Linker invocation for wasm32-unknown-unknown incompatibly changed between 1.66.1 and 1.67.1

[pnkfelix]

I ran this sequence of commands via rustup to install 1.66.1 and 1.67.1, as well as the target support for wasm32-unknown-unknown for each, and then tried to compile a cdylib for wasm32-unknown-unknown while also overriding the linker.

% rustup update 1.66.1
[...]
% rustup update 1.67.1
[...]
% rustup target add --toolchain 1.66.1 wasm32-unknown-unknown
[...]
% rustup target add --toolchain 1.67.1 wasm32-unknown-unknown
[...]
% echo > /tmp/hello.rs
% rustc +1.66.1 /tmp/hello.rs --crate-type cdylib --target wasm32-unknown-unknown -C linker=clang && echo success
success
% rustc +1.67.1 /tmp/hello.rs --crate-type cdylib --target wasm32-unknown-unknown -C linker=clang
error: linking with `clang` failed: exit status: 1
  |
  = note: "clang" "-Wl,-z" "-Wl,stack-size=1048576" "-Wl,--stack-first" "-Wl,--allow-undefined" "-Wl,--fatal-warnings" "-Wl,--no-demangle" "--target=wasm32-unknown-unknown" "-Wl,--no-entry" "hello.hello.4592d75c-cgu.0.rcgu.o" "hello.395fat3noigyayzs.rcgu.o" "-L" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libstd-4cc67ecdd38ec816.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libpanic_abort-d7aed05895fd3acd.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libdlmalloc-d614b689e44911c3.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/librustc_demangle-b67bdb405beac3e8.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libstd_detect-d35bb83dee339ae2.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libhashbrown-a91c75972ec9987b.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libminiz_oxide-cebf340576826f4d.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libadler-1dfefb8109e16570.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/librustc_std_workspace_alloc-fb56ca5e4fa7e885.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libunwind-7758a353f6e6c303.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libcfg_if-9fa968ba9e6d4f22.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/liblibc-6769bef7a67a1976.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/liballoc-5ef00dd564498f26.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/librustc_std_workspace_core-a025c7699298bef7.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libcore-028eb657858f49ce.rlib" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib/libcompiler_builtins-f9be42f11058bb31.rlib" "-L" "/home/pnkfelix/.rustup/toolchains/1.67.1-x86_64-unknown-linux-gnu/lib/rustlib/wasm32-unknown-unknown/lib" "-o" "hello.wasm" "-Wl,--gc-sections" "-shared" "-nodefaultlibs"
  = note: clang: warning: argument unused during compilation: '-shared' [-Wunused-command-line-argument]
          wasm-ld-14: error: cannot open crt1.o: No such file or directory
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

I expected to see this happen: a command sequence that worked under 1.66.1 to continue working under 1.67.1

Instead, this happened: A linker failure that is somewhat inscrutable to a layperson.

---

Note: There is a relatively easy workaround available. Under 1.66.1 when passing -C linker=clang, the linker invocation also included the argument -nostartfiles. For some reason, that argument has gone away under 1.67.1 with this invocation sequence, but one can re-add it manually by passing -C link-arg=-nostartfiles into rustc.

The questions I have are:

1. Why did this change between 1.66 and 1.67? Was it an expected change in behavior? (Why is this change good? Its not noted in the release notes that I saw.)
2. Should we bring back the old behavior, at least in a limited set of cases?
3. Is there a different invocation our users should be using when they override the linker, e.g. perhaps they should be using -C linker-flavor=xxx to tell rustc that it needs to pass along parameters compatible with using clang as a linker? (But if that is the case, I do not see clang listed as one of the available values for linker-flavor)
4. Should we start investing in post-processing the linker-errors from rustc itself -- and still spit out those linker errors (because in the general case, the user is going to need to see them in order to make sense of what to do in reaction), but potentially also provide hints as to things to look into (e.g. in this case, maybe the crt1.o in the error could be used as a hint to look at the -nostartfiles option as something to pass to the linker).

[cuviper]

Under 1.66.1 when passing -C linker=clang, the linker invocation also included the argument -nostartfiles. For some reason, that argument has gone away under 1.67.1 with this invocation sequence, but one can re-add it manually by passing -C link-arg=-nostartfiles into rustc.

I think that was #104137, which removed link_self_contained: LinkSelfContainedDefault::True from the common wasm_base.rs. When self_contained was true, fn add_order_independent_options called cmd.no_crt_objects(), which adds -nostartfiles.

[pnkfelix]

Indeed, cargo-bisect-rustc reports that commit e69336e is to blame here, which is the rollup that included #104137.

So that seems like strong evidence for what the relevant change was here.

My next question is: What should we do about this?

Should we readd link_self_contained: LinkSelfContainedDefault::True to the wasm_base.rs?

[bjorn3]

Explicitly adding -nostartfiles to the linker args makes more sense I think.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[pnkfelix]

Explicitly adding -nostartfiles to the linker args makes more sense I think.

Sorry, @bjorn3 , I don't know how to interpret your suggestion.

In particular, who should be responsible for adding -nostartfiles?

• The person who is overriding the link driver via the -C linker parameter? (In which case, should we have this in the release note for 1.67?)
• The compiler itself?

[bjorn3]

The compiler itself. The target specification has a list of linker arguments to pass. As I understand it we used to pass this before as side effect of using link-self-contained, but now it needs to be made explicit in the target spec.

[pnkfelix]

The compiler itself. The target specification has a list of linker arguments to pass. As I understand it we used to pass this before as side effect of using link-self-contained, but now it needs to be made explicit in the target spec.

Okay, great. That sounds like you're in favor then of @cuviper 's idea of having an explicit flag in the target spec, as they mentioned on zulip over here, specifically a no_crt_objects flag.

(On the other hand, on that same zulip topic thread, @sunfishcode advised not adding a new flag, and "just" reverting the change to wasm_base.rs so that it returns to using link_self_contained: LinkSelfContainedDefault::True for now...)

[cuviper]

Targets also have the ability to add explicit link args:

rust/compiler/rustc_target/src/spec/mod.rs

Lines 1772 to 1784 in 39f2657

	fn link_args(flavor: LinkerFlavor, args: &[&'static str]) -> LinkArgs {
	let mut link_args = LinkArgs::new();
	add_link_args(&mut link_args, flavor, args);
	link_args
	}
	fn add_pre_link_args(&mut self, flavor: LinkerFlavor, args: &[&'static str]) {
	add_link_args(&mut self.pre_link_args, flavor, args);
	}
	fn add_post_link_args(&mut self, flavor: LinkerFlavor, args: &[&'static str]) {
	add_link_args(&mut self.post_link_args, flavor, args);
	}

But given that we already have the Linker::no_crt_objects abstraction, I thought we might as well use a flag for that.