Revert `MRUBY_TARGET` mechanism; ref #5096

* In explanation of mruby, the expression `build_config.rb` is frequently
  used including official documents, so I think that it will not make sense
  if the file is no longer used.
* The `MRUBY_TARGET` mechanism seems to have little improvement, so I don't
  think it should be changed to avoid unnecessary confusion.
* `MRUBY_TARGET` and `MRuby.targets` represent somewhat different things,
  so using the same term "target" is a bit confusing.

The mechanism that can be written short when using a file under
`build_config` (renamed from `target`) directory remains
(`build_config/${MRUBY_CONFIG}.rb` is used if the path specified
in `MRUBY_CONFIG` doesn't exist).

31 files changed, 50 insertions, 81 deletions

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 95f7c40d8..56fbe86c7 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -6,7 +6,7 @@ jobs:
   Ubuntu-1604:
     runs-on: ubuntu-16.04
     env:
-      MRUBY_TARGET: travis
+      MRUBY_CONFIG: travis
     steps:
     - uses: actions/checkout@v1
     - name: apt
@@ -17,7 +17,7 @@ jobs:
   Ubuntu-1804-gcc:
     runs-on: ubuntu-18.04
     env:
-      MRUBY_TARGET: travis
+      MRUBY_CONFIG: travis
       CC: gcc
       CXX: g++
     steps:
@@ -30,7 +30,7 @@ jobs:
   Ubuntu-1804-clang:
     runs-on: ubuntu-18.04
     env:
-      MRUBY_TARGET: travis
+      MRUBY_CONFIG: travis
       CC: clang
       CXX: clang++
     steps:
@@ -43,7 +43,7 @@ jobs:
   macOS:
     runs-on: macos-latest
     env:
-      MRUBY_TARGET: travis
+      MRUBY_CONFIG: travis
     steps:
     - uses: actions/checkout@v1
     - name: brew
@@ -54,7 +54,7 @@ jobs:
   Windows-MinGW:
     runs-on: windows-latest
     env:
-      MRUBY_TARGET: travis
+      MRUBY_CONFIG: travis
       CFLAGS: -g -O1 -Wall -Wundef
     steps:
     - uses: actions/checkout@v1
@@ -66,7 +66,7 @@ jobs:
   Windows-Cygwin:
     runs-on: windows-latest
     env:
-      MRUBY_TARGET: travis
+      MRUBY_CONFIG: travis
     steps:
     - uses: actions/checkout@v1
     - uses: actions/cache@v1
@@ -91,7 +91,7 @@ jobs:
   Windows-VC:
     runs-on: windows-latest
     env:
-      MRUBY_TARGET: appveyor
+      MRUBY_CONFIG: appveyor
       # TODO(take-cheeze): Re-enable /O2
       CFLAGS: "/c /nologo /W3 /we4013 /Zi /MD /D_CRT_SECURE_NO_WARNINGS"
       CXXFLAGS: "/c /nologo /W3 /Zi /MD /EHs /D_CRT_SECURE_NO_WARNINGS"
diff --git a/.travis.yml b/.travis.yml
index 676ac27b5..7da3d40a9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,7 +8,7 @@ jobs:
       - export PATH="/usr/local/opt/bison/bin:$PATH"
 
 env:
-  - MRUBY_TARGET=travis
+  - MRUBY_CONFIG=travis
 
 script:
   - rake gensym && rake -m && rake test
diff --git a/README.md b/README.md
index 959abb0fc..5abf3445d 100644
--- a/README.md
+++ b/README.md
@@ -38,22 +38,10 @@ The URL of the mruby home-page is: https://mruby.org.
 
 We don't have a mailing list, but you can use [GitHub issues](https://github.com/mruby/mruby/issues).
 
-## How to compile and install (mruby and gems)
+## How to compile, test, and install (mruby and gems)
 
 See the [compile.md](https://github.com/mruby/mruby/blob/master/doc/guides/compile.md) file.
 
