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_trait_selection/messages.ftl

@@ -261,7 +261,6 @@ trait_selection_oc_fn_lang_correct_type = {$lang_item_name ->
         *[lang_item_name] lang item `{$lang_item_name}`
     } function has wrong type
 trait_selection_oc_fn_main_correct_type = `main` function has wrong type
-trait_selection_oc_fn_start_correct_type = `#[start]` function has wrong type
 trait_selection_oc_generic = mismatched types
 
 trait_selection_oc_if_else_different = `if` and `else` have incompatible types
@@ -396,7 +395,6 @@ trait_selection_subtype = ...so that the {$requirement ->
     [if_else_different] `if` and `else` have incompatible types
     [no_else] `if` missing an `else` returns `()`
     [fn_main_correct_type] `main` function has the correct type
-    [fn_start_correct_type] `#[start]` function has the correct type
     [fn_lang_correct_type] lang item function has the correct type
     [intrinsic_correct_type] intrinsic has the correct type
     [method_correct_type] method receiver has the correct type
@@ -410,7 +408,6 @@ trait_selection_subtype_2 = ...so that {$requirement ->
     [if_else_different] `if` and `else` have incompatible types
     [no_else] `if` missing an `else` returns `()`
     [fn_main_correct_type] `main` function has the correct type
-    [fn_start_correct_type] `#[start]` function has the correct type
     [fn_lang_correct_type] lang item function has the correct type
     [intrinsic_correct_type] intrinsic has the correct type
     [method_correct_type] method receiver has the correct type

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

@@ -2318,7 +2318,6 @@ impl<'tcx> ObligationCause<'tcx> {
             | ObligationCauseCode::MatchExpressionArm(_)
             | ObligationCauseCode::IfExpression { .. }
             | ObligationCauseCode::LetElse
-            | ObligationCauseCode::StartFunctionType
             | ObligationCauseCode::LangFunctionType(_)
             | ObligationCauseCode::IntrinsicType
             | ObligationCauseCode::MethodReceiver => FailureCode::Error0308,
@@ -2376,9 +2375,6 @@ impl<'tcx> ObligationCause<'tcx> {
             ObligationCauseCode::MainFunctionType => {
                 ObligationCauseFailureCode::FnMainCorrectType { span }
             }
-            ObligationCauseCode::StartFunctionType => {
-                ObligationCauseFailureCode::FnStartCorrectType { span, subdiags }
-            }
             &ObligationCauseCode::LangFunctionType(lang_item_name) => {
                 ObligationCauseFailureCode::FnLangCorrectType { span, subdiags, lang_item_name }
             }
@@ -2421,7 +2417,6 @@ impl<'tcx> ObligationCause<'tcx> {
                 "const is compatible with trait"
             }
             ObligationCauseCode::MainFunctionType => "`main` function has the correct type",
-            ObligationCauseCode::StartFunctionType => "`#[start]` function has the correct type",
             ObligationCauseCode::LangFunctionType(_) => "lang item function has the correct type",
             ObligationCauseCode::IntrinsicType => "intrinsic has the correct type",
             ObligationCauseCode::MethodReceiver => "method receiver has the correct type",
@@ -2442,7 +2437,6 @@ impl IntoDiagArg for ObligationCauseAsDiagArg<'_> {
                 "const_compat"
             }
             ObligationCauseCode::MainFunctionType => "fn_main_correct_type",
-            ObligationCauseCode::StartFunctionType => "fn_start_correct_type",
             ObligationCauseCode::LangFunctionType(_) => "fn_lang_correct_type",
             ObligationCauseCode::IntrinsicType => "intrinsic_correct_type",
             ObligationCauseCode::MethodReceiver => "method_correct_type",

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -2740,7 +2740,6 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             | ObligationCauseCode::IfExpression { .. }
             | ObligationCauseCode::IfExpressionWithNoElse
             | ObligationCauseCode::MainFunctionType
-            | ObligationCauseCode::StartFunctionType
             | ObligationCauseCode::LangFunctionType(_)
             | ObligationCauseCode::IntrinsicType
             | ObligationCauseCode::MethodReceiver

compiler/rustc_trait_selection/src/errors.rs

@@ -1695,13 +1695,6 @@ pub enum ObligationCauseFailureCode {
         #[primary_span]
         span: Span,
     },
-    #[diag(trait_selection_oc_fn_start_correct_type, code = E0308)]
-    FnStartCorrectType {
-        #[primary_span]
-        span: Span,
-        #[subdiagnostic]
-        subdiags: Vec<TypeErrorAdditionalDiags>,
-    },
     #[diag(trait_selection_oc_fn_lang_correct_type, code = E0308)]
     FnLangCorrectType {
         #[primary_span]