vala: Disable unity builds

Our approach to unity builds with vala is broken, you cannot unify the
generated C files, as they contain duplicate symbols. We would need to
instead combine the files while they are still in their vala form, then
convert that to C and compile the unified C file.

This does not fix the linked issue, as this removed the ability to do
vala unity builds, but it does allow running vala with `--unity=on`.

Related: #5280

3 files changed, 20 insertions, 14 deletions

diff --git a/docs/markdown/snippets/vala_unity_builds_disabled.md b/docs/markdown/snippets/vala_unity_builds_disabled.md
new file mode 100644
index 0000000..80e6523
--- /dev/null
+++ b/docs/markdown/snippets/vala_unity_builds_disabled.md
@@ -0,0 +1,7 @@
+## Unity build with Vala disabled
+
+The approach that meson has used for Vala unity builds is incorrect, we
+combine the generated C files like we would any other C file. This is very
+fragile however, as the Vala compiler generates helper functions and macros
+which work fine when each file is a separate translation unit, but fail when
+they are combined.
diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index f95ca4d..233173f 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -49,7 +49,7 @@ if T.TYPE_CHECKING:
 # Languages that can mix with C or C++ but don't support unity builds yet
 # because the syntax we use for unity builds is specific to C/++/ObjC/++.
 # Assembly files cannot be unitified and neither can LLVM IR files
-LANGS_CANT_UNITY = ('d', 'fortran')
+LANGS_CANT_UNITY = ('d', 'fortran', 'vala')
 
 class TestProtocol(enum.Enum):
 
diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 949b59d..ca17f19 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -812,25 +812,24 @@ int dummy;
         # Generate compilation targets for C sources generated from Vala
         # sources. This can be extended to other $LANG->C compilers later if
         # necessary. This needs to be separate for at least Vala
+        #
+        # Do not try to unity-build the generated c files from vala, as these
+        # often contain duplicate symbols and will fail to compile properly
         vala_generated_source_files = []
         for src in vala_generated_sources:
             dirpart, fnamepart = os.path.split(src)
             raw_src = File(True, dirpart, fnamepart)
-            if is_unity:
-                unity_src.append(os.path.join(self.environment.get_build_dir(), src))
+            # Generated targets are ordered deps because the must exist
+            # before the sources compiling them are used. After the first
+            # compile we get precise dependency info from dep files.
+            # This should work in all cases. If it does not, then just
+            # move them from orderdeps to proper deps.
+            if self.environment.is_header(src):
                 header_deps.append(raw_src)
             else:
-                # Generated targets are ordered deps because the must exist
-                # before the sources compiling them are used. After the first
-                # compile we get precise dependency info from dep files.
-                # This should work in all cases. If it does not, then just
-                # move them from orderdeps to proper deps.
-                if self.environment.is_header(src):
-                    header_deps.append(raw_src)
-                else:
-                    # We gather all these and generate compile rules below
-                    # after `header_deps` (above) is fully generated
-                    vala_generated_source_files.append(raw_src)
+                # We gather all these and generate compile rules below
+                # after `header_deps` (above) is fully generated
+                vala_generated_source_files.append(raw_src)
         for src in vala_generated_source_files:
             # Passing 'vala' here signifies that we want the compile
             # arguments to be specialized for C code generated by