remove support for the #[start] attribute

Author: RalfJung

Diff for: compiler/rustc_ast/src/entry.rs

@@ -18,12 +18,6 @@ pub enum EntryPointType {
     /// fn main() {}
     /// ```
     RustcMainAttr,
-    /// This is a function with the `#[start]` attribute.
-    /// ```ignore (clashes with test entrypoint)
-    /// #[start]
-    /// fn main() {}
-    /// ```
-    Start,
     /// This function is **not** an entrypoint but simply named `main` (not at the root).
     /// This is only used for diagnostics.
     /// ```
@@ -40,9 +34,7 @@ pub fn entry_point_type(
     at_root: bool,
     name: Option<Symbol>,
 ) -> EntryPointType {
-    if attr::contains_name(attrs, sym::start) {
-        EntryPointType::Start
-    } else if attr::contains_name(attrs, sym::rustc_main) {
+    if attr::contains_name(attrs, sym::rustc_main) {
         EntryPointType::RustcMainAttr
     } else if let Some(name) = name
         && name == sym::main

Diff for: compiler/rustc_ast_passes/src/feature_gate.rs

@@ -230,18 +230,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
             }
 
-            ast::ItemKind::Fn(..) => {
-                if attr::contains_name(&i.attrs, sym::start) {
-                    gate!(
-                        &self,
-                        start,
-                        i.span,
-                        "`#[start]` functions are experimental and their signature may change \
-                         over time"
-                    );
-                }
-            }
-
             ast::ItemKind::Struct(..) | ast::ItemKind::Enum(..) | ast::ItemKind::Union(..) => {
                 for attr in attr::filter_by_name(&i.attrs, sym::repr) {
                     for item in attr.meta_item_list().unwrap_or_else(ThinVec::new) {