cmake: Enable compilation database generation (spack#42353)

* cmake: Enable CMAKE_EXPORT_COMPILE_COMMANDS

Enabling this option causes CMake to generate a compile_commands.json file
containing a compilation database that can be used to drive third-party tools.

CMAKE_EXPORT_COMPILE_COMMANDS only exists for CMake >= 3.5

Exporting compilation databases is only supported for Makefile and Ninja
generators, so check these conditions as well.

CMAKE_EXPORT_COMPILE_COMMANDS is only enabled in supported configurations

lib/spack/spack/build_systems/cmake.py

@@ -58,6 +58,20 @@ def _maybe_set_python_hints(pkg: spack.package_base.PackageBase, args: List[str]
     )
 
 
+def _supports_compilation_databases(pkg: spack.package_base.PackageBase) -> bool:
+    """Check if this package (and CMake) can support compilation databases."""
+
+    # CMAKE_EXPORT_COMPILE_COMMANDS only exists for CMake >= 3.5
+    if not pkg.spec.satisfies("^cmake@3.5:"):
+        return False
+
+    # CMAKE_EXPORT_COMPILE_COMMANDS is only implemented for Makefile and Ninja generators
+    if not (pkg.spec.satisfies("generator=make") or pkg.spec.satisfies("generator=ninja")):
+        return False
+
+    return True
+
+
 def _conditional_cmake_defaults(pkg: spack.package_base.PackageBase, args: List[str]) -> None:
     """Set a few default defines for CMake, depending on its version."""
     cmakes = pkg.spec.dependencies("cmake", dt.BUILD)
@@ -95,6 +109,10 @@ def _conditional_cmake_defaults(pkg: spack.package_base.PackageBase, args: List[
         args.append(CMakeBuilder.define("CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY", False))
         args.append(CMakeBuilder.define("CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY", False))
 
+    # Export a compilation database if supported.
+    if _supports_compilation_databases(pkg):
+        args.append(CMakeBuilder.define("CMAKE_EXPORT_COMPILE_COMMANDS", True))
+
 
 def generator(*names: str, default: Optional[str] = None):
     """The build system generator to use.
@@ -284,7 +302,10 @@ class CMakeBuilder(BaseBuilder):
     @property
     def archive_files(self):
         """Files to archive for packages based on CMake"""
-        return [os.path.join(self.build_directory, "CMakeCache.txt")]
+        files = [os.path.join(self.build_directory, "CMakeCache.txt")]
+        if _supports_compilation_databases(self):
+            files.append(os.path.join(self.build_directory, "compile_commands.json"))
+        return files
 
     @property
     def root_cmakelists_dir(self):