meson: let Meson handle mixed-language linking of Rust and C objects

With the bump to Meson 1.10.0, C objects can be passed to rust targets.
This way, the Rust libstd will be added by rustc itself in its final
linker invocation.  Use that to eliminate the staticlib and allow
dynamic linking with libstd (also introduced by Meson 1.9.0, but not
for staticlib crates due to lack of support in rustc).

The main() function is still provided by C, which is possible by
declaring the main source file of the Rust executable (which is
still created by scripts/rust/rust_root_crate.sh) as #![no_main].

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

meson.build

@@ -4352,25 +4352,22 @@ foreach target : target_dirs
   arch_srcs += target_specific.sources()
   arch_deps += target_specific.dependencies()
 
+  main_rs = []
+  crates = []
   if have_rust and target_type == 'system'
     target_rust = rust_devices_ss.apply(config_target, strict: false)
-    crates = []
+    arch_deps += target_rust.dependencies()
     foreach dep : target_rust.dependencies()
       crates += dep.get_variable('crate')
     endforeach
     if crates.length() > 0
-      rlib_rs = custom_target('rust_' + target.underscorify() + '.rs',
+      main_rs = custom_target('rust_' + target.underscorify() + '.rs',
                               output: 'rust_' + target.underscorify() + '.rs',
                               command: [rust_root_crate, crates],
                               capture: true,
                               build_by_default: true,
                               build_always_stale: true)
-      rlib = static_library('rust_' + target.underscorify(),
-                            structured_sources([], {'.': rlib_rs}),
-                            dependencies: target_rust.dependencies(),
-                            override_options: ['rust_std=2021', 'build.rust_std=2021'],
-                            rust_abi: 'c')
-      arch_deps += declare_dependency(link_whole: [rlib])
+      main_rs = structured_sources(main_rs)
     endif
   endif
 
@@ -4394,14 +4391,14 @@ foreach target : target_dirs
     execs = [{
       'name': 'qemu-system-' + target_name,
       'win_subsystem': 'console',
-      'sources': files('system/main.c'),
+      'sources': [main_rs, files('system/main.c')],
       'dependencies': [sdl]
     }]
     if host_os == 'windows' and (sdl.found() or gtk.found())
       execs += [{
         'name': 'qemu-system-' + target_name + 'w',
         'win_subsystem': 'windows',
-        'sources': files('system/main.c'),
+        'sources': [main_rs, files('system/main.c')],
         'dependencies': [sdl]
       }]
     endif
@@ -4410,7 +4407,7 @@ foreach target : target_dirs
       execs += [{
         'name': 'qemu-fuzz-' + target_name,
         'win_subsystem': 'console',
-        'sources': specific_fuzz.sources(),
+        'sources': [main_rs, specific_fuzz.sources()],
         'dependencies': specific_fuzz.dependencies(),
       }]
     endif

scripts/rust/rust_root_crate.sh

@@ -6,6 +6,7 @@ cat <<EOF
 // @generated
 // This file is autogenerated by scripts/rust/rust_root_crate.sh
 
+#![no_main]
 EOF
 
 for crate in $*; do