rust: implement support for --edition

Using the std option, so now `rust_std=..` will work. I've chosen to use
"std" even though rust calls these "editions", as meson refers to
language versions as "standards", which makes meson feel more uniform,
and be less surprising.

Fixes: #5100

docs/markdown/snippets/rust_std_option.md

@@ -0,0 +1,5 @@
+## Rust now has a a standard option
+
+Rust calls these `editions`, however, meson generally refers to such language
+versions as "standards", or "std" for short. Rust uses "std" to keep normalize
+it with other languages.

mesonbuild/backend/ninjabackend.py

@@ -1524,10 +1524,13 @@ int dummy;
             for a in rustc.linker.get_always_args():
                 args += ['-C', 'link-arg={}'.format(a)]
 
+        opt_proxy = self.get_compiler_options_for_target(target)[rustc.language]
+
         args += ['--crate-name', target.name]
         args += rustc.get_buildtype_args(self.get_option_for_target('buildtype', target))
         args += rustc.get_debug_args(self.get_option_for_target('debug', target))
         args += rustc.get_optimization_args(self.get_option_for_target('optimization', target))
+        args += rustc.get_option_compile_args(opt_proxy)
         args += self.build.get_global_args(rustc, target.for_machine)
         args += self.build.get_project_args(rustc, target.subproject, target.for_machine)
         depfile = os.path.join(target.subdir, target.name + '.d')

mesonbuild/compilers/rust.py

@@ -16,15 +16,18 @@ import subprocess, os.path
 import textwrap
 import typing as T
 
+from .. import coredata
 from ..mesonlib import EnvironmentException, MachineChoice, Popen_safe
 from .compilers import Compiler, rust_buildtype_args, clike_debug_args
 
 if T.TYPE_CHECKING:
+    from ..coredata import OptionDictType
     from ..dependencies import ExternalProgram
     from ..envconfig import MachineInfo
     from ..environment import Environment  # noqa: F401
     from ..linkers import DynamicLinker
 
+
 rust_optimization_args = {
     '0': [],
     'g': ['-C', 'opt-level=0'],
@@ -122,3 +125,19 @@ class RustCompiler(Compiler):
     # Rust does not have a use_linker_args because it dispatches to a gcc-like
     # C compiler for dynamic linking, as such we invoke the C compiler's
     # use_linker_args method instead.
+
+    def get_options(self) -> 'OptionDictType':
+        return {
+            'std': coredata.UserComboOption(
+                'Rust Eddition to use',
+                ['none', '2015', '2018'],
+                'none',
+            ),
+        }
+
+    def get_option_compile_args(self, options: 'OptionDictType') -> T.List[str]:
+        args = []
+        std = options['std']
+        if std.value != 'none':
+            args.append('--edition=' + std.value)
+        return args

test cases/rust/10 language stds/2015.rs

@@ -0,0 +1,3 @@
+trait Foo {
+    fn foo(&self, Box<dyn Foo>);
+}

test cases/rust/10 language stds/2018.rs

@@ -0,0 +1,9 @@
+const fn foo(x: i32) -> i32 {
+    return x + 1;
+}
+
+const VALUE: i32 = foo(-1);
+
+pub fn main() {
+    std::process::exit(VALUE);
+}

test cases/rust/10 language stds/meson.build

@@ -0,0 +1,18 @@
+project('rust std options', 'rust')
+
+# this only works in 2018
+new = executable(
+  'new',
+  '2018.rs',
+  override_options : ['rust_std=2018'],
+)
+
+# this only works in 2015
+old = static_library(
+  'old',
+  '2015.rs',
+  override_options : ['rust_std=2015'],
+)
+
+
+test('2018 std', new)