rust: add rust_dynamic_std option

As an initial implementation, simply adding "-C prefer-dynamic" works
for binary crates (as well as dylib and proc-macro that already used it).
In the future this could be extended to other crate types.  For more
information see the comment in the changed file, as well as
#8828 and
#14215.

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

docs/markdown/snippets/rust-dynamic-std.md

@@ -0,0 +1,8 @@
+## New experimental option `rust_dynamic_std`
+
+A new option `rust_dynamic_std` can be used to link Rust programs so
+that they use a dynamic library for the Rust `libstd`.
+
+Right now, C ABI crates (corresponding to Rust crate types `cdylib` and
+`staticlib`) cannot be produced if `rust_dynamic_std` is true, but this
+may change in the future.

mesonbuild/backend/ninjabackend.py

@@ -2115,13 +2115,26 @@ def _link_library(libname: str, static: bool, bundle: bool = False):
                                    and dep.rust_crate_type == 'dylib'
                                    for dep in target_deps)
 
-        if target.rust_crate_type in {'dylib', 'proc-macro'} or has_rust_shared_deps:
+        if target.rust_crate_type in {'cdylib', 'staticlib'} \
+                and target.get_option(OptionKey('rust_dynamic_std')):
+            # cdylib and staticlib crates always include a copy of the Rust
+            # libstd, therefore it is not possible to also link it dynamically.
+            # The options to avoid this (-Z staticlib-allow-rdylib-deps and
+            # -Z staticlib-prefer-dynamic) are not yet stable; alternatively,
+            # one could use "--emit obj" (implemented in the pull request at
+            # https://github.com/mesonbuild/meson/pull/11213) or "--emit rlib"
+            # (officially not recommended for linking with C programs).
+            raise MesonException('rust_dynamic_std does not support cdylib and staticlib crates yet')
+
+        if target.rust_crate_type in {'dylib', 'proc-macro'} or has_rust_shared_deps \
+                or target.get_option(OptionKey('rust_dynamic_std')):
             # add prefer-dynamic if any of the Rust libraries we link
             # against are dynamic or this is a dynamic library itself,
             # otherwise we'll end up with multiple implementations of libstd.
             args += ['-C', 'prefer-dynamic']
 
-        if isinstance(target, build.SharedLibrary) or has_shared_deps:
+        if isinstance(target, build.SharedLibrary) or has_shared_deps \
+                or target.get_option(OptionKey('rust_dynamic_std')):
             args += self.get_build_rpath_args(target, rustc)
 
         proc_macro_dylib_path = None

mesonbuild/options.py

@@ -638,6 +638,7 @@ def add_to_argparse(self, name: str, parser: argparse.ArgumentParser, help_suffi
     (OptionKey('layout'),          BuiltinOption(UserComboOption, 'Build directory layout', 'mirror', choices=['mirror', 'flat'])),
     (OptionKey('optimization'),    BuiltinOption(UserComboOption, 'Optimization level', '0', choices=['plain', '0', 'g', '1', '2', '3', 's'])),
     (OptionKey('prefer_static'),   BuiltinOption(UserBooleanOption, 'Whether to try static linking before shared linking', False)),
+    (OptionKey('rust_dynamic_std'), BuiltinOption(UserBooleanOption, 'Whether to link Rust programs to a dynamic libstd', False)),
     (OptionKey('stdsplit'),        BuiltinOption(UserBooleanOption, 'Split stdout and stderr in test logs', True)),
     (OptionKey('strip'),           BuiltinOption(UserBooleanOption, 'Strip targets on install', False)),
     (OptionKey('unity'),           BuiltinOption(UserComboOption, 'Unity build', 'off', choices=['on', 'off', 'subprojects'])),

test cases/rust/1 basic/meson.build

@@ -6,6 +6,12 @@ e = executable('rust-program', 'prog.rs',
 )
 test('rusttest', e)
 
+e = executable('rust-dynamic', 'prog.rs',
+  override_options: {'rust_dynamic_std': true},
+  install : true
+)
+test('rusttest-dynamic', e)
+
 subdir('subdir')
 
 # this should fail due to debug_assert

test cases/rust/1 basic/test.json

@@ -3,6 +3,8 @@
     {"type": "exe", "file": "usr/bin/rust-program"},
     {"type": "pdb", "file": "usr/bin/rust-program"},
     {"type": "exe", "file": "usr/bin/rust-program2"},
-    {"type": "pdb", "file": "usr/bin/rust-program2"}
+    {"type": "pdb", "file": "usr/bin/rust-program2"},
+    {"type": "exe", "file": "usr/bin/rust-dynamic"},
+    {"type": "pdb", "file": "usr/bin/rust-dynamic"}
   ]
 }