Missing `Reflect` on a child leads to linker errors on Windows

[kleinesfilmroellchen]

Bevy version

0.14

What you did

I used #[derive(Reflect)] on several components that were missing a Reflect implementation on a child type. While this is of course not possible, I would expect a nicer error message than some linker errors that had me searching for about half an hour.

What went wrong

The build fails with linker errors like this one:

error: linking with `link.exe` failed: exit code: 1120
<long and sensitive linker invocation line omitted>

  = note: lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_108_$LT$impl$u20$bevy_reflect..type_registry..GetTypeRegistration$u20$for$u20$cmp..model..nav..NavComponent$GT$21get_type_registration17haeef134c9a08e10bE".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_108_$LT$impl$u20$bevy_reflect..type_registry..GetTypeRegistration$u20$for$u20$cmp..model..nav..NavComponent$GT$26register_type_dependencies17hd5b05f4b2184b1feE".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_90_$LT$impl$u20$bevy_reflect..reflect..Reflect$u20$for$u20$cmp..model..nav..NavComponent$GT$18reflect_partial_eq17h7193ed9efffbe4edE".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_90_$LT$impl$u20$bevy_reflect..type_info..Typed$u20$for$u20$cmp..model..nav..NavComponent$GT$9type_info17h1383952cb944e417E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_94_$LT$impl$u20$bevy_reflect..struct_trait..Struct$u20$for$u20$cmp..model..nav..NavComponent$GT$11iter_fields17h374bad68a829126dE".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_94_$LT$impl$u20$bevy_reflect..struct_trait..Struct$u20$for$u20$cmp..model..nav..NavComponent$GT$12field_at_mut17ha33376bd44e99291E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_94_$LT$impl$u20$bevy_reflect..struct_trait..Struct$u20$for$u20$cmp..model..nav..NavComponent$GT$13clone_dynamic17h76e402f00bd92585E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_94_$LT$impl$u20$bevy_reflect..struct_trait..Struct$u20$for$u20$cmp..model..nav..NavComponent$GT$5field17hcd641fb8c9a404e5E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_94_$LT$impl$u20$bevy_reflect..struct_trait..Struct$u20$for$u20$cmp..model..nav..NavComponent$GT$7name_at17h885325772a3bb243E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_94_$LT$impl$u20$bevy_reflect..struct_trait..Struct$u20$for$u20$cmp..model..nav..NavComponent$GT$8field_at17h8b4d6a71a905aafaE".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_94_$LT$impl$u20$bevy_reflect..struct_trait..Struct$u20$for$u20$cmp..model..nav..NavComponent$GT$9field_len17hac564c17a54c82edE".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_94_$LT$impl$u20$bevy_reflect..struct_trait..Struct$u20$for$u20$cmp..model..nav..NavComponent$GT$9field_mut17h014f952078d75a88E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp5model3nav1_99_$LT$impl$u20$bevy_reflect..from_reflect..FromReflect$u20$for$u20$cmp..model..nav..NavComponent$GT$12from_reflect17h0317fca958b30622E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp8graphics1_81_$LT$impl$u20$bevy_reflect..reflect..Reflect$u20$for$u20$cmp..graphics..Sides$GT$18reflect_partial_eq17h895ddd3c776b4042E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp8graphics1_81_$LT$impl$u20$bevy_reflect..type_info..Typed$u20$for$u20$cmp..graphics..Sides$GT$9type_info17h2d4f02562378d1b3E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp8graphics1_90_$LT$impl$u20$bevy_reflect..from_reflect..FromReflect$u20$for$u20$cmp..graphics..Sides$GT$12from_reflect17h8373a3f9dd382b4fE".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp8graphics1_90_$LT$impl$u20$bevy_reflect..tuple_struct..TupleStruct$u20$for$u20$cmp..graphics..Sides$GT$13clone_dynamic17h3353cfaa8143ce63E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp8graphics1_90_$LT$impl$u20$bevy_reflect..tuple_struct..TupleStruct$u20$for$u20$cmp..graphics..Sides$GT$5field17h13206a0d6892ed5eE".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp8graphics1_90_$LT$impl$u20$bevy_reflect..tuple_struct..TupleStruct$u20$for$u20$cmp..graphics..Sides$GT$9field_mut17h14a60af13883f93cE".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp8graphics1_99_$LT$impl$u20$bevy_reflect..type_registry..GetTypeRegistration$u20$for$u20$cmp..graphics..Sides$GT$21get_type_registration17hdec5b11f70576bd1E".␍
          lib.def : error LNK2001: Nicht aufgelöstes externes Symbol "_ZN3cmp8graphics1_99_$LT$impl$u20$bevy_reflect..type_registry..GetTypeRegistration$u20$for$u20$cmp..graphics..Sides$GT$26register_type_dependencies17ha68b80319c65fe54E".␍

Additional information

In my case, Sides is the type that doesn't have Reflect, and NavComponent is a struct with a Sides field, but NavComponent itself has a Reflect derive.

[MrGVSV]

Could you provide a minimum reproducible example of the code that causes this, as well as steps to reproduce the error?

And just to confirm, is this being built on Windows? lol I didn't fully read the title apparently

[kleinesfilmroellchen]

Currently working on that MWE with slow progress. I'm fairly confident this is related to my linking setup: The crate that contains the Reflect-ed structs is dynamically linked into the executable. (If I put the structs into the executable's own crate, I get a normal error)

[kleinesfilmroellchen]

MWE coming in a bit, but it seems to be related to the following:

• #![feature(trivial_bounds)] (which the Rust compiler recommends you to use when it detects that Reflect trait bounds are not satisfied)
• (possibly relevant) rustc 1.82.0-nightly (612a33f20 2024-07-29)
• the crate with both the feature and the problematic reflected structs is a lib crate that is dynamically linked into a bin crate

So this might not be worth investigating right now, given that it depends on unstable Rust features to happen.

[kleinesfilmroellchen]

MWE:

#### Workspace Cargo.toml
[workspace]
resolver = "2"               # Important! wgpu/Bevy needs this!
members = ["cmp", "client"]
default-members = ["client"]

[workspace.package]
version = "0.1.0"
edition = "2021"

[profile.dev]
opt-level = 1

[profile.dev.package."*"]
opt-level = 3

#### cmp/Cargo.toml
[package]
name = "cmp"
edition.workspace = true
version.workspace = true

[lib]
crate-type = ["dylib"]

[dependencies]
bevy = { version = "0.14", features = ["dynamic_linking"] }

#### client/Cargo.toml
[package]
name = "client"
version.workspace = true
edition.workspace = true

[dependencies]
cmp = { path = "../cmp" }

//// client/src/main.rs (minimal bin stub)
fn main() {}

//// cmp/src/lib.rs
#![feature(trivial_bounds)]

use bevy::prelude::*;

#[derive(Reflect)]
pub struct MweReflected {
	pub field: MweUnreflected,
}

pub struct MweUnreflected;

[soqb]

this looks like a plain mistake on Rust's behalf. !#[feature(trivial_bounds)] is an incorrect suggestion here and that's what's causing the linker errors.

[kleinesfilmroellchen]

@soqb this means the problem is twofold:

• trivial_bounds has a bug where it may cause linker errors by not emitting certain symbols (or something similar)
• the compiler's suggestion to use the trivial_bounds feature is imprecise

I'll report this on rust-lang/rust then.

[kleinesfilmroellchen]

Moved to rust-lang/rust#128682