Pick the max DWARF version when LTO'ing modules with different versions

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -97,7 +97,11 @@ impl<'ll, 'tcx> CodegenUnitDebugContext<'ll, 'tcx> {
                 // Android has the same issue (#22398)
                 llvm::add_module_flag_u32(
                     self.llmod,
-                    llvm::ModuleFlagMergeBehavior::Warning,
+                    // In the case where multiple CGUs with different dwarf version
+                    // values are being merged together, such as with cross-crate
+                    // LTO, then we want to use the highest version of dwarf
+                    // we can. This matches Clang's behavior as well.
+                    llvm::ModuleFlagMergeBehavior::Max,
                     "Dwarf Version",
                     sess.dwarf_version(),
                 );

tests/assembly/auxiliary/dwarf-mixed-versions-lto-aux.rs

@@ -0,0 +1,5 @@
+//@ compile-flags: -g --crate-type=rlib -Zdwarf-version=4
+
+pub fn check_is_even(number: &u64) -> bool {
+    number % 2 == 0
+}

tests/assembly/dwarf-mixed-versions-lto.rs

@@ -0,0 +1,19 @@
+// This test ensures that if LTO occurs between crates with different DWARF versions, we
+// will choose the highest DWARF version for the final binary. This matches Clang's behavior.
+
+//@ only-linux
+//@ aux-build:dwarf-mixed-versions-lto-aux.rs
+//@ compile-flags: -C lto -g -Zdwarf-version=5
+//@ assembly-output: emit-asm
+//@ no-prefer-dynamic
+
+extern crate dwarf_mixed_versions_lto_aux;
+
+fn main() {
+    dwarf_mixed_versions_lto_aux::check_is_even(&0);
+}
+
+// CHECK: .section .debug_info
+// CHECK-NOT: {{\.(short|hword)}} 2
+// CHECK-NOT: {{\.(short|hword)}} 4
+// CHECK: {{\.(short|hword)}} 5

tests/ui/lto/auxiliary/dwarf-mixed-versions-lto-aux.rs

@@ -0,0 +1,5 @@
+//@ compile-flags: -g --crate-type=rlib -Zdwarf-version=4
+
+pub fn say_hi() {
+    println!("hello there")
+}

tests/ui/lto/dwarf-mixed-versions-lto.rs

@@ -0,0 +1,15 @@
+// This test verifies that we do not produce a warning when performing LTO on a
+// crate graph that contains a mix of different DWARF version settings. This
+// matches Clang's behavior.
+
+//@ ignore-msvc Platform must use DWARF
+//@ aux-build:dwarf-mixed-versions-lto-aux.rs
+//@ compile-flags: -C lto -g -Zdwarf-version=5
+//@ no-prefer-dynamic
+//@ build-pass
+
+extern crate dwarf_mixed_versions_lto_aux;
+
+fn main() {
+    dwarf_mixed_versions_lto_aux::say_hi();
+}