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, 24 insertions, 4 deletions

diff --git a/tests/codegen/gdb_debug_script_load.rs b/tests/codegen/gdb_debug_script_load.rs
index 30d518c0bcb..3e92eba10b1 100644
--- a/tests/codegen/gdb_debug_script_load.rs
+++ b/tests/codegen/gdb_debug_script_load.rs
@@ -4,14 +4,34 @@
 //@ ignore-wasm
 //@ ignore-emscripten
 
-//@ compile-flags: -g -C no-prepopulate-passes
+//@ compile-flags: -g -C no-prepopulate-passes -Cpanic=abort
 
-#![feature(start)]
+#![feature(lang_items)]
+#![no_std]
 
+#[panic_handler]
+fn panic_handler(_: &core::panic::PanicInfo) -> ! {
+    loop {}
+}
+
+#[no_mangle]
+extern "C" fn rust_eh_personality() {
+    loop {}
+}
+
+// Needs rustc to generate `main` as that's where the magic load is inserted.
+// IOW, we cannot write this test with `#![no_main]`.
 // CHECK-LABEL: @main
 // CHECK: load volatile i8, {{.+}} @__rustc_debug_gdb_scripts_section__
 
-#[start]
-fn start(_: isize, _: *const *const u8) -> isize {
+#[lang = "start"]
+fn lang_start<T: 'static>(
+    _main: fn() -> T,
+    _argc: isize,
+    _argv: *const *const u8,
+    _sigpipe: u8,
+) -> isize {
     return 0;
 }
+
+fn main() {}