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/Builtin-options.md

@@ -290,6 +290,7 @@ or compiler being used:
 | cpp_thread_count | 4             | integer value ≥ 0                        | Number of threads to use with emcc when using threads |
 | cpp_winlibs      | see below     | free-form comma-separated list           | Standard Windows libs to link against |
 | fortran_std      | none          | [none, legacy, f95, f2003, f2008, f2018] | Fortran language standard to use |
+| rust_dynamic_std | false         | true, false                              | Whether to link dynamically to the Rust standard library *(Added in 1.8.0)* |
 | cuda_ccbindir    |               | filesystem path                          | CUDA non-default toolchain directory to use (-ccbin) *(Added in 0.57.1)* |
 
 The default values of `c_winlibs` and `cpp_winlibs` are in

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/compilers/rust.py

@@ -243,6 +243,12 @@ def get_options(self) -> MutableKeyedOptionDictType:
             'none',
             choices=['none', '2015', '2018', '2021', '2024'])
 
+        key = self.form_compileropt_key('dynamic_std')
+        opts[key] = options.UserBooleanOption(
+            self.make_option_name(key),
+            'Whether to link Rust programs to a dynamic libstd',
+            False)
+
         return opts
 
     def get_dependency_compile_args(self, dep: 'Dependency') -> T.List[str]:

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"}
   ]
 }