Add #[alias] attribute to allow symbol aliasing

[carbotaniuman]

Proposal

ELF platforms along with Windows allow specifying aliases for symbols (I am unsure about Mac). The semantics of aliases are that is simply another name to the same symbol, with no extra layers of indirection, and is available in C as a GCC extension:
int __attribute__((weak, alias("__read"))) read();.

The attribute is similar to the #[linkage] attribute, in that is not designed to be exposed stably, but portions of it may be stabilized depending on use-cases. The syntax would be as follows, applied to an existing function declaration or static, and accepted values for linkage would be external and weak.

The attribute can be repeated to allow for multiple aliases, but each name field must be globally unique to avoid duplicate symbol definitions.

#[alias(name = "read_from_a_thingie")] // should imply `linkage = "external"`
#[alias(name = "read", linkage = "weak")]
#[no_mangle]
extern "C" fn __read() {
...
}

The main use-case for this feature would be current work on Mustang, a libc implemented in Rust, which unfortunately requires a lot of platform-specific shenanigans. This would "properly" fix rust-lang/compiler-builtins#70, although that appears to already have a working workaround.

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Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[apiraino]

Closing MCP (reason: superseded by a T-lang initiative, as mentioned on the Zulip stream), see MCP process for closing MCPs.