From 860813dd9d63a2cf160801b59295ebb0d2063385 Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Fri, 8 May 2026 15:17:44 +0200
Subject: [PATCH] Require EIIs to be defined when we compile a rust dylib

The linker will complain about undefined symbols otherwise and on object
file formats with two level namespaces (Mach-O and PE/COFF) it is
required to know which dylib a symbol will be imported from when linking
a dylib or executable.
---
 compiler/rustc_passes/src/eii.rs     |  4 ++--
 tests/ui/eii/dylib_needs_impl.rs     |  7 +++++++
 tests/ui/eii/dylib_needs_impl.stderr | 10 ++++++++++
 3 files changed, 19 insertions(+), 2 deletions(-)
 create mode 100644 tests/ui/eii/dylib_needs_impl.rs
 create mode 100644 tests/ui/eii/dylib_needs_impl.stderr

diff --git a/compiler/rustc_passes/src/eii.rs b/compiler/rustc_passes/src/eii.rs
index f3e84665f21d..2916f5355e7b 100644
--- a/compiler/rustc_passes/src/eii.rs
+++ b/compiler/rustc_passes/src/eii.rs
@@ -18,8 +18,8 @@ enum CheckingMode {
 }
 
 fn get_checking_mode(tcx: TyCtxt<'_>) -> CheckingMode {
-    // if any of the crate types is not rlib or dylib, we must check for existence.
-    if tcx.crate_types().iter().any(|i| !matches!(i, CrateType::Rlib | CrateType::Dylib)) {
+    // if any of the crate types is not rlib, we must check for existence.
+    if tcx.crate_types().iter().any(|i| !matches!(i, CrateType::Rlib)) {
         CheckingMode::CheckExistence
     } else {
         CheckingMode::CheckDuplicates
diff --git a/tests/ui/eii/dylib_needs_impl.rs b/tests/ui/eii/dylib_needs_impl.rs
new file mode 100644
index 000000000000..bb1ec22b0488
--- /dev/null
+++ b/tests/ui/eii/dylib_needs_impl.rs
@@ -0,0 +1,7 @@
+//@ no-prefer-dynamic
+//@ needs-crate-type: dylib
+#![crate_type = "dylib"]
+#![feature(extern_item_impls)]
+
+#[eii(eii1)] //~ ERROR `#[eii1]` required, but not found
+fn decl1(x: u64);
diff --git a/tests/ui/eii/dylib_needs_impl.stderr b/tests/ui/eii/dylib_needs_impl.stderr
new file mode 100644
index 000000000000..60ef656c3017
--- /dev/null
+++ b/tests/ui/eii/dylib_needs_impl.stderr
@@ -0,0 +1,10 @@
+error: `#[eii1]` required, but not found
+  --> $DIR/dylib_needs_impl.rs:6:7
+   |
+LL | #[eii(eii1)]
+   |       ^^^^ expected because `#[eii1]` was declared here in crate `dylib_needs_impl`
+   |
+   = help: expected at least one implementation in crate `dylib_needs_impl` or any of its dependencies
+
+error: aborting due to 1 previous error
+