime.

spec/helpers/example.cr

@@ -0,0 +1,71 @@
+require "ecr"
+require "json"
+require "./result"
+
+module Spectator::SpecHelpers
+  # Wrapper for compiling and running an example at runtime and getting a result.
+  class Example
+    # Creates the example.
+    # The *spec_helper_path* is the path to spec_helper.cr file.
+    # The name or ID of the example is given by *example_id*.
+    # Lastly, the source code for the example is given by *example_code*.
+    def initialize(@spec_helper_path : String, @example_id : String, @example_code : String)
+    end
+
+    # Instructs the Crystal compiler to compile the test.
+    # Returns an instance of `JSON::Any`.
+    # This will be the outcome and information about the test.
+    # Output will be surpressed for the test.
+    # If an error occurs while attempting to compile and run the test, an error will be raised.
+    def compile
+      # Create a temporary file containing the test.
+      with_tempfile do |source_file|
+        args = ["run", "--no-color", source_file, "--", "--json"]
+        Process.run(crystal_executable, args) do |process|
+          JSON.parse(process.output)
+        rescue JSON::ParseException
+          raise "Compilation of example #{@example_id} failed\n\n#{process.error.gets_to_end}"
+        end
+      end
+    end
+
+    # Same as `#compile`, but returns the result of the first example in the test.
+    # Returns a `SpectatorHelpers::Result` instance.
+    def result
+      output = compile
+      example = output["examples"][0]
+      Result.from_json_any(example)
+    end
+
+    # Constructs the string representation of the example.
+    # This produces the Crystal source code.
+    # *io* is the file handle to write to.
+    # The *dir* is the directory of the file being written to.
+    # This is needed to resolve the relative path to the spec_helper.cr file.
+    private def write(io, dir)
+      spec_helper_path = Path[@spec_helper_path].relative_to(dir)
+      ECR.embed(__DIR__ + "/example.ecr", io)
+    end
+
+    # Creates a temporary file containing the compilable example code.
+    # Yields the path of the temporary file.
+    # Ensures the file is deleted after it is done being used.
+    private def with_tempfile
+      tempfile = File.tempfile("_#{@example_id}_spec.cr") do |file|
+        dir = File.dirname(file.path)
+        write(file, dir)
+      end
+
+      begin
+        yield tempfile.path
+      ensure
+        tempfile.delete
+      end
+    end
+
+    # Attempts to find the Crystal compiler on the system or raises an error.
+    private def crystal_executable
+      Process.find_executable("crystal") || raise("Could not find Crystal compiler")
+    end
+  end
+end