Use failure location in output if available

Fixes https://gitlab.com/arctic-fox/spectator/-/issues/57

src/spectator/fail_result.cr

@@ -1,4 +1,6 @@
 require "json"
+require "./example_failed"
+require "./location"
 require "./result"
 
 module Spectator
@@ -36,6 +38,15 @@ module Spectator
       true
     end
 
+    # Attempts to retrieve the location where the example failed.
+    # This only works if the location of the failed expectation was reported.
+    # If available, returns a `Location`, otherwise `nil`.
+    def source : Location?
+      return unless error = @error.as?(ExampleFailed)
+
+      error.location?
+    end
+
     # One-word description of the result.
     def to_s(io)
       io << "fail"

src/spectator/formatting/components/example_command.cr

@@ -1,11 +1,14 @@
 require "../../example"
+require "../../location"
 require "./comment"
 
 module Spectator::Formatting::Components
   # Provides syntax for running a specific example from the command-line.
   struct ExampleCommand
     # Creates the component with the specified example.
-    def initialize(@example : Example)
+    # The location can be overridden, for instance, pointing to a problematic line in the example.
+    # Otherwise the example's location is used.
+    def initialize(@example : Example, @location : Location? = nil)
     end
 
     # Produces output for running the previously specified example.
@@ -14,7 +17,7 @@ module Spectator::Formatting::Components
 
       # Use location for argument if it's available, since it's simpler.
       # Otherwise, use the example name filter argument.
-      if location = @example.location?
+      if location = (@location || @example.location?)
         io << location
       else
         io << "-e " << @example

src/spectator/formatting/components/failure_command_list.cr

@@ -14,7 +14,12 @@ module Spectator::Formatting::Components
       io.puts "Failed examples:"
       io.puts
       @failures.each do |failure|
-        io.puts ExampleCommand.new(failure).colorize(:red)
+        # Use failed location if it's available.
+        if (result = failure.result).responds_to?(:source)
+          location = result.source
+        end
+
+        io.puts ExampleCommand.new(failure, location).colorize(:red)
       end
     end
   end