Do not forget to pass DWARF fragment information to LLVM.

compiler/rustc_codegen_ssa/src/mir/debuginfo.rs

@@ -445,9 +445,23 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             bx.store(place.llval, alloca.llval, alloca.align);
 
             // Point the debug info to `*alloca` for the current variable
-            bx.dbg_var_addr(dbg_var, dbg_loc, alloca.llval, Size::ZERO, &[Size::ZERO], None);
+            bx.dbg_var_addr(
+                dbg_var,
+                dbg_loc,
+                alloca.llval,
+                Size::ZERO,
+                &[Size::ZERO],
+                var.fragment,
+            );
         } else {
-            bx.dbg_var_addr(dbg_var, dbg_loc, base.llval, direct_offset, &indirect_offsets, None);
+            bx.dbg_var_addr(
+                dbg_var,
+                dbg_loc,
+                base.llval,
+                direct_offset,
+                &indirect_offsets,
+                var.fragment,
+            );
         }
     }
 
@@ -570,17 +584,22 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         }
 
                         let place = fragment.contents;
+                        let fragment = if fragment_layout.size == Size::ZERO {
+                            // Fragment is a ZST, so does not represent anything.
+                            continue;
+                        } else if fragment_layout.size == var_layout.size {
+                            // Fragment covers entire variable, so as far as
+                            // DWARF is concerned, it's not really a fragment.
+                            None
+                        } else {
+                            Some(fragment_start..fragment_start + fragment_layout.size)
+                        };
+
                         per_local[place.local].push(PerLocalVarDebugInfo {
                             name: var.name,
                             source_info: var.source_info,
                             dbg_var,
-                            fragment: if fragment_layout.size == var_layout.size {
-                                // Fragment covers entire variable, so as far as
-                                // DWARF is concerned, it's not really a fragment.
-                                None
-                            } else {
-                                Some(fragment_start..fragment_start + fragment_layout.size)
-                            },
+                            fragment,
                             projection: place.projection,
                         });
                     }

tests/codegen/sroa-fragment-debuginfo.rs

@@ -0,0 +1,46 @@
+// compile-flags: -g -Zmir-opt-level=0 -Zmir-enable-passes=+ScalarReplacementOfAggregates
+// compile-flags: -Cno-prepopulate-passes
+//
+// Tested offsets are only correct for x86_64.
+// only-x86_64
+
+#![crate_type = "lib"]
+
+pub struct ExtraSlice<'input> {
+    slice: &'input [u8],
+    extra: u32,
+}
+
+#[no_mangle]
+pub fn extra(s: &[u8]) {
+// CHECK: void @extra(
+// CHECK: %slice.dbg.spill1 = alloca i32,
+// CHECK: %slice.dbg.spill = alloca { ptr, i64 },
+// CHECK: %s.dbg.spill = alloca { ptr, i64 },
+// CHECK: call void @llvm.dbg.declare(metadata ptr %s.dbg.spill, metadata ![[S_EXTRA:.*]], metadata !DIExpression()),
+// CHECK: call void @llvm.dbg.declare(metadata ptr %slice.dbg.spill, metadata ![[SLICE_EXTRA:.*]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 128)),
+// CHECK: call void @llvm.dbg.declare(metadata ptr %slice.dbg.spill1, metadata ![[SLICE_EXTRA]], metadata !DIExpression(DW_OP_LLVM_fragment, 128, 32)),
+    let slice = ExtraSlice { slice: s, extra: s.len() as u32 };
+}
+
+struct Zst;
+
+pub struct ZstSlice<'input> {
+    slice: &'input [u8],
+    extra: Zst,
+}
+
+#[no_mangle]
+pub fn zst(s: &[u8]) {
+    // The field `extra` is a ZST. The fragment for the field `slice` encompasses the whole
+    // variable, so is not a fragment. In that case, the variable must have no fragment.
+
+// CHECK: void @zst(
+// CHECK-NOT: call void @llvm.dbg.declare(metadata ptr %slice.dbg.spill, metadata !{}, metadata !DIExpression(DW_OP_LLVM_fragment,
+// CHECK: call void @llvm.dbg.declare(metadata ptr %{{.*}}, metadata ![[SLICE_ZST:.*]], metadata !DIExpression()),
+// CHECK-NOT: call void @llvm.dbg.declare(metadata ptr %{{.*}}, metadata ![[SLICE_ZST]],
+    let slice = ZstSlice { slice: s, extra: Zst };
+}
+
+// CHECK: ![[S_EXTRA]] = !DILocalVariable(name: "s",
+// CHECK: ![[SLICE_EXTRA]] = !DILocalVariable(name: "slice",

tests/ui/debuginfo/sroa-fragment-debuginfo.rs

@@ -0,0 +1,31 @@
+// Verify that we do not trigger a LLVM assertion by creating zero-sized DWARF fragments.
+//
+// build-pass
+// compile-flags: -g -Zmir-opt-level=0 -Zmir-enable-passes=+ScalarReplacementOfAggregates
+// compile-flags: -Cno-prepopulate-passes
+
+#![crate_type = "lib"]
+
+pub struct ExtraSlice<'input> {
+    slice: &'input [u8],
+    extra: u32,
+}
+
+#[no_mangle]
+pub fn extra(s: &[u8]) {
+    let slice = ExtraSlice { slice: s, extra: s.len() as u32 };
+}
+
+struct Zst;
+
+pub struct ZstSlice<'input> {
+    slice: &'input [u8],
+    extra: Zst,
+}
+
+#[no_mangle]
+pub fn zst(s: &[u8]) {
+    // The field `extra` is a ZST. The fragment for the field `slice` encompasses the whole
+    // variable, so is not a fragment. In that case, the variable must have no fragment.
+    let slice = ZstSlice { slice: s, extra: Zst };
+}