Add cross-compilation support for `rustc`

This patch is largely modeled on the relatively-straightforward code
for Fortran cross-compilation, so there might be some intricacies
missing.

6 files changed, 62 insertions, 19 deletions

diff --git a/cross/ubuntu-armhf.txt b/cross/ubuntu-armhf.txt
index 367eba3..6246ffe 100644
--- a/cross/ubuntu-armhf.txt
+++ b/cross/ubuntu-armhf.txt
@@ -3,6 +3,7 @@
 # when cross compiled binaries can't be run we don't do that
 c = '/usr/bin/arm-linux-gnueabihf-gcc-7'
 cpp = '/usr/bin/arm-linux-gnueabihf-g++-7'
+rust = ['rustc', '--target', 'arm-unknown-linux-gnueabihf', '-C', 'linker=/usr/bin/arm-linux-gnueabihf-gcc-7']
 ar = '/usr/arm-linux-gnueabihf/bin/ar'
 strip = '/usr/arm-linux-gnueabihf/bin/strip'
 pkgconfig = '/usr/bin/arm-linux-gnueabihf-pkg-config'
diff --git a/docs/markdown/snippets/rust-cross.md b/docs/markdown/snippets/rust-cross.md
new file mode 100644
index 0000000..7f18c44
--- /dev/null
+++ b/docs/markdown/snippets/rust-cross.md
@@ -0,0 +1,16 @@
+## Rust cross-compilation
+
+Cross-compilation is now supported for Rust targets. Like other
+cross-compilers, the Rust binary must be specified in your cross
+file. It should specify a `--target` (as installed by `rustup target`)
+and a custom linker pointing to your C cross-compiler. For example:
+
+```
+[binaries]
+c = '/usr/bin/arm-linux-gnueabihf-gcc-7'
+rust = [
+    'rustc',
+    '--target', 'arm-unknown-linux-gnueabihf',
+    '-C', 'linker=/usr/bin/arm-linux-gnueabihf-gcc-7',
+]
+```
diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 77c7d50..e1f3909 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -1291,7 +1291,11 @@ int dummy;
             # installations
             for rpath_arg in rpath_args:
                 args += ['-C', 'link-arg=' + rpath_arg + ':' + os.path.join(rustc.get_sysroot(), 'lib')]
-        element = NinjaBuildElement(self.all_outputs, target_name, 'rust_COMPILER', relsrc)
+        crstr = ''
+        if target.is_cross:
+            crstr = '_CROSS'
+        compiler_name = 'rust%s_COMPILER' % crstr
+        element = NinjaBuildElement(self.all_outputs, target_name, compiler_name, relsrc)
         if len(orderdeps) > 0:
             element.add_orderdep(orderdeps)
         element.add_item('ARGS', args)
@@ -1579,8 +1583,11 @@ int dummy;
         outfile.write(restat)
         outfile.write('\n')
 
-    def generate_rust_compile_rules(self, compiler, outfile):
-        rule = 'rule %s_COMPILER\n' % compiler.get_language()
+    def generate_rust_compile_rules(self, compiler, outfile, is_cross):
+        crstr = ''
+        if is_cross:
+            crstr = '_CROSS'
+        rule = 'rule %s%s_COMPILER\n' % (compiler.get_language(), crstr)
         invoc = ' '.join([ninja_quote(i) for i in compiler.get_exelist()])
         command = ' command = %s $ARGS $in\n' % invoc
         description = ' description = Compiling Rust source $in.\n'
@@ -1671,8 +1678,7 @@ rule FORTRAN_DEP_HACK
                 self.generate_vala_compile_rules(compiler, outfile)
             return
         if langname == 'rust':
-            if not is_cross:
-                self.generate_rust_compile_rules(compiler, outfile)
+            self.generate_rust_compile_rules(compiler, outfile, is_cross)
             return
         if langname == 'swift':
             if not is_cross:
diff --git a/mesonbuild/compilers/rust.py b/mesonbuild/compilers/rust.py
index b93289f..d1a05ed 100644
--- a/mesonbuild/compilers/rust.py
+++ b/mesonbuild/compilers/rust.py
@@ -19,9 +19,11 @@ from ..mesonlib import EnvironmentException, Popen_safe
 from .compilers import Compiler, rust_buildtype_args
 
