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

compiler/rustc_feature/src/builtin_attrs.rs

@@ -448,7 +448,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     // Entry point:
-    ungated!(start, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(no_start, CrateLevel, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(no_main, CrateLevel, template!(Word), WarnFollowing, EncodeCrossCrate::No),
 

compiler/rustc_feature/src/removed.rs

@@ -220,8 +220,9 @@ declare_features! (
     (removed, rustc_diagnostic_macros, "1.38.0", None, None),
     /// Allows identifying crates that contain sanitizer runtimes.
     (removed, sanitizer_runtime, "1.17.0", None, None),
-    (removed, simd, "1.0.0", Some(27731),
-     Some("removed in favor of `#[repr(simd)]`")),
+    (removed, simd, "1.0.0", Some(27731), Some("removed in favor of `#[repr(simd)]`")),
+    /// Allows using `#[start]` on a function indicating that it is the program entrypoint.
+    (removed, start, "1.0.0", Some(29633), Some("not portable enough and never RFC'd")),
     /// Allows `#[link(kind = "static-nobundle", ...)]`.
     (removed, static_nobundle, "1.16.0", Some(37403),
      Some(r#"subsumed by `#[link(kind = "static", modifiers = "-bundle", ...)]`"#)),

compiler/rustc_feature/src/unstable.rs

@@ -300,8 +300,6 @@ declare_features! (
     (internal, rustdoc_internals, "1.58.0", Some(90418)),
     /// Allows using the `rustdoc::missing_doc_code_examples` lint
     (unstable, rustdoc_missing_doc_code_examples, "1.31.0", Some(101730)),
-    /// Allows using `#[start]` on a function indicating that it is the program entrypoint.
-    (unstable, start, "1.0.0", Some(29633)),
     /// Allows using `#[structural_match]` which indicates that a type is structurally matchable.
     /// FIXME: Subsumed by trait `StructuralPartialEq`, cannot move to removed until a library
     /// feature with the same name exists.