Factor out the two entry_point_type functions.

They are very similar, and each one has a comment about the importance
of being kept in sync with the other. This commit removes the
duplication.

compiler/rustc_ast/src/entry.rs

@@ -1,3 +1,7 @@
+use crate::{attr, Attribute};
+use rustc_span::symbol::sym;
+use rustc_span::Symbol;
+
 #[derive(Debug)]
 pub enum EntryPointType {
     None,
@@ -6,3 +10,26 @@ pub enum EntryPointType {
     Start,
     OtherMain, // Not an entry point, but some other function named main
 }
+
+pub fn entry_point_type(
+    attrs: &[Attribute],
+    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) {
+        EntryPointType::RustcMainAttr
+    } else {
+        if let Some(name) = name && name == sym::main {
+            if at_root {
+                // This is a top-level function so it can be `main`.
+                EntryPointType::MainNamed
+            } else {
+                EntryPointType::OtherMain
+            }
+        } else {
+            EntryPointType::None
+        }
+    }
+}