Do not fail if --emit≠link and there’s no main fn

Fixes #21813

Author: nagisa

src/librustc/middle/entry.rs

@@ -50,7 +50,10 @@ pub fn find_entry_point(session: &Session, ast_map: &ast_map::Map) {
     let any_exe = session.crate_types.borrow().iter().any(|ty| {
         *ty == config::CrateTypeExecutable
     });
-    if !any_exe {
+    let emit_link = session.opts.output_types.iter().any(|ty| {
+        *ty == config::OutputTypeExe
+    });
+    if !any_exe || !emit_link {
         // No need to find a main function
         return
     }

src/librustc/session/config.rs

@@ -82,7 +82,7 @@ pub struct Options {
     pub debuginfo: DebugInfoLevel,
     pub lint_opts: Vec<(String, lint::Level)>,
     pub describe_lints: bool,
-    pub output_types: Vec<OutputType> ,
+    pub output_types: Vec<OutputType>,
     // This was mutable for rustpkg, which updates search paths based on the
     // parsed code. It remains mutable in case its replacements wants to use
     // this.