test for immediate symbol name hashing

src/test/run-make/a-b-a-linker-guard/Makefile

@@ -0,0 +1,7 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) a.rs --cfg x -C prefer-dynamic
+	$(RUSTC) b.rs -C prefer-dynamic
+	$(RUSTC) a.rs --cfg y -C prefer-dynamic
+	$(call RUN,b) 2>&1 | grep "undefined symbol"

src/test/run-make/a-b-a-linker-guard/a.rs

@@ -0,0 +1,10 @@
+#![crate_name = "a"]
+#![crate_type = "dylib"]
+
+#[cfg(x)]
+pub fn foo(x: u32) { }
+
+#[cfg(y)]
+pub fn foo(x: i32) { }
+
+

src/test/run-make/a-b-a-linker-guard/b.rs

@@ -0,0 +1,7 @@
+#![crate_name = "b"]
+
+extern crate a;
+
+fn main() {
+    a::foo(22_u32);
+}