From fe3cd2d1941a726a0e49b2553de3d4c73a7b4a6d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Mi=C4=85sko?= <tomasz.miasko@gmail.com>
Date: Sun, 27 Aug 2023 00:00:00 +0000
Subject: [PATCH] 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      | 13 +++++--------
 tests/ui/mir/mir-inlining/always-encode-mirs.rs | 12 ++++++++++++
 tests/ui/mir/mir-inlining/auxiliary/internal.rs |  7 +++++++
 3 files changed, 24 insertions(+), 8 deletions(-)
 create mode 100644 tests/ui/mir/mir-inlining/always-encode-mirs.rs
 create mode 100644 tests/ui/mir/mir-inlining/auxiliary/internal.rs

diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 9551c7a2a8d6b..7d4c4a823a8db 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/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() {
diff --git a/tests/ui/mir/mir-inlining/always-encode-mirs.rs b/tests/ui/mir/mir-inlining/always-encode-mirs.rs
new file mode 100644
index 0000000000000..f3731996cf29b
--- /dev/null
+++ b/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());
+}
diff --git a/tests/ui/mir/mir-inlining/auxiliary/internal.rs b/tests/ui/mir/mir-inlining/auxiliary/internal.rs
new file mode 100644
index 0000000000000..fa0f9c2da4130
--- /dev/null
+++ b/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
+}