-## How to Build
-
-To build mruby, execute the following from the project's root directory.
-
-  $ rake TARGET=<target> test
-
-The default `TARGET` is `host`. `TARGET` will be loaded from `target` directory.
-
-Note: `bison` bundled with MacOS is too old to compile `mruby`.
-Try `brew install bison` and follow the instuction shown to update
-the `$PATH` to compile `mruby`.
-
 ## Building documentation
 
 There are two sets of documentation in mruby: the mruby API (generated by yard) and C API (Doxygen)
diff --git a/Rakefile b/Rakefile
index 5c5a6e8eb..036894592 100644
--- a/Rakefile
+++ b/Rakefile
@@ -14,14 +14,7 @@ require "mruby-core-ext"
 require "mruby/build"
 
 # load configuration file
-if ENV['MRUBY_CONFIG']
-  MRUBY_CONFIG = ENV['MRUBY_CONFIG']
-  MRUBY_TARGET = File.basename(MRUBY_CONFIG, ".rb")
-else
-  MRUBY_TARGET = ENV['MRUBY_TARGET'] || ENV['TARGET'] || "host"
-  MRUBY_CONFIG = "#{MRUBY_ROOT}/target/#{MRUBY_TARGET}.rb"
-end
-load MRUBY_CONFIG
+MRUBY_CONFIG = MRuby::Build.load_config
 
 # load basic rules
 MRuby.each_target do |build|
diff --git a/appveyor.yml b/appveyor.yml
index e2fa0e900..e2a79f20d 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -40,7 +40,7 @@ init:
 
 
 build_script:
-  - set MRUBY_TARGET=appveyor
+  - set MRUBY_CONFIG=appveyor
   - rake gensym
   - rake -m all
   - rake -E $stdout.sync=true test
diff --git a/target/host.rb b/build_config.rb
index 0a92b9108..0a92b9108 100644
--- a/target/host.rb
+++ b/build_config.rb
diff --git a/target/ArduinoDue.rb b/build_config/ArduinoDue.rb
index 5859f8508..5859f8508 100644
--- a/target/ArduinoDue.rb
+++ b/build_config/ArduinoDue.rb
diff --git a/target/IntelEdison.rb b/build_config/IntelEdison.rb
index a22f9cfd2..a22f9cfd2 100644
--- a/target/IntelEdison.rb
+++ b/build_config/IntelEdison.rb
diff --git a/target/IntelGalileo.rb b/build_config/IntelGalileo.rb
index e996efa5b..e996efa5b 100644
--- a/target/IntelGalileo.rb
+++ b/build_config/IntelGalileo.rb
diff --git a/target/RX630.rb b/build_config/RX630.rb
index 01c883958..01c883958 100644
--- a/target/RX630.rb
+++ b/build_config/RX630.rb
diff --git a/target/android_arm64-v8a.rb b/build_config/android_arm64-v8a.rb
index 7fab2f063..7fab2f063 100644
--- a/target/android_arm64-v8a.rb
+++ b/build_config/android_arm64-v8a.rb
diff --git a/target/android_armeabi.rb b/build_config/android_armeabi.rb
index 41a657c3b..41a657c3b 100644
--- a/target/android_armeabi.rb
+++ b/build_config/android_armeabi.rb
diff --git a/target/android_armeabi_v7a_neon_hard.rb b/build_config/android_armeabi_v7a_neon_hard.rb
index 6129b238e..6129b238e 100644
--- a/target/android_armeabi_v7a_neon_hard.rb
+++ b/build_config/android_armeabi_v7a_neon_hard.rb
diff --git a/target/appveyor.rb b/build_config/appveyor.rb
index e084a6a36..e084a6a36 100644
--- a/target/appveyor.rb
+++ b/build_config/appveyor.rb
diff --git a/target/bench.rb b/build_config/bench.rb
index 30ec56ad3..30ec56ad3 100644
--- a/target/bench.rb
+++ b/build_config/bench.rb
diff --git a/target/boxing.rb b/build_config/boxing.rb
index 97c8e8ca9..97c8e8ca9 100644
--- a/target/boxing.rb
+++ b/build_config/boxing.rb
diff --git a/target/chipKITMax32.rb b/build_config/chipKITMax32.rb
index bbee221f5..bbee221f5 100644
--- a/target/chipKITMax32.rb
+++ b/build_config/chipKITMax32.rb
diff --git a/target/clang-asan.rb b/build_config/clang-asan.rb
index e693332c2..e693332c2 100644
--- a/target/clang-asan.rb
+++ b/build_config/clang-asan.rb
diff --git a/target/cross-32bit.rb b/build_config/cross-32bit.rb
index 82d5d023f..82d5d023f 100644
--- a/target/cross-32bit.rb
+++ b/build_config/cross-32bit.rb
diff --git a/target/dreamcast_shelf.rb b/build_config/dreamcast_shelf.rb
index 3ae53184d..3ae53184d 100644
--- a/target/dreamcast_shelf.rb
+++ b/build_config/dreamcast_shelf.rb
diff --git a/target/host-cxx.rb b/build_config/host-cxx.rb
index ae05e63c2..ae05e63c2 100644
--- a/target/host-cxx.rb
+++ b/build_config/host-cxx.rb
diff --git a/target/host-debug.rb b/build_config/host-debug.rb
index ad4106065..ad4106065 100644
--- a/target/host-debug.rb
+++ b/build_config/host-debug.rb
diff --git a/target/host-gprof.rb b/build_config/host-gprof.rb
index 31b952920..31b952920 100644
--- a/target/host-gprof.rb
+++ b/build_config/host-gprof.rb
diff --git a/target/host-m32.rb b/build_config/host-m32.rb
index 39b2c952f..39b2c952f 100644
--- a/target/host-m32.rb
+++ b/build_config/host-m32.rb
diff --git a/target/host-shared.rb b/build_config/host-shared.rb
index 8b5028837..8b5028837 100644
--- a/target/host-shared.rb
+++ b/build_config/host-shared.rb
diff --git a/target/travis.rb b/build_config/travis.rb
index 58a739ce6..58a739ce6 100644
--- a/target/travis.rb
+++ b/build_config/travis.rb
diff --git a/doc/guides/compile.md b/doc/guides/compile.md
index 233c3e594..ad57970e3 100644
--- a/doc/guides/compile.md
+++ b/doc/guides/compile.md
@@ -24,40 +24,29 @@ Optional:
 
