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Add a used attribute #1459

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62 changes: 62 additions & 0 deletions text/0000-used.md
Original file line number Diff line number Diff line change
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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.
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I think that this drawbacks section should probably at least mention that "may be discarded by the linker" can in some cases be confusing. In annotating a symbol as #[preserve] one might expect it to be preserved but it may not make it all the way through.

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One might expect that #[inline] inlines the function but that's not true either.


# Alternatives
[alternatives]: #alternatives

The name of the attribute could be changed.

# Unresolved questions
[unresolved]: #unresolved-questions

None.