Make FnDef 1-ZST in LLVM debuginfo.

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -335,12 +335,20 @@ fn build_subroutine_type_di_node<'ll, 'tcx>(
 
     // This is actually a function pointer, so wrap it in pointer DI.
     let name = compute_debuginfo_type_name(cx.tcx, fn_ty, false);
+    let (size, align) = match fn_ty.kind() {
+        ty::FnDef(..) => (0, 1),
+        ty::FnPtr(..) => (
+            cx.tcx.data_layout.pointer_size.bits(),
+            cx.tcx.data_layout.pointer_align.abi.bits() as u32,
+        ),
+        _ => unreachable!(),
+    };
     let di_node = unsafe {
         llvm::LLVMRustDIBuilderCreatePointerType(
             DIB(cx),
             fn_di_node,
-            cx.tcx.data_layout.pointer_size.bits(),
-            cx.tcx.data_layout.pointer_align.abi.bits() as u32,
+            size,
+            align,
             0, // Ignore DWARF address space.
             name.as_ptr().cast(),
             name.len(),

tests/codegen/debug-fndef-size.rs

@@ -0,0 +1,18 @@
+// Verify that `i32::cmp` FnDef type is declared with size 0 and align 1 in LLVM debuginfo.
+// compile-flags: -O -g -Cno-prepopulate-passes
+// ignore-msvc the types are mangled differently
+
+use std::cmp::Ordering;
+
+fn foo<F: FnOnce(&i32, &i32) -> Ordering>(v1: i32, v2: i32, compare: F) -> Ordering {
+    compare(&v1, &v2)
+}
+
+pub fn main() {
+    foo(0, 1, i32::cmp);
+}
+
+// CHECK: %compare.dbg.spill = alloca {}, align 1
+// CHECK: call void @llvm.dbg.declare(metadata ptr %compare.dbg.spill, metadata ![[VAR:.*]], metadata !DIExpression()), !dbg !{{.*}}
+// CHECK: ![[TYPE:.*]] = !DIDerivedType(tag: DW_TAG_pointer_type, name: "fn(&i32, &i32) -> core::cmp::Ordering", baseType: !{{.*}}, align: 1, dwarfAddressSpace: {{.*}})
+// CHECK: ![[VAR]] = !DILocalVariable(name: "compare", scope: !{{.*}}, file: !{{.*}}, line: {{.*}}, type: ![[TYPE]], align: 1)