0.63.1 breaks D builds with LDC when external linker flags are set
Hi!
At the moment all D packages that use Meson (which is quite a bunch!) fail to build in Debian with the upgrade to Meson 0.63.1 (0.63.0 was fine). Debian sets the `LDFLAGS` env var to set specific linker flags, which Meson passes through to the *compiler* in verbatim. The LDC D compiler can't understand GNU linker flags on its own though, they need to be preprocessed as done in the D compiler backend of Meson: https://github.com/mesonbuild/meson/blob/master/mesonbuild/compilers/d.py#L286

The commit that highly likely broke this is https://github.com/mesonbuild/meson/commit/0ec039d2597526c670f7e0478a7b5008e14ff474, see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1017087 for the Debian bugreport.

**To Reproduce**
Just set `LDFLAGS=-Wl,-z,relro` and try to build the D examples from the testsuite.

**Expected behavior**
To determine the linker, Meson should either not pass through all linker flags from the environment, or (probably better), if they are sent to the compiler give the compiler backend the chance to sanitize them into a form that the compiler can actually understand.

Cheers,
    Matthias

diff --git a/mesonbuild/compilers/compilers.py b/mesonbuild/compilers/compilers.py
index 5aab9c1da..fab9fc17c 100644
--- a/mesonbuild/compilers/compilers.py
+++ b/mesonbuild/compilers/compilers.py
@@ -692,10 +692,14 @@ class Compiler(HoldableObject, metaclass=abc.ABCMeta):
         """
         raise EnvironmentException('Language %s does not support function checks.' % self.get_display_language())
 
-    def unix_args_to_native(self, args: T.List[str]) -> T.List[str]:
+    @classmethod
+    def _unix_args_to_native(cls, args: T.List[str], info: MachineInfo) -> T.List[str]:
         "Always returns a copy that can be independently mutated"
         return args.copy()
 
+    def unix_args_to_native(self, args: T.List[str]) -> T.List[str]:
+        return self._unix_args_to_native(args, self.info)
+
     @classmethod
     def native_args_to_unix(cls, args: T.List[str]) -> T.List[str]:
         "Always returns a copy that can be independently mutated"
diff --git a/mesonbuild/compilers/d.py b/mesonbuild/compilers/d.py
index f4fd0cd40..54f0d9a0c 100644
--- a/mesonbuild/compilers/d.py
+++ b/mesonbuild/compilers/d.py
@@ -283,7 +283,8 @@ class DmdLikeCompilerMixin(CompilerMixinBase):
         return super().build_rpath_args(
             env, build_dir, from_dir, rpath_paths, build_rpath, install_rpath)
 
-    def _translate_args_to_nongnu(self, args: T.List[str]) -> T.List[str]:
+    @classmethod
+    def _translate_args_to_nongnu(cls, args: T.List[str], info: MachineInfo, link_id: str) -> T.List[str]:
         # Translate common arguments to flags the LDC/DMD compilers
         # can understand.
         # The flags might have been added by pkg-config files,
@@ -298,10 +299,10 @@ class DmdLikeCompilerMixin(CompilerMixinBase):
         for arg in args:
             # Translate OS specific arguments first.
             osargs = []  # type: T.List[str]
-            if self.info.is_windows():
-                osargs = self.translate_arg_to_windows(arg)
-            elif self.info.is_darwin():
-                osargs = self._translate_arg_to_osx(arg)
+            if info.is_windows():
+                osargs = cls.translate_arg_to_windows(arg)
+            elif info.is_darwin():
+                osargs = cls._translate_arg_to_osx(arg)
             if osargs:
                 dcargs.extend(osargs)
                 continue
@@ -386,7 +387,7 @@ class DmdLikeCompilerMixin(CompilerMixinBase):
                     continue
 
                 # linker flag such as -L=/DEBUG must pass through
-                if self.linker.id == 'link' and self.info.is_windows() and suffix.startswith('/'):
+                if info.is_windows() and link_id == 'link' and suffix.startswith('/'):
                     dcargs.append(arg)
                     continue
 