 class RustCompiler(Compiler):
-    def __init__(self, exelist, version):
+    def __init__(self, exelist, version, is_cross, exe_wrapper=None):
         self.language = 'rust'
         super().__init__(exelist, version)
+        self.is_cross = is_cross
+        self.exe_wrapper = exe_wrapper
         self.id = 'rustc'
 
     def needs_static_linker(self):
@@ -41,7 +43,16 @@ class RustCompiler(Compiler):
         pc.wait()
         if pc.returncode != 0:
             raise EnvironmentException('Rust compiler %s can not compile programs.' % self.name_string())
-        if subprocess.call(output_name) != 0:
+        if self.is_cross:
+            if self.exe_wrapper is None:
+                # Can't check if the binaries run so we have to assume they do
+                return
+            cmdlist = self.exe_wrapper + [output_name]
+        else:
+            cmdlist = [output_name]
+        pe = subprocess.Popen(cmdlist, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+        pe.wait()
+        if pe.returncode != 0:
             raise EnvironmentException('Executables created by Rust compiler %s are not runnable.' % self.name_string())
 
     def get_dependency_gen_args(self, outfile):
diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index e5aa43e..0e5dff0 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -278,6 +278,7 @@ class Environment:
         self.default_objc = ['cc']
         self.default_objcpp = ['c++']
         self.default_fortran = ['gfortran', 'g95', 'f95', 'f90', 'f77', 'ifort']
+        self.default_rust = ['rustc']
         self.default_static_linker = ['ar']
         self.vs_static_linker = ['lib']
         self.gcc_static_linker = ['gcc-ar']
@@ -688,16 +689,24 @@ class Environment:
             return ValaCompiler(exelist, version)
         raise EnvironmentException('Unknown compiler "' + ' '.join(exelist) + '"')
 
-    def detect_rust_compiler(self):
-        exelist = ['rustc']
-        try:
-            p, out = Popen_safe(exelist + ['--version'])[0:2]
-        except OSError:
-            raise EnvironmentException('Could not execute Rust compiler "%s"' % ' '.join(exelist))
-        version = search_version(out)
-        if 'rustc' in out:
-            return RustCompiler(exelist, version)
-        raise EnvironmentException('Unknown compiler "' + ' '.join(exelist) + '"')
+    def detect_rust_compiler(self, want_cross):
+        popen_exceptions = {}
+        compilers, ccache, is_cross, exe_wrap = self._get_compilers('rust', 'RUSTC', want_cross)
+        for compiler in compilers:
+            if isinstance(compiler, str):
+                compiler = [compiler]
+            try:
+                p, out = Popen_safe(compiler + ['--version'])[0:2]
+            except OSError as e:
+                popen_exceptions[compiler] = e
+                continue
+
+            version = search_version(out)
+
+            if 'rustc' in out:
+                return RustCompiler(compiler, version, is_cross, exe_wrap)
+
+        self._handle_exceptions(popen_exceptions, compilers)
 
     def detect_d_compiler(self, want_cross):
         is_cross = False
diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index c759892..fb29fe9 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -1992,9 +1992,9 @@ to directly access options of other subprojects.''')
             if need_cross_compiler:
                 cross_comp = self.environment.detect_d_compiler(True)
         elif lang == 'rust':
-            comp = self.environment.detect_rust_compiler()
+            comp = self.environment.detect_rust_compiler(False)
             if need_cross_compiler:
-                cross_comp = comp  # FIXME, not correct.
+                cross_comp = self.environment.detect_rust_compiler(True)
         elif lang == 'fortran':
             comp = self.environment.detect_fortran_compiler(False)
             if need_cross_compiler: