rust: Accept generated sources for main.rs

There are still caveats here. Rust/cargo handles generated sources by
writing out all targets of a single repo into a single output directory,
setting a path to that via a build-time environment variable, and then
include those files via a set of functions and macros. Meson's build
layout is naturally different, and ninja makes working with environment
variables at compile time difficult.

Fixes #8157

mesonbuild/backend/ninjabackend.py

@@ -45,7 +45,7 @@ from ..mesonlib import (
 )
 from ..mesonlib import get_compiler_for_source, has_path_sep, OptionKey
 from .backends import CleanTrees
-from ..build import InvalidArguments
+from ..build import GeneratedList, InvalidArguments
 from ..interpreter import Interpreter
 
 if T.TYPE_CHECKING:
@@ -1581,12 +1581,23 @@ int dummy;
         args = rustc.compiler_args()
         # Compiler args for compiling this target
         args += compilers.get_base_compile_args(base_proxy, rustc)
+        self.generate_generator_list_rules(target)
         main_rust_file = None
         for i in target.get_sources():
             if not rustc.can_compile(i):
                 raise InvalidArguments('Rust target {} contains a non-rust source file.'.format(target.get_basename()))
             if main_rust_file is None:
                 main_rust_file = i.rel_to_builddir(self.build_to_src)
+        for g in target.get_generated_sources():
+            for i in g.get_outputs():
+                if not rustc.can_compile(i):
+                    raise InvalidArguments('Rust target {} contains a non-rust source file.'.format(target.get_basename()))
+                if isinstance(g, GeneratedList):
+                    fname = os.path.join(self.get_target_private_dir(target), i)
+                else:
+                    fname = i
+                if main_rust_file is None:
+                    main_rust_file = fname
         if main_rust_file is None:
             raise RuntimeError('A Rust target has no Rust sources. This is weird. Also a bug. Please report')
         target_name = os.path.join(target.subdir, target.get_filename())

mesonbuild/build.py

@@ -569,7 +569,7 @@ class BuildTarget(Target):
         unity_opt = environment.coredata.get_option(OptionKey('unity'))
         self.is_unity = unity_opt == 'on' or (unity_opt == 'subprojects' and subproject != '')
         self.environment = environment
-        self.sources = []
+        self.sources: T.List[File] = []
         self.compilers = OrderedDict() # type: OrderedDict[str, Compiler]
         self.objects = []
         self.external_deps = []
@@ -588,7 +588,7 @@ class BuildTarget(Target):
         self.need_install = False
         self.pch = {}
         self.extra_args: T.Dict[str, T.List['FileOrString']] = {}
-        self.generated = []
+        self.generated: T.Sequence[T.Union[GeneratedList, CustomTarget, CustomTargetIndex]] = []
         self.d_features = {}
         self.pic = False
         self.pie = False
@@ -1414,7 +1414,14 @@ You probably should put it in link_with instead.''')
 
     def uses_rust(self) -> bool:
         """Is this target a rust target."""
-        return self.sources and self.sources[0].fname.endswith('.rs')
+        if self.sources:
+            first_file = self.sources[0]
+            if first_file.fname.endswith('.rs'):
+                return True
+        elif self.generated:
+            if self.generated[0].get_outputs()[0].endswith('.rs'):
+                return True
+        return False
 
     def get_using_msvc(self):
         '''
@@ -1434,11 +1441,13 @@ You probably should put it in link_with instead.''')
         2. If the target contains only objects, process_compilers guesses and
            picks the first compiler that smells right.
         '''
-        compiler, _ = self.get_clink_dynamic_linker_and_stdlibs()
+        # Rustc can use msvc style linkers
+        if self.uses_rust():
+            compiler = self.environment.coredata.compilers[self.for_machine]['rust']
+        else:
+            compiler, _ = self.get_clink_dynamic_linker_and_stdlibs()
         # Mixing many languages with MSVC is not supported yet so ignore stdlibs.
-        if compiler and compiler.get_linker_id() in {'link', 'lld-link', 'xilink', 'optlink'}:
-            return True
-        return False
+        return compiler and compiler.get_linker_id() in {'link', 'lld-link', 'xilink', 'optlink'}
 
     def check_module_linking(self):
         '''

test cases/rust/11 generated main/gen.py

@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+
+import argparse
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('out')
+    args = parser.parse_args()
+
+    with open(args.out, 'w') as f:
+        f.write('fn main() { println!("I prefer tarnish, actually.") }')
+
+
+if __name__ == "__main__":
+    main()

test cases/rust/11 generated main/meson.build

@@ -0,0 +1,16 @@
+project('generated rust main', 'rust')
+
+gen = find_program('gen.py')
+
+c = custom_target(
+  'custom_target',
+  command : [gen, '@OUTPUT@'],
+  output : ['main.rs'],
+)
+
+executable('custom_target_main', c)
+executable('custom_target_index_main', c[0])
+
+gen = generator(gen, arguments : ['@OUTPUT@'], output : '@BASENAME@.rs')
+# Doesn't actually use gen.py as input, just a limitation of generators
+executable('generator_main', gen.process(['gen.py']))