[vcpkg] Add the possibility to load vcvars.bat with external toolchain. (#10399)

* Add VCPKG_LOAD_VCVARS_ENV
allow loading vcvars.bat if an external toolchain file is used.

* add documentation

docs/users/triplets.md

@@ -125,6 +125,10 @@ Valid settings:
 * The Visual Studio 2017 platform toolset is `v141`.
 * The Visual Studio 2015 platform toolset is `v140`.
 
+### VCPKG_LOAD_VCVARS_ENV
+If `VCPKG_CHAINLOAD_TOOLCHAIN_FILE` is used, VCPKG will not setup the Visual Studio environment. 
+Setting `VCPKG_LOAD_VCVARS_ENV` to (true|1|on) changes this behavior so that the Visual Studio environment is setup following the same rules as if `VCPKG_CHAINLOAD_TOOLCHAIN_FILE` was not set.
+
 ## MacOS Variables
 
 ### VCPKG_INSTALL_NAME_DIR

scripts/vcpkg_get_tags.cmake

@@ -23,6 +23,7 @@ e1e74b5c-18cb-4474-a6bd-5c1c8bc81f3f")
     message("c35112b6-d1ba-415b-aa5d-81de856ef8eb
 VCPKG_PUBLIC_ABI_OVERRIDE=${VCPKG_PUBLIC_ABI_OVERRIDE}
 VCPKG_ENV_PASSTHROUGH=${VCPKG_ENV_PASSTHROUGH}
+VCPKG_LOAD_VCVARS_ENV=${VCPKG_LOAD_VCVARS_ENV}
 e1e74b5c-18cb-4474-a6bd-5c1c8bc81f3f
 8c504940-be29-4cba-9f8f-6cd83e9d87b7")
 endfunction()

toolsrc/include/vcpkg/build.h

@@ -153,6 +153,7 @@ namespace vcpkg::Build
                      Triplet triplet,
                      const std::unordered_map<std::string, std::string>& cmakevars);
 
+        bool load_vcvars_env;
         std::string triplet_abi_tag;
         std::string target_architecture;
         std::string cmake_system_name;
@@ -179,6 +180,7 @@ namespace vcpkg::Build
         BUILD_TYPE,
         ENV_PASSTHROUGH,
         PUBLIC_ABI_OVERRIDE,
+        LOAD_VCVARS_ENV,
     };
 
     const std::unordered_map<std::string, VcpkgTripletVar> VCPKG_OPTIONS = {
@@ -191,6 +193,7 @@ namespace vcpkg::Build
         {"VCPKG_BUILD_TYPE", VcpkgTripletVar::BUILD_TYPE},
         {"VCPKG_ENV_PASSTHROUGH", VcpkgTripletVar::ENV_PASSTHROUGH},
         {"VCPKG_PUBLIC_ABI_OVERRIDE", VcpkgTripletVar::PUBLIC_ABI_OVERRIDE},
+        {"VCPKG_LOAD_VCVARS_ENV", VcpkgTripletVar::LOAD_VCVARS_ENV},
     };
 
     struct ExtendedBuildResult

toolsrc/src/vcpkg/build.cpp

@@ -274,7 +274,7 @@ namespace vcpkg::Build
 
     std::string make_build_env_cmd(const PreBuildInfo& pre_build_info, const Toolset& toolset)
     {
-        if (pre_build_info.external_toolchain_file.has_value()) return "";
+        if (pre_build_info.external_toolchain_file.has_value() && !pre_build_info.load_vcvars_env) return "";
         if (!pre_build_info.cmake_system_name.empty() && pre_build_info.cmake_system_name != "WindowsStore") return "";
 
         const char* tonull = " >nul";
@@ -1052,6 +1052,25 @@ namespace vcpkg::Build
                 case VcpkgTripletVar::PUBLIC_ABI_OVERRIDE:
                     public_abi_override = variable_value.empty() ? nullopt : Optional<std::string>{variable_value};
                     break;
+                case VcpkgTripletVar::LOAD_VCVARS_ENV:
+                        if (variable_value.empty()) 
+                        {
+                            load_vcvars_env = true;
+                            if(external_toolchain_file)
+                                load_vcvars_env = false;
+                        }
+                        else if (Strings::case_insensitive_ascii_equals(variable_value, "1") ||
+                                 Strings::case_insensitive_ascii_equals(variable_value, "on") ||
+                                 Strings::case_insensitive_ascii_equals(variable_value, "true"))
+                            load_vcvars_env = true;
+                        else if (Strings::case_insensitive_ascii_equals(variable_value, "0") ||
+                                 Strings::case_insensitive_ascii_equals(variable_value, "off") ||
+                                 Strings::case_insensitive_ascii_equals(variable_value, "false"))
+                            load_vcvars_env = false;
+                        else
+                            Checks::exit_with_message(
+                                VCPKG_LINE_INFO, "Unknown boolean setting for VCPKG_LOAD_VCVARS_ENV: %s", variable_value);
+                        break;
             }
         }
 

toolsrc/src/vcpkg/vcpkgpaths.cpp

@@ -193,7 +193,7 @@ namespace vcpkg
 
     const Toolset& VcpkgPaths::get_toolset(const Build::PreBuildInfo& prebuildinfo) const
     {
-        if (prebuildinfo.external_toolchain_file ||
+        if ((prebuildinfo.external_toolchain_file && !prebuildinfo.load_vcvars_env) ||
             (!prebuildinfo.cmake_system_name.empty() && prebuildinfo.cmake_system_name != "WindowsStore"))
         {
             static Toolset external_toolset = []() -> Toolset {