fix MesonToolchain usages of VirtualBuildEnv

conan/tools/meson/toolchain.py

@@ -181,7 +181,11 @@ def __init__(self, conanfile, backend=None):
         compilers_by_conf = self._conanfile.conf.get("tools.build:compiler_executables", default={},
                                                      check_type=dict)
         # Read the VirtualBuildEnv to update the variables
+        # FIXME: This VirtualBuildEnv instance is breaking things!!
+        # FIXME: It shouldn't be used here, not intended for this use case
+        prev_status = self._conanfile.virtualbuildenv
         build_env = VirtualBuildEnv(self._conanfile).vars()
+        self._conanfile.virtualbuildenv = prev_status
         #: Defines the Meson ``c`` variable. Defaulted to ``CC`` build environment value
         self.c = compilers_by_conf.get("c") or build_env.get("CC") or default_comp
         #: Defines the Meson ``cpp`` variable. Defaulted to ``CXX`` build environment value

conans/test/integration/toolchains/meson/test_mesontoolchain.py

@@ -51,6 +51,10 @@ def generate(self):
             "host_prof": cross})
 
     t.run("install . -pr:h host_prof -pr:b build_prof")
+    # Checking that the global conanbuild aggregator includes conanbuildenv-xxx file
+    # it should have been generated by implicit VirtualBuildEnv generator
+    env_file = t.load("conanbuild.sh")
+    assert "conanbuildenv" in env_file
     content = t.load(MesonToolchain.cross_filename)
     assert "c_args = ['-isysroot', '/other/sdk/path', '-arch', 'myarch', '-otherminversion=10.7']" in content
     assert "c_link_args = ['-isysroot', '/other/sdk/path', '-arch', 'myarch', '-otherminversion=10.7']" in content