Add a used attribute

text/0000-used.md

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+- Feature Name: preserve
+- Start Date: 2016-01-11
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Add a `preserve` attribute to prevent symbols from being discarded. This
+attribute is similar to GCC's `used` attribute.
+
+# Motivation
+[motivation]: #motivation
+
+The compiler cannot always see all references to a symbol. For example, if a
+symbol is only referenced from inline assembly, the compiler might think that
+the symbol is unused.
+
+If the compiler thinks that a symbol is unused, it might optimize the symbol
+away which might change the behavior of the program or cause the linking step to
+fail. For example:
+
+```rust
+#![allow(dead_code)]
+
+#[link_section = ".init_array"]
+static F: extern fn() = f;
+
+extern fn f() {
+    println!("test");
+}
+
+fn main() { }
+```
+
+When this program is compiled without optimization, it prints "test". But if it
+is compiled with optimization, it prints nothing.
+
+# Detailed design
+[design]: #detailed-design
+
+A `#[preserve]` attribute is added. This attribute can be applied to static
+variables and functions without type parameters.
+
+If an item is marked with this attribute, the compiler generates the
+corresponding symbol and the symbol is not discarded by the compilation process.
+It may be discarded by the linking process.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+None.
+
+# Alternatives
+[alternatives]: #alternatives
+
+None.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.