Use the linker to determine if we are using MSVC

This is much more accurate since this is actually what determines what
file naming to use and whether there will be PDB debugging information
or not.

Closes #1169

mesonbuild/build.py

@@ -63,17 +63,6 @@ known_lib_kwargs.update({'version' : True, # Only for shared libs
                          'pic' : True, # Only for static libs
                         })
 
-def compilers_are_msvc(compilers):
-    """
-    Check if all the listed compilers are MSVC. Used by Executable,
-    StaticLibrary, and SharedLibrary for deciding when to use MSVC-specific
-    file naming.
-    """
-    for compiler in compilers.values():
-        if compiler.get_id() != 'msvc':
-            return False
-    return True
-
 
 class InvalidArguments(MesonException):
     pass
@@ -818,6 +807,29 @@ class BuildTarget():
             if l in self.compilers:
                 return self.compilers[l]
 
+    def get_using_msvc(self):
+        '''
+        Check if the dynamic linker is MSVC. Used by Executable, StaticLibrary,
+        and SharedLibrary for deciding when to use MSVC-specific file naming
+        and debug filenames.
+
+        If at least some code is built with MSVC and the final library is
+        linked with MSVC, we can be sure that some debug info will be
+        generated. We only check the dynamic linker here because the static
+        linker is guaranteed to be of the same type.
+
+        Interesting cases:
+        1. The Vala compiler outputs C code to be compiled by whatever
+           C compiler we're using, so all objects will still be created by the
+           MSVC compiler.
+        2. If the target contains only objects, process_compilers guesses and
+           picks the first compiler that smells right.
+        '''
+        linker = self.get_clike_dynamic_linker()
+        if linker and linker.get_id() == 'msvc':
+            return True
+        return False
+
 
 class Generator():
     def __init__(self, args, kwargs):
@@ -942,7 +954,7 @@ class Executable(BuildTarget):
             self.filename += '.' + self.suffix
         # See determine_debug_filenames() in build.SharedLibrary
         buildtype = environment.coredata.get_builtin_option('buildtype')
-        if compilers_are_msvc(self.compilers) and buildtype.startswith('debug'):
+        if self.get_using_msvc() and buildtype.startswith('debug'):
             self.debug_filename = self.prefix + self.name + '.pdb'
 
     def type_suffix(self):
@@ -973,7 +985,7 @@ class StaticLibrary(BuildTarget):
         self.filename = self.prefix + self.name + '.' + self.suffix
         # See determine_debug_filenames() in build.SharedLibrary
         buildtype = environment.coredata.get_builtin_option('buildtype')
-        if compilers_are_msvc(self.compilers) and buildtype.startswith('debug'):
+        if self.get_using_msvc() and buildtype.startswith('debug'):
             self.debug_filename = self.prefix + self.name + '.pdb'
 
     def type_suffix(self):
@@ -1049,7 +1061,7 @@ class SharedLibrary(BuildTarget):
             suffix = 'dll'
             self.vs_import_filename = '{0}.lib'.format(self.name)
             self.gcc_import_filename = 'lib{0}.dll.a'.format(self.name)
-            if compilers_are_msvc(self.compilers):
+            if self.get_using_msvc():
                 # Shared library is of the form foo.dll
                 prefix = ''
                 # Import library is called foo.lib
@@ -1096,7 +1108,7 @@ class SharedLibrary(BuildTarget):
         determine_filenames() above.
         """
         buildtype = env.coredata.get_builtin_option('buildtype')
-        if compilers_are_msvc(self.compilers) and buildtype.startswith('debug'):
+        if self.get_using_msvc() and buildtype.startswith('debug'):
             # Currently we only implement separate debug symbol files for MSVC
             # since the toolchain does it for us. Other toolchains embed the
             # debugging symbols in the file itself by default.