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.

2 files changed, 23 insertions, 35 deletions

diff --git a/compiler/rustc_passes/src/entry.rs b/compiler/rustc_passes/src/entry.rs
index d52092f2aa9..b43c8282db1 100644
--- a/compiler/rustc_passes/src/entry.rs
+++ b/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()
         {
diff --git a/compiler/rustc_passes/src/errors.rs b/compiler/rustc_passes/src/errors.rs
index 743faf54560..65cad82cc8c 100644
--- a/compiler/rustc_passes/src/errors.rs
+++ b/compiler/rustc_passes/src/errors.rs
@@ -1207,22 +1207,6 @@ pub struct NakedFunctionsMustUseNoreturn {
 }
 
 #[derive(Diagnostic)]
-#[diag(passes_attr_only_on_main)]
-pub struct AttrOnlyOnMain {
-    #[primary_span]
-    pub span: Span,
-    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 {
     #[primary_span]