Auto merge of #25365 - MeFisto94:autolaunch-vcvars, r=jdm

Launch vcvarsall.bat for the recognized VS Installation Directory from python instead of making mach.bat try that on hardcoded paths.

Move the Execution of vcvars (which sets up the environment for visual studio tools) from mach.bat to python, so that ./mach works under mozilla-build and that #25300 can be used.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #25360 #25336
- [X] These changes do not require tests because changes to build infra

mach.bat

@@ -1,65 +1,8 @@
 @echo off
 
-setlocal
-
-if EXIST "%VCINSTALLDIR%" (
-  GOTO mach
-)
-
-pushd .
-
-IF EXIST "%ProgramFiles(x86)%" (
-  set "ProgramFiles32=%ProgramFiles(x86)%"
-) ELSE (
-  set "ProgramFiles32=%ProgramFiles%"
-)
-
-for %%v in (2019 2017) do (
-  for %%e in (Enterprise Professional Community BuildTools) do (
-    IF EXIST "%ProgramFiles32%\Microsoft Visual Studio\%%v\%%e\VC\Auxiliary\Build\vcvarsall.bat" (
-      set "VS_VCVARS=%ProgramFiles32%\Microsoft Visual Studio\%%v\%%e\VC\Auxiliary\Build\vcvarsall.bat"
-      GOTO vcvars
-    )
-  )
-)
-
-set VC14VARS=%VS140COMNTOOLS%..\..\VC\vcvarsall.bat
-IF EXIST "%VC14VARS%" (
-  set "VS_VCVARS=%VC14VARS%"
-)
-
-:vcvars
-IF EXIST "%VS_VCVARS%" (
-  IF NOT DEFINED Platform (
-    IF EXIST "%ProgramFiles(x86)%" (
-      call "%VS_VCVARS%" x64
-    ) ELSE (
-      ECHO 32-bit Windows is currently unsupported.
-      GOTO bad_exit
-    )
-  )
-) ELSE (
-  ECHO Visual Studio 2015, 2017, or 2019 is not installed.
-  ECHO Download and install Visual Studio from https://www.visualstudio.com/
-  GOTO bad_exit
-)
-
-popd
-
-:mach
 where /Q py.exe
 IF %ERRORLEVEL% NEQ 0 (
   python mach %*
 ) ELSE (
   py -2 mach %*
 )
-
-GOTO exit
-
-:bad_exit
-endlocal
-EXIT /B 1
-
-:exit
-endlocal
-exit /B

python/servo/build_commands.py

@@ -10,6 +10,7 @@
 from __future__ import print_function, unicode_literals
 
 import datetime
+import locale
 import os
 import os.path as path
 import platform
@@ -310,6 +311,16 @@ class MachCommands(CommandBase):
                 "lib", "pkgconfig"
             )
 
+        if 'windows' in host:
+            process = subprocess.Popen('("%s" %s > nul) && "python" -c "import os; print(repr(os.environ))"' %
+                                       (os.path.join(vs_dirs['vcdir'], "Auxiliary", "Build", "vcvarsall.bat"), "x64"),
+                                       stdout=subprocess.PIPE, shell=True)
+            stdout, _ = process.communicate()
+            exitcode = process.wait()
+            encoding = locale.getpreferredencoding()  # See https://stackoverflow.com/a/9228117
+            if exitcode == 0:
+                os.environ.update(eval(stdout.decode(encoding)))
+
         # Ensure that GStreamer libraries are accessible when linking.
         if 'windows' in target_triple:
             gst_root = gstreamer_root(target_triple, env)