 ## Build
 
-To compile `mruby`, just call `rake` inside of the mruby source
-root. To generate and execute the test tools call `rake test`. To
-clean all build files call `rake clean`. To see full command line on
-build, call `rake -v`.
-
-If you want to compile for the specific configuration, specify
-`MRUBY_TARGET` or `TARGET` environment variable, e.g
-
-```sh
-rake TAGRET=host
-```
-
-The default target is `host`.  The compilation target desciption files
-(with `.rb` suffix) are contained in the `target` directory.
-
-A build description file contains the build configuration of mruby and
-looks like the following for example:
-
+Inside of the root directory of the mruby source a file exists
+called *build_config.rb*. This file contains the build configuration
+of mruby and looks like this for example:
 ```ruby
 MRuby::Build.new do |conf|
   toolchain :gcc
 end
 ```
 
-All tools necessary to compile mruby can be set or modified here. In
-case you want to try different configuration, you can create a new
-configuration file under `target` and specify the configuration using
-the `MRUBY_TARGET` environment variable.
+All tools necessary to compile mruby can be set or modified here. In case
+you want to maintain an additional *build_config.rb* you can define a
+customized path using the *$MRUBY_CONFIG* environment variable. If the
+path doesn't exist, *build_config/${MRUBY_CONFIG}.rb* is used.
+
+To compile just call `rake` inside of the mruby source root. To
+generate and execute the test tools call `rake test`. To clean
+all build files call `rake clean`. To see full command line on
+build, call `rake -v`.
 
