Various improvements to entrypoint code

This moves some code around and adds some documentation comments to make
it easier to understand what's going on with the entrypoint logic, which
is a bit complicated.

The only change in behavior is consolidating the error messages for
unix_sigpipe to make the code slightly simpler.

compiler/rustc_ast/src/entry.rs

@@ -4,11 +4,34 @@ use rustc_span::Symbol;
 
 #[derive(Debug)]
 pub enum EntryPointType {
+    /// This function is not an entrypoint.
     None,
+    /// This is a function called `main` at the root level.
+    /// ```
+    /// fn main() {}
+    /// ```
     MainNamed,
+    /// This is a function with the `#[rustc_main]` attribute.
+    /// Used by the testing harness to create the test entrypoint.
+    /// ```ignore (clashes with test entrypoint)
+    /// #[rustc_main]
+    /// fn main() {}
+    /// ```
     RustcMainAttr,
+    /// This is a function with the `#[start]` attribute.
+    /// ```ignore (clashes with test entrypoint)
+    /// #[start]
+    /// fn main() {}
+    /// ```
     Start,
-    OtherMain, // Not an entry point, but some other function named main
+    /// This function is **not** an entrypoint but simply named `main` (not at the root).
+    /// This is only used for diagnostics.
+    /// ```
+    /// mod meow {
+    ///     fn main() {}
+    /// }
+    /// ```
+    OtherMain,
 }
 
 pub fn entry_point_type(

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -266,7 +266,7 @@ fn generate_test_harness(
 ///
 /// By default this expands to
 ///
-/// ```ignore UNSOLVED (I think I still need guidance for this one. Is it correct? Do we try to make it run? How do we nicely fill it out?)
+/// ```ignore (messes with test internals)
 /// #[rustc_main]
 /// pub fn main() {
 ///     extern crate test;

compiler/rustc_passes/messages.ftl

@@ -50,10 +50,7 @@ passes_attr_only_in_functions =
     `{$attr}` attribute can only be used on functions
 
 passes_attr_only_on_main =
-    `{$attr}` attribute can only be used on `fn main()`
-
-passes_attr_only_on_root_main =
-    `{$attr}` attribute can only be used on root `fn main()`
+    `{$attr}` attribute can only be used on the root `fn main()`
 
 passes_both_ffi_const_and_pure =
     `#[ffi_const]` function cannot be `#[ffi_pure]`

compiler/rustc_passes/src/entry.rs

@@ -18,10 +18,10 @@ use crate::errors::{
 struct EntryContext<'tcx> {
     tcx: TyCtxt<'tcx>,
 
-    /// The function that has attribute named `main`.
-    attr_main_fn: Option<(LocalDefId, Span)>,
+    /// The function has the `#[rustc_main]` attribute.
+    rustc_main_fn: Option<(LocalDefId, Span)>,
 
-    /// The function that has the attribute 'start' on it.
+    /// 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.
@@ -42,10 +42,10 @@ fn entry_fn(tcx: TyCtxt<'_>, (): ()) -> Option<(DefId, EntryFnType)> {
     }
 
     let mut ctxt =
-        EntryContext { tcx, attr_main_fn: None, start_fn: None, non_main_fns: Vec::new() };
+        EntryContext { tcx, rustc_main_fn: None, start_fn: None, non_main_fns: Vec::new() };
 
     for id in tcx.hir().items() {
-        find_item(id, &mut ctxt);
+        check_and_search_item(id, &mut ctxt);
     }
 
     configure_main(tcx, &ctxt)
@@ -56,7 +56,16 @@ fn attr_span_by_symbol(ctxt: &EntryContext<'_>, id: ItemId, sym: Symbol) -> Opti
     attr::find_by_name(attrs, sym).map(|attr| attr.span)
 }
 
-fn find_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
+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] {
+            if let Some(span) = attr_span_by_symbol(ctxt, id, attr) {
+                ctxt.tcx.dcx().emit_err(AttrOnlyInFunctions { span, attr });
+            }
+        }
+        return;
+    }
+
     let at_root = ctxt.tcx.opt_local_parent(id.owner_id.def_id) == Some(CRATE_DEF_ID);
 
     let attrs = ctxt.tcx.hir().attrs(id.hir_id());
@@ -65,26 +74,20 @@ fn find_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
         at_root,
         ctxt.tcx.opt_item_name(id.owner_id.to_def_id()),
     );
+
     match entry_point_type {
-        EntryPointType::None => (),
-        _ if !matches!(ctxt.tcx.def_kind(id.owner_id), DefKind::Fn) => {
-            for attr in [sym::start, sym::rustc_main] {
-                if let Some(span) = attr_span_by_symbol(ctxt, id, attr) {
-                    ctxt.tcx.dcx().emit_err(AttrOnlyInFunctions { span, attr });
-                }
-            }
-        }
-        EntryPointType::MainNamed => (),
+        EntryPointType::None => {}
+        EntryPointType::MainNamed => {}
         EntryPointType::OtherMain => {
             ctxt.non_main_fns.push(ctxt.tcx.def_span(id.owner_id));
         }
         EntryPointType::RustcMainAttr => {
-            if ctxt.attr_main_fn.is_none() {
-                ctxt.attr_main_fn = Some((id.owner_id.def_id, ctxt.tcx.def_span(id.owner_id)));
+            if ctxt.rustc_main_fn.is_none() {
+                ctxt.rustc_main_fn = Some((id.owner_id.def_id, ctxt.tcx.def_span(id.owner_id)));
             } else {
                 ctxt.tcx.dcx().emit_err(MultipleRustcMain {
                     span: ctxt.tcx.def_span(id.owner_id.to_def_id()),
-                    first: ctxt.attr_main_fn.unwrap().1,
+                    first: ctxt.rustc_main_fn.unwrap().1,
                     additional: ctxt.tcx.def_span(id.owner_id.to_def_id()),
                 });
             }
@@ -107,10 +110,11 @@ fn find_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
 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.attr_main_fn {
+    } else 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 {
+        // The actual resolution of main happens in the resolver, this here
         if let Some(main_def) = tcx.resolutions(()).main_def
             && let Some(def_id) = main_def.opt_fn_def_id()
         {

compiler/rustc_passes/src/errors.rs

@@ -1214,14 +1214,6 @@ pub struct AttrOnlyOnMain {
     pub attr: Symbol,
 }
 
-#[derive(Diagnostic)]
-#[diag(passes_attr_only_on_root_main)]
-pub struct AttrOnlyOnRootMain {
-    #[primary_span]
-    pub span: Span,
-    pub attr: Symbol,
-}
-
 #[derive(Diagnostic)]
 #[diag(passes_attr_only_in_functions)]
 pub struct AttrOnlyInFunctions {