rust: build: move rustc_args.py invocation to qemu-api crate

Only qemu-api needs access to the symbols in config-host.h.  Remove
the temptation to use them elsewhere by limiting the --cfg arguments to
the qemu-api crate.

Per-crate invocation of the script will also be needed to add --check-cfg
options for each crate's features (when more complex, build-time
configurable devices are added in the future).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -120,7 +120,30 @@ if have_rust
 endif
 
 if have_rust
+  rustc_args = find_program('scripts/rust/rustc_args.py')
   rustfmt = find_program('rustfmt', required: false)
+
+  # Prohibit code that is forbidden in Rust 2024
+  rustc_lint_args = ['-D', 'unsafe_op_in_unsafe_fn']
+
+  # Occasionally, we may need to silence warnings and clippy lints that
+  # were only introduced in newer Rust compiler versions.  Do not croak
+  # in that case; a CI job with rust_strict_lints == true ensures that
+  # we do not have misspelled allow() attributes.
+  if not get_option('strict_rust_lints')
+    rustc_lint_args += ['-A', 'unknown_lints']
+  endif
+
+  # Apart from procedural macros, our Rust executables will often link
+  # with C code, so include all the libraries that C code needs.  This
+  # is safe; https://github.com/rust-lang/rust/pull/54675 says that
+  # passing -nodefaultlibs to the linker "was more ideological to
+  # start with than anything".
+  add_project_arguments(rustc_lint_args +
+      ['--cfg', 'MESON', '-C', 'default-linker-libraries'],
+      native: false, language: 'rust')
+  add_project_arguments(rustc_lint_args + ['--cfg', 'MESON'],
+      native: true, language: 'rust')
 endif
 
 dtrace = not_found
@@ -3399,37 +3422,8 @@ endif
 # Generated sources #
 #####################
 
-genh += configure_file(output: 'config-host.h', configuration: config_host_data)
+config_host_h = configure_file(output: 'config-host.h', configuration: config_host_data)
-
+genh += config_host_h
-if have_rust
-  rustc_args = run_command(
-    find_program('scripts/rust/rustc_args.py'),
-    '--config-headers', meson.project_build_root() / 'config-host.h',
-    capture : true,
-    check: true).stdout().strip().split()
-
-  # Prohibit code that is forbidden in Rust 2024
-  rustc_args += ['-D', 'unsafe_op_in_unsafe_fn']
-
-  # Occasionally, we may need to silence warnings and clippy lints that
-  # were only introduced in newer Rust compiler versions.  Do not croak
-  # in that case; a CI job with rust_strict_lints == true ensures that
-  # we do not have misspelled allow() attributes.
-  if not get_option('strict_rust_lints')
-    rustc_args += ['-A', 'unknown_lints']
-  endif
-
-  # Apart from procedural macros, our Rust executables will often link
-  # with C code, so include all the libraries that C code needs.  This
-  # is safe; https://github.com/rust-lang/rust/pull/54675 says that
-  # passing -nodefaultlibs to the linker "was more ideological to
-  # start with than anything".
-  add_project_arguments(rustc_args +
-      ['--cfg', 'MESON', '-C', 'default-linker-libraries'],
-      native: false, language: 'rust')
-  add_project_arguments(rustc_args + ['--cfg', 'MESON'],
-      native: true, language: 'rust')
-endif
 
 hxtool = find_program('scripts/hxtool')
 shaderinclude = find_program('scripts/shaderinclude.py')

rust/qemu-api/meson.build

@@ -1,4 +1,7 @@
-_qemu_api_cfg = []
+_qemu_api_cfg = run_command(rustc_args,
+  '--config-headers', config_host_h,
+  capture: true, check: true).stdout().strip().split()
+
 # _qemu_api_cfg += ['--cfg', 'feature="allocator"']
 if rustc.version().version_compare('>=1.77.0')
   _qemu_api_cfg += ['--cfg', 'has_offset_of']