Print build errors to stderr

Remove unnecessary I/O flush, it's already done in the `main`.

Note: The notation `command 2>&1 >/dev/null` will close stdout, then
redirect stderr to (a new) stdout

spec/compiler/crystal/tools/init_spec.cr

@@ -163,27 +163,27 @@ end
     it "prints error if a directory already present" do
       Dir.mkdir_p("#{__DIR__}/tmp")
 
-      `bin/crystal init lib "#{__DIR__}/tmp" 2>/dev/null`.should contain("file or directory #{__DIR__}/tmp already exists")
+      `bin/crystal init lib "#{__DIR__}/tmp" 2>&1 >/dev/null`.should contain("file or directory #{__DIR__}/tmp already exists")
 
       `rm -rf #{__DIR__}/tmp`
     end
 
     it "prints error if a file already present" do
       File.open("#{__DIR__}/tmp", "w")
 
-      `bin/crystal init lib "#{__DIR__}/tmp" 2>/dev/null`.should contain("file or directory #{__DIR__}/tmp already exists")
+      `bin/crystal init lib "#{__DIR__}/tmp" 2>&1 >/dev/null`.should contain("file or directory #{__DIR__}/tmp already exists")
 
       File.delete("#{__DIR__}/tmp")
     end
 
     it "honors the custom set directory name" do
       Dir.mkdir_p("tmp")
 
-      `bin/crystal init lib tmp 2>/dev/null`.should contain("file or directory tmp already exists")
+      `bin/crystal init lib tmp 2>&1 >/dev/null`.should contain("file or directory tmp already exists")
 
-      `bin/crystal init lib tmp "#{__DIR__}/fresh-new-tmp" 2>/dev/null`.should_not contain("file or directory tmp already exists")
+      `bin/crystal init lib tmp "#{__DIR__}/fresh-new-tmp" 2>&1 >/dev/null`.should_not contain("file or directory tmp already exists")
 
-      `bin/crystal init lib tmp "#{__DIR__}/fresh-new-tmp" 2>/dev/null`.should contain("file or directory #{__DIR__}/fresh-new-tmp already exists")
+      `bin/crystal init lib tmp "#{__DIR__}/fresh-new-tmp" 2>&1 >/dev/null`.should contain("file or directory #{__DIR__}/fresh-new-tmp already exists")
 
       `rm -rf tmp #{__DIR__}/fresh-new-tmp`
     end

src/compiler/crystal/command.cr

@@ -116,19 +116,19 @@ class Crystal::Command
   rescue ex : Crystal::Exception
     ex.color = @color
     if @config.try(&.output_format) == "json"
-      puts ex.to_json
+      STDERR.puts ex.to_json
     else
-      puts ex
+      STDERR.puts ex
     end
     exit 1
   rescue ex : OptionParser::Exception
     error ex.message
   rescue ex
-    puts ex
+    STDERR.puts ex
     ex.backtrace.each do |frame|
-      puts frame
+      STDERR.puts frame
     end
-    puts
+    STDERR.puts
     error "you've found a bug in the Crystal compiler. Please open an issue, including source code that will allow us to reproduce the bug: https://github.com/crystal-lang/crystal/issues"
   end
 
@@ -422,7 +422,7 @@ class Crystal::Command
     end
 
     if filenames.size == 0 || (cursor_command && cursor_location.nil?)
-      puts option_parser
+      STDERR.puts option_parser
       exit 1
     end
 

src/compiler/crystal/command/format.cr

@@ -84,23 +84,21 @@ class Crystal::Command
       exit(result == source ? 0 : 1) if check_files
 
       print result
-      STDOUT.flush
     rescue ex : InvalidByteSequenceError
-      print "Error: ".colorize.toggle(@color).red.bold
-      print "source is not a valid Crystal source file: ".colorize.toggle(@color).bold
-      puts ex.message
+      STDERR.print "Error: ".colorize.toggle(@color).red.bold
+      STDERR.print "source is not a valid Crystal source file: ".colorize.toggle(@color).bold
+      STDERR.puts ex.message
       exit 1
     rescue ex : Crystal::SyntaxException
       if @format == "json"
-        puts ex.to_json
+        STDERR.puts ex.to_json
       else
-        puts ex
+        STDERR.puts ex
       end
       exit 1
     rescue ex
       couldnt_format "STDIN"
       STDERR.puts
-      STDERR.flush
       exit 1
     end
   end
@@ -114,21 +112,20 @@ class Crystal::Command
 
