Replace __ONCE__ with Cargo links key

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"

src/lib.rs

@@ -689,6 +689,10 @@ pub use macros::exception;
 /// [rfc1414]: https://github.com/rust-lang/rfcs/blob/master/text/1414-rvalue_static_promotion.md
 pub use macros::pre_init;
 
+// We export this static with an informative name so that if an application attempts to link
+// two copies of cortex-m-rt together, linking will fail. We also declare a links key in
+// Cargo.toml which is the more modern way to solve the same problem, but we have to keep
+// __ONCE__ around to prevent linking with versions before the links key was added.
 #[export_name = "error: cortex-m-rt appears more than once in the dependency graph"]
 #[doc(hidden)]
 pub static __ONCE__: () = ();