@@ -440,6 +441,10 @@ class DmdLikeCompilerMixin(CompilerMixinBase):
             args.append('-L=' + arg)
         return args
 
+    @classmethod
+    def _unix_args_to_native(cls, args: T.List[str], info: MachineInfo, link_id: str = '') -> T.List[str]:
+        return cls._translate_args_to_nongnu(args, info, link_id)
+
     def get_debug_args(self, is_debug: bool) -> T.List[str]:
         ddebug_args = []
         if is_debug:
@@ -899,7 +904,7 @@ class LLVMDCompiler(DmdLikeCompilerMixin, DCompiler):
         return self._get_crt_args(crt_val, buildtype)
 
     def unix_args_to_native(self, args: T.List[str]) -> T.List[str]:
-        return self._translate_args_to_nongnu(args)
+        return self._unix_args_to_native(args, self.info, self.linker.id)
 
     def get_optimization_args(self, optimization_level: str) -> T.List[str]:
         return ldc_optimization_args[optimization_level]
@@ -988,7 +993,7 @@ class DmdDCompiler(DmdLikeCompilerMixin, DCompiler):
         return self._get_crt_args(crt_val, buildtype)
 
     def unix_args_to_native(self, args: T.List[str]) -> T.List[str]:
-        return self._translate_args_to_nongnu(args)
+        return self._unix_args_to_native(args, self.info, self.linker.id)
 
     def get_optimization_args(self, optimization_level: str) -> T.List[str]:
         return dmd_optimization_args[optimization_level]
diff --git a/mesonbuild/compilers/mixins/ccrx.py b/mesonbuild/compilers/mixins/ccrx.py
index d87769eb3..f4c5e06c2 100644
--- a/mesonbuild/compilers/mixins/ccrx.py
+++ b/mesonbuild/compilers/mixins/ccrx.py
@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from __future__ import annotations
 
 """Representations specific to the Renesas CC-RX compiler family."""
 
@@ -20,6 +21,7 @@ import typing as T
 from ...mesonlib import EnvironmentException
 
 if T.TYPE_CHECKING:
+    from ...envconfig import MachineInfo
     from ...environment import Environment
     from ...compilers.compilers import Compiler
 else:
@@ -105,7 +107,7 @@ class CcrxCompiler(Compiler):
         return ccrx_debug_args[is_debug]
 
     @classmethod
-    def unix_args_to_native(cls, args: T.List[str]) -> T.List[str]:
+    def _unix_args_to_native(cls, args: T.List[str], info: MachineInfo) -> T.List[str]:
         result = []
         for i in args:
             if i.startswith('-D'):
diff --git a/mesonbuild/compilers/mixins/compcert.py b/mesonbuild/compilers/mixins/compcert.py
index 5e2ba0de5..f1de8f16c 100644
--- a/mesonbuild/compilers/mixins/compcert.py
+++ b/mesonbuild/compilers/mixins/compcert.py
@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from __future__ import annotations
 
 """Representations specific to the CompCert C compiler family."""
 
@@ -19,6 +20,7 @@ import re
 import typing as T
 
 if T.TYPE_CHECKING:
+    from envconfig import MachineInfo
     from ...environment import Environment
     from ...compilers.compilers import Compiler
 else:
@@ -87,7 +89,8 @@ class CompCertCompiler(Compiler):
     def get_pch_use_args(self, pch_dir: str, header: str) -> T.List[str]:
         return []
 
-    def unix_args_to_native(self, args: T.List[str]) -> T.List[str]:
+    @classmethod
+    def _unix_args_to_native(cls, args: T.List[str], info: MachineInfo) -> T.List[str]:
         "Always returns a copy that can be independently mutated"
         patched_args = []  # type: T.List[str]
         for arg in args:
diff --git a/mesonbuild/compilers/mixins/ti.py b/mesonbuild/compilers/mixins/ti.py
index cbad3004f..a98e6d07e 100644
--- a/mesonbuild/compilers/mixins/ti.py
+++ b/mesonbuild/compilers/mixins/ti.py
@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from __future__ import annotations
 
 """Representations specific to the Texas Instruments compiler family."""
 
