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

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

1 files changed, 6 insertions, 6 deletions

diff --git a/tests/ui/array-slice-vec/vec-macro-no-std.rs b/tests/ui/array-slice-vec/vec-macro-no-std.rs
index 1b5ab536dcb..ea0df0bea71 100644
--- a/tests/ui/array-slice-vec/vec-macro-no-std.rs
+++ b/tests/ui/array-slice-vec/vec-macro-no-std.rs
@@ -1,21 +1,21 @@
 //@ run-pass
-
 //@ ignore-emscripten no no_std executables
+//@ ignore-wasm different `main` convention
 
-#![feature(lang_items, start, rustc_private)]
 #![no_std]
+#![no_main]
 
+// Import global allocator and panic handler.
 extern crate std as other;
 
-#[macro_use]
-extern crate alloc;
+#[macro_use] extern crate alloc;
 
 use alloc::vec::Vec;
 
 // Issue #16806
 
-#[start]
-fn start(_argc: isize, _argv: *const *const u8) -> isize {
+#[no_mangle]
+extern "C" fn main(_argc: core::ffi::c_int, _argv: *const *const u8) -> core::ffi::c_int {
     let x: Vec<u8> = vec![0, 1, 2];
     match x.last() {
         Some(&2) => (),