       File.write(filename, result)
     rescue ex : InvalidByteSequenceError
-      print "Error: ".colorize.toggle(@color).red.bold
-      print "file '#{Crystal.relative_filename(filename)}' is not a valid Crystal source file: ".colorize.toggle(@color).bold
-      puts ex.message
+      STDERR.print "Error: ".colorize.toggle(@color).red.bold
+      STDERR.print "file '#{Crystal.relative_filename(filename)}' is not a valid Crystal source file: ".colorize.toggle(@color).bold
+      STDERR.puts ex.message
       exit 1
     rescue ex : Crystal::SyntaxException
       if @format == "json"
-        puts ex.to_json
+        STDERR.puts ex.to_json
       else
-        puts ex
+        STDERR.puts ex
       end
       exit 1
     rescue ex
       couldnt_format "'#{filename}'"
       STDERR.puts
-      STDERR.flush
       exit 1
     end
   end
@@ -168,23 +165,22 @@ class Crystal::Command
       if check_files
         check_files << FormatResult.new(filename, FormatResult::Code::INVALID_BYTE_SEQUENCE)
       else
-        print "Error: ".colorize.toggle(@color).red.bold
-        print "file '#{Crystal.relative_filename(filename)}' is not a valid Crystal source file: ".colorize.toggle(@color).bold
-        puts ex.message
+        STDERR.print "Error: ".colorize.toggle(@color).red.bold
+        STDERR.print "file '#{Crystal.relative_filename(filename)}' is not a valid Crystal source file: ".colorize.toggle(@color).bold
+        STDERR.puts ex.message
       end
     rescue ex : Crystal::SyntaxException
       if check_files
         check_files << FormatResult.new(filename, FormatResult::Code::SYNTAX)
       else
-        STDOUT << "Syntax Error:".colorize(:yellow).toggle(@color) << " " << ex.message << " at " << filename << ":" << ex.line_number << ":" << ex.column_number << "\n"
+        STDERR << "Syntax Error:".colorize(:yellow).toggle(@color) << " " << ex.message << " at " << filename << ":" << ex.line_number << ":" << ex.column_number << "\n"
       end
     rescue ex
       if check_files
         check_files << FormatResult.new(filename, FormatResult::Code::BUG)
       else
         couldnt_format "'#{filename}'"
         STDERR.puts
-        STDERR.flush
       end
     end
   end

src/compiler/crystal/compiler.cr

@@ -208,9 +208,9 @@ module Crystal
       parser.wants_doc = wants_doc?
       parser.parse
     rescue ex : InvalidByteSequenceError
-      stdout.print colorize("Error: ").red.bold
-      stdout.print colorize("file '#{Crystal.relative_filename(source.filename)}' is not a valid Crystal source file: ").bold
-      stdout.puts ex.message
+      stderr.print colorize("Error: ").red.bold
+      stderr.print colorize("file '#{Crystal.relative_filename(source.filename)}' is not a valid Crystal source file: ").bold
+      stderr.puts ex.message
       exit 1
     end
 
@@ -431,9 +431,9 @@ module Crystal
         TargetMachine.create(triple, @mcpu || "", @mattr || "", @release)
       end
     rescue ex : ArgumentError
-      stdout.print colorize("Error: ").red.bold
-      stdout.print "llc: "
-      stdout.puts ex.message
+      stderr.print colorize("Error: ").red.bold
+      stderr.print "llc: "
+      stderr.puts ex.message
       exit 1
     end
 

src/compiler/crystal/tools/init.cr

@@ -67,7 +67,7 @@ module Crystal
     def self.fetch_directory(args, project_name)
       directory = args.empty? ? project_name : args.shift
       if Dir.exists?(directory) || File.exists?(directory)
-        puts "file or directory #{directory} already exists"
+        STDERR.puts "file or directory #{directory} already exists"
         exit 1
       end
       directory
@@ -76,17 +76,17 @@ module Crystal
     def self.fetch_skeleton_type(opts, args)
       skeleton_type = fetch_required_parameter(opts, args, "TYPE")
       unless {"lib", "app"}.includes?(skeleton_type)
-        puts "invalid TYPE value: #{skeleton_type}"
-        puts opts
+        STDERR.puts "invalid TYPE value: #{skeleton_type}"
+        STDERR.puts opts
         exit 1
       end
       skeleton_type
     end
 
     def self.fetch_required_parameter(opts, args, name)
       if args.empty?
-        puts "#{name} is missing"
-        puts opts
+        STDERR.puts "#{name} is missing"
+        STDERR.puts opts
         exit 1
       end
       args.shift

src/compiler/crystal/tools/print_hierarchy.cr

@@ -278,7 +278,6 @@ module Crystal
           print_type(@program.object, json)
         end
       end
-      STDOUT.flush
     end
 
     def print_subtypes(types, json)