Spurious "dependency loop detected" and "missing struct field" errors

[dweiller]

Zig Version

0.12.0-dev.494+a8d2ed806

Steps to Reproduce and Observed Behavior

Clone https://github.com/dweiller/zimalloc and run

git checkout d577c4a3
zig build

~I will try to make a smaller repro case and bisect the Zig commit introducing it when I get time (and llvm17 finishes compiling).~ A minimal repro case is below in https://github.com/ziglang/zig/issues/17255#issuecomment-1763884168.

Here is the full error message:

Build Summary: 1/4 steps succeeded; 1 failed (disable with --summary none)
test transitive failure
└─ run test transitive failure
   └─ zig test Debug native 3 errors
src/Segment.zig:9:5: error: dependency loop detected
pub const Ptr = *align(segment_alignment) @This();
~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/zimalloc.zig:50:32: error: missing struct field: thread_heaps
    var gpa = Allocator(config){};
              ~~~~~~~~~~~~~~~~~^~
src/zimalloc.zig:50:32: note: missing struct field: thread_heaps_lock
src/allocator.zig:8:12: note: struct 'allocator.Allocator(.{.thread_data_prealloc = 128, .thread_safe = true, .safety_checks = false})' declared here
    return struct {
           ^~~~~~
src/allocator.zig:10:79: error: missing struct field: prealloc_segment
        thread_heaps: std.SegmentedList(Heap, config.thread_data_prealloc) = .{},
                                                                             ~^~
src/allocator.zig:10:79: note: missing struct field: dynamic_segments
src/allocator.zig:10:79: note: missing struct field: len
/path/to/zig/zig-linux-x86_64-0.12.0-dev.494+a8d2ed806/lib/std/segmented_list.zig:79:12: note: struct 'segmented_list.SegmentedList(Heap,128)' declared here
    return struct {
           ^~~~~~

These errors are not encountered when compiling with 0.12.0-dev.464+a63a1c5cb (the commit that merged llvm17, though I can't find the tarball for this to test locally now, but my CI worked using it), so it seems the issue was introduced between that commit and when #17172 got merged.

The dependency loop issue shouldn't be the same issue as other dependency issue in the issue tracker (e.g. #16932, #16419, #14353) which are all about function pointers and affect earlier compiler versions than 0.12.0-dev.464+a63a1c5cb.

Expected Behavior

The code should compile as far as I can tell - the dependency loop is (according to the compile error) caused by a decl that is creates a type alias like pub const Ptr = *align(alignment) @This(); and the missing struct fields all have default values.

[Vexu]

a8d2ed806 merged #17172 which is likely to be the cause.

[dweiller]

I haven't managed to produce a minimal repro case yet, but have found a patch to workaround the issue:

--- a/src/Heap.zig
+++ b/src/Heap.zig
@@ -1,6 +1,6 @@
 thread_id: std.Thread.Id,
 pages: [size_class_count]Page.List,
-segments: ?Segment.Ptr,
+segments: ?*align(constants.segment_alignment) Segment,
 huge_allocations: HugeAllocTable,

Note that Segment.Ptr is defined in Segment.zig like this:

const pub const Ptr = *align(alignment) @This();
const alignment = constants.segment_alignment;

I have also tested the changes in #17490 as the described changes to structs seemed possibly related, but unfortunately that PR doesn't seem to affect this issue (as of commit 3a8932445 in the PR).

[dweiller]

The error seems to be related to the use of the align property on the pointers. Instead of the above workaround, the compiler does not error with this patch:

--- a/src/Segment.zig
+++ b/src/Segment.zig
@@ -6,8 +6,8 @@ heap: *Heap,
 next: ?Ptr,
 prev: ?Ptr,

-pub const Ptr = *align(segment_alignment) @This();
-pub const ConstPtr = *align(segment_alignment) const @This();
+pub const Ptr = *@This();
+pub const ConstPtr = *const @This();

 pub const PageSize = union(enum) {
     small,
@@ -92,7 +92,7 @@ fn allocateSegment() ?*align(segment_alignment) [segment_size]u8 {
 }

 fn deallocateSegment(self: Ptr) void {
-    const ptr: *align(segment_alignment) [segment_size]u8 = @ptrCast(self);
+    const ptr: *align(segment_alignment) [segment_size]u8 = @ptrCast(@alignCast(self));
     huge_alignment.deallocate(ptr);
 }

With the added @alignCast, the compiler errors with the align(segment_alignment) property on Ptr and ConstPtr and does not error if the align(segment_alignment)s are removed.

[dweiller]

I have managed to make a minimal repro case:

const Slab = struct {
    pub const Ptr = *align(1 << 16) Slab;

    pub const List = struct {
        head: ?Ptr = null,
    };

    next: Ptr,
};

const MeshingPool = struct {
    partial_slabs: Slab.List = .{},
};

test {
    _ = MeshingPool{};
}

If you run zig test on the above the compiler reports:

repro.zig:2:9: error: dependency loop detected
    pub const Ptr = *align(1 << 16) Slab;
    ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I have 6 different 1-line diffs that allow the above case to compile successfully:

@@ -1,5 +1,5 @@
 const Slab = struct {
-    pub const Ptr = *align(1 << 16) Slab;
+    pub const Ptr = *Slab;

     pub const List = struct {
         head: ?Ptr = null,

@@ -2,7 +2,6 @@
     pub const Ptr = *align(1 << 16) Slab;

     pub const List = struct {
-        head: ?Ptr = null,
     };

@@ -2,7 +2,7 @@
     pub const Ptr = *align(1 << 16) Slab;

     pub const List = struct {
-        head: ?Ptr = null,
+        head: ?*align(1 << 16) Slab = null,
     };

     next: Ptr,

@@ -5,7 +5,6 @@
         head: ?Ptr = null,
     };

-    next: Ptr,
 };

@@ -5,7 +5,7 @@
         head: ?Ptr = null,
     };

-    next: Ptr,
+    next: *align(1 << 16) Slab,
 };

 const MeshingPool = struct {

@@ -13,5 +13,5 @@
 };

 test {
-    _ = MeshingPool{};
+    _ = MeshingPool;
 }

(I wasn't sure if the it's surprising that the last diff changes whether it compiles or not).