@@ -20,6 +21,7 @@ import typing as T
 from ...mesonlib import EnvironmentException
 
 if T.TYPE_CHECKING:
+    from ...envconfig import MachineInfo
     from ...environment import Environment
     from ...compilers.compilers import Compiler
 else:
@@ -120,7 +122,7 @@ class TICompiler(Compiler):
         return ['-I=' + path]
 
     @classmethod
-    def unix_args_to_native(cls, args: T.List[str]) -> T.List[str]:
+    def _unix_args_to_native(cls, args: T.List[str], info: MachineInfo) -> T.List[str]:
         result = []
         for i in args:
             if i.startswith('-D'):
diff --git a/mesonbuild/compilers/mixins/xc16.py b/mesonbuild/compilers/mixins/xc16.py
index 243356105..db7a33737 100644
--- a/mesonbuild/compilers/mixins/xc16.py
+++ b/mesonbuild/compilers/mixins/xc16.py
@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from __future__ import annotations
 
 """Representations specific to the Microchip XC16 C compiler family."""
 
@@ -20,6 +21,7 @@ import typing as T
 from ...mesonlib import EnvironmentException
 
 if T.TYPE_CHECKING:
+    from ...envconfig import MachineInfo
     from ...environment import Environment
     from ...compilers.compilers import Compiler
 else:
@@ -104,7 +106,7 @@ class Xc16Compiler(Compiler):
         return xc16_debug_args[is_debug]
 
     @classmethod
-    def unix_args_to_native(cls, args: T.List[str]) -> T.List[str]:
+    def _unix_args_to_native(cls, args: T.List[str], info: MachineInfo) -> T.List[str]:
         result = []
         for i in args:
             if i.startswith('-D'):
diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index 17133f51b..8dcf26b4f 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -757,10 +757,10 @@ class CoreData:
             raise type(e)(('Validation failed for option %s: ' % option_name) + str(e)) \\
                 .with_traceback(sys.exc_info()[2])
 
-    def get_external_args(self, for_machine: MachineChoice, lang: str) -> T.Union[str, T.List[str]]:
+    def get_external_args(self, for_machine: MachineChoice, lang: str) -> T.List[str]:
         return self.options[OptionKey('args', machine=for_machine, lang=lang)].value
 
-    def get_external_link_args(self, for_machine: MachineChoice, lang: str) -> T.Union[str, T.List[str]]:
+    def get_external_link_args(self, for_machine: MachineChoice, lang: str) -> T.List[str]:
         return self.options[OptionKey('link_args', machine=for_machine, lang=lang)].value
 
     def update_project_options(self, options: 'MutableKeyedOptionDictType') -> None:
diff --git a/mesonbuild/linkers/detect.py b/mesonbuild/linkers/detect.py
index 684328b4c..9080444d4 100644
--- a/mesonbuild/linkers/detect.py
+++ b/mesonbuild/linkers/detect.py
@@ -140,7 +140,9 @@ def guess_nix_linker(env: 'Environment', compiler: T.List[str], comp_class: T.Ty
     """
     env.coredata.add_lang_args(comp_class.language, comp_class, for_machine, env)
     extra_args = extra_args or []
-    extra_args += env.coredata.get_external_link_args(for_machine, comp_class.language)
+
+    ldflags = env.coredata.get_external_link_args(for_machine, comp_class.language)
+    extra_args += comp_class._unix_args_to_native(ldflags, env.machines[for_machine])
 
     if isinstance(comp_class.LINKER_PREFIX, str):
         check_args = [comp_class.LINKER_PREFIX + '--version'] + extra_args
['mesonbuild/compilers/compilers.py', 'mesonbuild/linkers/detect.py', 'mesonbuild/compilers/mixins/compcert.py', 'mesonbuild/compilers/mixins/ccrx.py', 'mesonbuild/compilers/mixins/xc16.py', 'mesonbuild/coredata.py', 'mesonbuild/compilers/mixins/ti.py', 'mesonbuild/compilers/d.py']