Allow generic foreign functions.

Generic extern functions written in Rust have their names mangled, as well as their internal clownshoe __rust_abi functions. This allows e.g. specific monomorphizations of these functions to be used as callbacks.

Closes #12502.

src/test/compile-fail/generic-extern.rs

@@ -8,8 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-extern "C" fn foo<T>() {}  //~ERROR foreign functions may not use type parameters
+extern {
+    fn foo<T>(); //~ ERROR foreign items may not have type parameters
+}
 
 fn main() {
-    let _ = foo::<int>;
+    foo::<i32>();
 }

src/test/run-make/extern-fn-generic/Makefile

@@ -0,0 +1,8 @@
+-include ../tools.mk
+
+all:
+	$(CC) -std=c99 test.c -c -o $(TMPDIR)/test.o
+	$(AR) rcs $(TMPDIR)/libtest.a $(TMPDIR)/test.o
+	$(RUSTC) testcrate.rs -L $(TMPDIR)
+	$(RUSTC) test.rs -L $(TMPDIR)
+	$(call RUN,test) || exit 1

src/test/run-make/extern-fn-generic/test.c

@@ -0,0 +1,16 @@
+#include <stdint.h>
+
+typedef struct TestStruct {
+	uint8_t x;
+	int32_t y;
+} TestStruct;
+
+typedef int callback(TestStruct s);
+
+uint32_t call(callback *c) {
+	TestStruct s;
+	s.x = 'a';
+	s.y = 3;
+
+	return c(s);
+}

src/test/run-make/extern-fn-generic/test.rs

@@ -0,0 +1,32 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+extern crate testcrate;
+
+extern "C" fn bar<T>(ts: testcrate::TestStruct<T>) -> T { ts.y }
+
+#[link(name = "test")]
+extern {
+    fn call(c: extern "C" fn(testcrate::TestStruct<i32>) -> i32) -> i32;
+}
+
+fn main() {
+    // Let's test calling it cross crate
+    let back = unsafe {
+        testcrate::call(testcrate::foo::<i32>)
+    };
+    assert_eq!(3, back);
+
+    // And just within this crate
+    let back = unsafe {
+        call(bar::<i32>)
+    };
+    assert_eq!(3, back);
+}

src/test/run-make/extern-fn-generic/testcrate.rs

@@ -0,0 +1,24 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_type = "lib"]
+
+#[repr(C)]
+pub struct TestStruct<T> {
+    pub x: u8,
+    pub y: T
+}
+
+pub extern "C" fn foo<T>(ts: TestStruct<T>) -> T { ts.y }
+
+#[link(name = "test")]
+extern {
+    pub fn call(c: extern "C" fn(TestStruct<i32>) -> i32) -> i32;
+}

src/test/run-pass/generic-extern-mangle.rs

@@ -0,0 +1,16 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+extern "C" fn foo<T: Int>(a: T, b: T) -> T { a + b }
+
+fn main() {
+    assert_eq!(99u8, foo(255u8, 100u8));
+    assert_eq!(99u16, foo(65535u16, 100u16));
+}