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](#29633 (comment)) and so far nobody spoke up in that issue with a usecase that would require keeping the attribute.*

Closes #29633

try-job: x86_64-gnu-nopt
try-job: x86_64-msvc-1
try-job: x86_64-msvc-2
try-job: test-various

Author: bors

tests/ui/borrow_as_ptr_no_std.fixed

@@ -1,22 +1,13 @@
 #![warn(clippy::borrow_as_ptr)]
-#![feature(lang_items, start, libc)]
 #![no_std]
+#![crate_type = "lib"]
 
 #[clippy::msrv = "1.75"]
-#[start]
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let val = 1;
     let _p = core::ptr::addr_of!(val);
 
     let mut val_mut = 1;
     let _p_mut = core::ptr::addr_of_mut!(val_mut);
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

tests/ui/borrow_as_ptr_no_std.rs

@@ -1,22 +1,13 @@
 #![warn(clippy::borrow_as_ptr)]
-#![feature(lang_items, start, libc)]
 #![no_std]
+#![crate_type = "lib"]
 
 #[clippy::msrv = "1.75"]
-#[start]
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let val = 1;
     let _p = &val as *const i32;
 
     let mut val_mut = 1;
     let _p_mut = &mut val_mut as *mut i32;
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

tests/ui/borrow_as_ptr_no_std.stderr

@@ -1,5 +1,5 @@
 error: borrow as raw pointer
-  --> tests/ui/borrow_as_ptr_no_std.rs:9:14
+  --> tests/ui/borrow_as_ptr_no_std.rs:8:14
    |
 LL |     let _p = &val as *const i32;
    |              ^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::addr_of!(val)`
@@ -8,7 +8,7 @@ LL |     let _p = &val as *const i32;
    = help: to override `-D warnings` add `#[allow(clippy::borrow_as_ptr)]`
 
 error: borrow as raw pointer
-  --> tests/ui/borrow_as_ptr_no_std.rs:12:18
+  --> tests/ui/borrow_as_ptr_no_std.rs:11:18
    |
 LL |     let _p_mut = &mut val_mut as *mut i32;
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::addr_of_mut!(val_mut)`

tests/ui/box_default_no_std.rs

@@ -1,6 +1,6 @@
-#![feature(lang_items, start, libc)]
 #![warn(clippy::box_default)]
 #![no_std]
+#![crate_type = "lib"]
 
 pub struct NotBox<T> {
     _value: T,
@@ -18,16 +18,7 @@ impl<T: Default> Default for NotBox<T> {
     }
 }
 
-#[start]
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let _p = NotBox::new(isize::default());
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

tests/ui/crashes/ice-7410.rs

@@ -1,7 +1,7 @@
 //@compile-flags: -Clink-arg=-nostartfiles
 //@ignore-target: apple windows
 
-#![feature(lang_items, start, libc)]
+#![crate_type = "lib"]
 #![no_std]
 #![allow(clippy::if_same_then_else)]
 #![allow(clippy::redundant_pattern_matching)]
@@ -15,18 +15,9 @@ impl Drop for S {
     fn drop(&mut self) {}
 }
 
-#[start]
-fn main(argc: isize, argv: *const *const u8) -> isize {
+pub fn main(argc: isize, argv: *const *const u8) -> isize {
     if let Some(_) = Some(S) {
     } else {
     }
     0
 }
-
-#[panic_handler]
-fn panic(_info: &PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

tests/ui/crate_level_checks/no_std_main_recursion.rs

@@ -1,11 +1,10 @@
 #![no_std]
-#![feature(lang_items, start, libc)]
 #![crate_type = "lib"]
 
 use core::panic::PanicInfo;
 
 #[warn(clippy::all)]
-fn main() {
+pub fn main() {
     let mut a = 42;
     let mut b = 1337;
 

tests/ui/crate_level_checks/no_std_swap.fixed

@@ -1,11 +1,10 @@
 #![no_std]
-#![feature(lang_items, start, libc)]
 #![crate_type = "lib"]
 
 use core::panic::PanicInfo;
 
 #[warn(clippy::all)]
-fn main() {
+pub fn main() {
     let mut a = 42;
     let mut b = 1337;
 

tests/ui/crate_level_checks/no_std_swap.rs

@@ -1,5 +1,5 @@
 error: this looks like you are trying to swap `a` and `b`
-  --> tests/ui/crate_level_checks/no_std_swap.rs:12:5
+  --> tests/ui/crate_level_checks/no_std_swap.rs:11:5
    |
 LL | /     a = b;
 ...  |

tests/ui/crate_level_checks/no_std_swap.stderr

@@ -1,6 +1,6 @@
 //@ignore-target: apple
 
-#![feature(no_core, lang_items, start)]
+#![feature(no_core, lang_items)]
 #![no_core]
 #![allow(clippy::missing_safety_doc)]