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Auto merge of #134299 - RalfJung:remove-start, r=compiler-errors

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remove support for the (unstable) #[start] attribute

As explained by `@Noratrieb:`
`#[start]` should be deleted. It's nothing but an accidentally leaked implementation detail that's a not very useful mix between "portable" entrypoint logic and bad abstraction.

I think the way the stable user-facing entrypoint should work (and works today on stable) is pretty simple:
- `std`-using cross-platform programs should use `fn main()`. the compiler, together with `std`, will then ensure that code ends up at `main` (by having a platform-specific entrypoint that gets directed through `lang_start` in `std` to `main` - but that's just an implementation detail)
- `no_std` platform-specific programs should use `#![no_main]` and define their own platform-specific entrypoint symbol with `#[no_mangle]`, like `main`, `_start`, `WinMain` or `my_embedded_platform_wants_to_start_here`. most of them only support a single platform anyways, and need cfg for the different platform's ways of passing arguments or other things *anyways*

`#[start]` is in a super weird position of being neither of those two. It tries to pretend that it's cross-platform, but its signature is  a total lie. Those arguments are just stubbed out to zero on ~~Windows~~ wasm, for example. It also only handles the platform-specific entrypoints for a few platforms that are supported by `std`, like Windows or Unix-likes. `my_embedded_platform_wants_to_start_here` can't use it, and neither could a libc-less Linux program.
So we have an attribute that only works in some cases anyways, that has a signature that's a total lie (and a signature that, as I might want to add, has changed recently, and that I definitely would not be comfortable giving *any* stability guarantees on), and where there's a pretty easy way to get things working without it in the first place.

Note that this feature has **not** been RFCed in the first place.

*This comment was posted [in May](https://github.com/rust-lang/rust/issues/29633#issuecomment-2088596042) and so far nobody spoke up in that issue with a usecase that would require keeping the attribute.*

Closes https://github.com/rust-lang/rust/issues/29633

try-job: x86_64-gnu-nopt
try-job: x86_64-msvc-1
try-job: x86_64-msvc-2
try-job: test-various
This commit is contained in:
bors 2025-01-21 19:46:20 +00:00
commit ed43cbcb88
176 changed files with 454 additions and 1260 deletions
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11
compiler/rustc_codegen_ssa/src/base.rs
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@ -490,8 +490,8 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
let ptr_ty = cx.type_ptr();
let (arg_argc, arg_argv) = get_argc_argv(&mut bx);
let (start_fn, start_ty, args, instance) = if let EntryFnType::Main { sigpipe } = entry_type
{
let EntryFnType::Main { sigpipe } = entry_type;
let (start_fn, start_ty, args, instance) = {
let start_def_id = cx.tcx().require_lang_item(LangItem::Start, None);
let start_instance = ty::Instance::expect_resolve(
cx.tcx(),
@ -512,10 +512,6 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
vec![rust_main, arg_argc, arg_argv, arg_sigpipe],
Some(start_instance),
)
} else {
debug!("using user-defined start fn");
let start_ty = cx.type_func(&[isize_ty, ptr_ty], isize_ty);
(rust_main, start_ty, vec![arg_argc, arg_argv], None)
};
let result = bx.call(start_ty, None, None, start_fn, &args, None, instance);
@ -530,7 +526,8 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
}
}
/// Obtain the `argc` and `argv` values to pass to the rust start function.
/// Obtain the `argc` and `argv` values to pass to the rust start function
/// (i.e., the "start" lang item).
fn get_argc_argv<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(bx: &mut Bx) -> (Bx::Value, Bx::Value) {
if bx.cx().sess().target.os.contains("uefi") {
// Params for UEFI