tests/rust: Add rust case linking to libm in rust static library

When building on Linux, I see:

rustc -C linker=cc --color=always --crate-type rlib --crate-name rs_math -g --emit dep-info=rs_math.d --emit link -L /usr/lib/x86_64-linux-gnu -o librs_math.rlib -l static=m ../rs_math.rs
error: failed to add native library /usr/lib/x86_64-linux-gnu/libm.a: file too small to be an archive

I think the "file too small to be an archive" message is coming from
libLLVM, and is a case of LLVM not handling this type of "script
archive". So, possibly this is just LLVM not handling a linking case.

The rust_args usage in meson.build is invalid, but required to
reproduce the issue in the test case. Perhaps meson should
automatically add the library include path via the dep_m meson object,
or maybe the meson.build can add the link path in a better way.

Changing '-l static=m' to '-l dylib=m' appears to fix this case. (See
comments in meson.build.)

test cases/rust/14 external libm/meson.build

@@ -0,0 +1,24 @@
+project('rust linking to libm', 'c', 'rust')
+
+if host_machine.system() == 'darwin'
+  error('MESON_SKIP_TEST: doesnt work right on macos, please fix!')
+endif
+
+cc = meson.get_compiler('c')
+dep_m = cc.find_library('m', required : false, static : get_option('static'))
+if not dep_m.found()
+  error('MESON_SKIP_TEST: Could not find a @0@ libm'.format(get_option('static') ? 'static' : 'shared'))
+endif
+
+librs_math = static_library(
+  'rs_math',
+  'rs_math.rs',
+  dependencies : [dep_m],
+)
+
+e = executable(
+  'prog', 'prog.rs',
+  link_with : [librs_math],
+)
+
+test('cdepstest', e)

test cases/rust/14 external libm/meson_options.txt

@@ -0,0 +1,2 @@
+option('static', type : 'boolean')
+option('method', type : 'string')

test cases/rust/14 external libm/prog.rs

@@ -0,0 +1,5 @@
+extern crate rs_math;
+
+fn main() {
+    assert_eq!(rs_math::rs_log2(8.0), 3.0);
+}

test cases/rust/14 external libm/rs_math.rs

@@ -0,0 +1,12 @@
+#![crate_name = "rs_math"]
+
+use std::os::raw::c_double;
+
+extern "C" {
+    fn log2(n: c_double) -> c_double;
+}
+
+#[no_mangle]
+pub extern fn rs_log2(n: c_double) -> c_double {
+    unsafe { log2(n) }
+}

test cases/rust/14 external libm/test.json

@@ -0,0 +1,10 @@
+{
+  "matrix": {
+    "options": {
+      "static": [
+        { "val": true },
+        { "val": false }
+      ]
+    }
+  }
+}