Replace __ONCE__ with Cargo links key

Author: adamgreig

cortex-m-rt/Cargo.toml

@@ -14,6 +14,7 @@ readme = "README.md"
 repository = "https://github.com/rust-embedded/cortex-m-rt"
 version = "0.6.11"
 autoexamples = true
+links = "cortex-m-rt" # Prevent multiple versions of cortex-m-rt being linked
 
 [dependencies]
 r0 = "1.0"

cortex-m-rt/src/lib.rs

@@ -689,10 +689,6 @@ pub use macros::exception;
 /// [rfc1414]: https://github.com/rust-lang/rfcs/blob/master/text/1414-rvalue_static_promotion.md
 pub use macros::pre_init;
 
-#[export_name = "error: cortex-m-rt appears more than once in the dependency graph"]
-#[doc(hidden)]
-pub static __ONCE__: () = ();
-
 /// Registers stacked (pushed onto the stack) during an exception.
 #[derive(Clone, Copy)]
 #[repr(C)]