 ## Build Configuration
 
-To create a new configuration, copy the existing configuration in the
-`target` directory, and modify it. We wish you submit a pull-request,
-once you created a new configuration for a new platform.
+We wish you submit a pull-request to *build_config/**PLATFORM**.rb*, once you
+created a new configuration for a new platform.
 
 Inside of the configuration file, the following options can be
 configured based on your environment.
diff --git a/doc/mruby3.md b/doc/mruby3.md
index 029c9887e..e611a1b16 100644
--- a/doc/mruby3.md
+++ b/doc/mruby3.md
@@ -3,45 +3,29 @@ User visible changes in `mruby3`
 
 # Build System
 
-You can specify `TARGET` option to `rake` via a command line
-option, or via an environment variable, e.g.
-
-`rake TARGET=host all`
-
-or
-
-`TARGET=host rake all`
-
-It's much easier to switch multiple targets than the
-previous `build_config.rb` system.
-
 ## `presym` target
 
 The first compilation of `mruby` may require generation of a
 static symbol table named `build/presym`. You can generate
 the table by `rake gensym`.
 
-## `target` directory
+## `build_config` directory
 
-Build target specification files are loaded from `target`
-directory. The default `TARGET` is `host` which is described
-in `target/host.rb`. There are many other targets for example:
+Typical build configuration files are located in `build_config`
+directory. For examples:
 
 * `host-gprof`: compiles with `gprof` for performance tuning
 * `host-m32`: compiles in gcc 32bit mode on 64bit platforms
 * `boxing`: compiles all three boxing options
 * `clang-asan`: compiles with `clang`'s Address Sanitizer
 
-`target/host.rb` comes  with comments to help  writing a new
-target description.
-
-If you want to have your target description out of the
-source tree, you can specify the path to the description
-file in `MRUBY_CONFIG`.
+`build_config/${MRUBY_CONFIG}.rb` is used if the path specified
+in `MRUBY_CONFIG` doesn't exist, so you can specify it as
+`rake MRUBY_CONFIG=boxing`.
 
-# Build Target Contribution
+# Build Configuration Contribution
 
-When you write a new target description, please
+When you write a new build configuration description, please
 contribute. We welcome your contribution as a GitHub
 pull-request.
 
diff --git a/lib/mruby/build.rb b/lib/mruby/build.rb
index fa3853342..d07890621 100644
--- a/lib/mruby/build.rb
+++ b/lib/mruby/build.rb
@@ -39,6 +39,17 @@ module MRuby
   class Build
     class << self
       attr_accessor :current
+      def load_config
+        path = ENV['MRUBY_CONFIG']
+        if path.nil? || path.empty?
+          path = "#{MRUBY_ROOT}/build_config.rb"
+        elsif !File.file?(path) && !Pathname.new(path).absolute?
+          f = "#{MRUBY_ROOT}/build_config/#{path}.rb"
+          path = File.exist?(f) ? f : File.extname(path).empty? ? f : path
+        end
+        load path
+        path
+      end
     end
     include Rake::DSL
     include LoadGems
diff --git a/lib/mruby/lockfile.rb b/lib/mruby/lockfile.rb
index 5d4313649..5abf804b4 100644
--- a/lib/mruby/lockfile.rb
+++ b/lib/mruby/lockfile.rb
@@ -26,7 +26,7 @@ module MRuby
       end
 
       def instance
-        @instance ||= new("#{MRUBY_ROOT}/build/#{MRUBY_TARGET}.lock")
+        @instance ||= new("#{MRUBY_CONFIG}.lock")
       end
     end
 
diff --git a/tasks/benchmark.rake b/tasks/benchmark.rake
index 661fd2ada..e7500ccd9 100644
--- a/tasks/benchmark.rake
+++ b/tasks/benchmark.rake
@@ -9,7 +9,11 @@ def bm_files
 end
 
 def build_config_name
-  MRUBY_TARGET
+  if !ENV['MRUBY_CONFIG'].to_s.empty?
+    File.basename(ENV['MRUBY_CONFIG'], '.rb').gsub('build_config_', '')
+  else
+    "build"
+  end
 end
 
 def plot_file