Fix inlining with -Zalways-encode-mir

compiler/rustc_mir_transform/src/inline.rs

@@ -388,14 +388,11 @@ impl<'tcx> Inliner<'tcx> {
             return Err("never inline hint");
         }
 
-        // Only inline local functions if they would be eligible for cross-crate
-        // inlining. This is to ensure that the final crate doesn't have MIR that
-        // reference unexported symbols
-        if callsite.callee.def_id().is_local() {
-            let is_generic = callsite.callee.args.non_erasable_generics().next().is_some();
-            if !is_generic && !callee_attrs.requests_inline() {
-                return Err("not exported");
-            }
+        // Reachability pass defines which functions are eligible for inlining. Generally inlining
+        // other functions is incorrect because they could reference symbols that aren't exported.
+        let is_generic = callsite.callee.args.non_erasable_generics().next().is_some();
+        if !is_generic && !callee_attrs.requests_inline() {
+            return Err("not exported");
         }
 
         if callsite.fn_sig.c_variadic() {

tests/ui/mir/mir-inlining/always-encode-mirs.rs

@@ -0,0 +1,12 @@
+// Regression test for MIR inlining with -Zalways-encode-mir enabled in the auxiliary crate.
+// Previously we inlined function not eligible for inlining which lead to linking error:
+// undefined reference to `internal::S'
+//
+// aux-build:internal.rs
+// build-pass
+// compile-flags: -O
+extern crate internal;
+
+fn main() {
+    println!("{}", internal::f());
+}

tests/ui/mir/mir-inlining/auxiliary/internal.rs

@@ -0,0 +1,7 @@
+// compile-flags: -Zalways-encode-mir
+
+static S: usize = 42;
+
+pub fn f() -> &'static usize {
+    &S
+}