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

4 files changed, 4 insertions, 41 deletions

diff --git a/compiler/rustc_passes/messages.ftl b/compiler/rustc_passes/messages.ftl
index 3ed600a717f..133d84572e6 100644
--- a/compiler/rustc_passes/messages.ftl
+++ b/compiler/rustc_passes/messages.ftl
@@ -502,11 +502,6 @@ passes_multiple_rustc_main =
     .first = first `#[rustc_main]` function
     .additional = additional `#[rustc_main]` function
 
-passes_multiple_start_functions =
-    multiple `start` functions
-    .label = multiple `start` functions
-    .previous = previous `#[start]` function here
-
 passes_must_not_suspend =
     `must_not_suspend` attribute should be applied to a struct, enum, union, or trait
     .label = is not a struct, enum, union, or trait
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index 1b2b8ac5dd9..dbb87443eed 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -275,7 +275,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     | sym::lang
                     | sym::needs_allocator
                     | sym::default_lib_allocator
-                    | sym::start
                     | sym::custom_mir,
                     ..
                 ] => {}
@@ -2655,7 +2654,6 @@ fn check_invalid_crate_level_attr(tcx: TyCtxt<'_>, attrs: &[Attribute]) {
         sym::repr,
         sym::path,
         sym::automatically_derived,
-        sym::start,
         sym::rustc_main,
         sym::derive,
         sym::test,
diff --git a/compiler/rustc_passes/src/entry.rs b/compiler/rustc_passes/src/entry.rs
index 4949a4316a7..22291c9282d 100644
--- a/compiler/rustc_passes/src/entry.rs
+++ b/compiler/rustc_passes/src/entry.rs
@@ -10,9 +10,7 @@ use rustc_session::RemapFileNameExt;
 use rustc_session::config::{CrateType, EntryFnType, RemapPathScopeComponents, sigpipe};
 use rustc_span::{Span, Symbol, sym};
 
-use crate::errors::{
-    AttrOnlyInFunctions, ExternMain, MultipleRustcMain, MultipleStartFunctions, NoMainErr,
-};
+use crate::errors::{AttrOnlyInFunctions, ExternMain, MultipleRustcMain, NoMainErr};
 
 struct EntryContext<'tcx> {
     tcx: TyCtxt<'tcx>,
@@ -20,9 +18,6 @@ struct EntryContext<'tcx> {
     /// The function has the `#[rustc_main]` attribute.
     rustc_main_fn: Option<(LocalDefId, Span)>,
 
-    /// The function that has the attribute `#[start]` on it.
-    start_fn: Option<(LocalDefId, Span)>,
-
     /// The functions that one might think are `main` but aren't, e.g.
     /// main functions not defined at the top level. For diagnostics.
     non_main_fns: Vec<Span>,
@@ -40,8 +35,7 @@ fn entry_fn(tcx: TyCtxt<'_>, (): ()) -> Option<(DefId, EntryFnType)> {
         return None;
     }
 
-    let mut ctxt =
-        EntryContext { tcx, rustc_main_fn: None, start_fn: None, non_main_fns: Vec::new() };
+    let mut ctxt = EntryContext { tcx, rustc_main_fn: None, non_main_fns: Vec::new() };
 
     for id in tcx.hir().items() {
         check_and_search_item(id, &mut ctxt);
@@ -57,7 +51,7 @@ fn attr_span_by_symbol(ctxt: &EntryContext<'_>, id: ItemId, sym: Symbol) -> Opti
 
 fn check_and_search_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
     if !matches!(ctxt.tcx.def_kind(id.owner_id), DefKind::Fn) {
-        for attr in [sym::start, sym::rustc_main] {
+        for attr in [sym::rustc_main] {
             if let Some(span) = attr_span_by_symbol(ctxt, id, attr) {
                 ctxt.tcx.dcx().emit_err(AttrOnlyInFunctions { span, attr });
             }
@@ -91,24 +85,11 @@ fn check_and_search_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
                 });
             }
         }
-        EntryPointType::Start => {
-            if ctxt.start_fn.is_none() {
-                ctxt.start_fn = Some((id.owner_id.def_id, ctxt.tcx.def_span(id.owner_id)));
-            } else {
-                ctxt.tcx.dcx().emit_err(MultipleStartFunctions {
-                    span: ctxt.tcx.def_span(id.owner_id),
-                    labeled: ctxt.tcx.def_span(id.owner_id.to_def_id()),
-                    previous: ctxt.start_fn.unwrap().1,
-                });
-            }
-        }
     }
 }
 
 fn configure_main(tcx: TyCtxt<'_>, visitor: &EntryContext<'_>) -> Option<(DefId, EntryFnType)> {
-    if let Some((def_id, _)) = visitor.start_fn {
-        Some((def_id.to_def_id(), EntryFnType::Start))
-    } else if let Some((local_def_id, _)) = visitor.rustc_main_fn {
+    if let Some((local_def_id, _)) = visitor.rustc_main_fn {
         let def_id = local_def_id.to_def_id();
         Some((def_id, EntryFnType::Main { sigpipe: sigpipe(tcx) }))
     } else {
diff --git a/compiler/rustc_passes/src/errors.rs b/compiler/rustc_passes/src/errors.rs
index c3043ac60aa..3d38b00e99f 100644
--- a/compiler/rustc_passes/src/errors.rs
+++ b/compiler/rustc_passes/src/errors.rs
@@ -1314,17 +1314,6 @@ pub(crate) struct MultipleRustcMain {
 }
 
 #[derive(Diagnostic)]
-#[diag(passes_multiple_start_functions, code = E0138)]
-pub(crate) struct MultipleStartFunctions {
-    #[primary_span]
-    pub span: Span,
-    #[label]
-    pub labeled: Span,
-    #[label(passes_previous)]
-    pub previous: Span,
-}
-
-#[derive(Diagnostic)]
 #[diag(passes_extern_main)]
 pub(crate) struct ExternMain {
     #[primary_span]