Fix inlining with -Zalways-encode-mir

Only inline functions that are considered eligible for inlining
by the reachability pass.

This constraint was previously indirectly enforced by only exporting MIR
of eligible functions, but that approach doesn't work with
-Zalways-encode-mir enabled.

compiler/rustc_mir_transform/src/inline.rs

@@ -388,14 +388,11 @@ fn check_codegen_attributes(
             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
+}