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Merged
rust-bors merged 1 commit into from
Apr 4, 2026
+8 −10
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rustc: Stop passing --allow-undefined on wasm targets #149868

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10 changes: 0 additions & 10 deletions compiler/rustc_target/src/spec/base/wasm.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,16 +28,6 @@ pub(crate) fn options() -> TargetOptions {
// stack overflow will be guaranteed to trap as it underflows instead of
// corrupting static data.
concat!($prefix, "--stack-first"),
// FIXME we probably shouldn't pass this but instead pass an explicit list
// of symbols we'll allow to be undefined. We don't currently have a
// mechanism of knowing, however, which symbols are intended to be imported
// from the environment and which are intended to be imported from other
// objects linked elsewhere. This is a coarse approximation but is sure to
// hide some bugs and frustrate someone at some point, so we should ideally
// work towards a world where we can explicitly list symbols that are
// supposed to be imported and have all other symbols generate errors if
// they remain undefined.
concat!($prefix, "--allow-undefined"),
// LLD only implements C++-like demangling, which doesn't match our own
// mangling scheme. Tell LLD to not demangle anything and leave it up to
// us to demangle these symbols later. Currently rustc does not perform
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2 changes: 2 additions & 0 deletions tests/run-make/wasm-stringify-ints-small/foo.rs
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@@ -1,12 +1,14 @@
#![crate_type = "cdylib"]

#[link(wasm_import_module = "the-world")]
extern "C" {
fn observe(ptr: *const u8, len: usize);
}

macro_rules! s {
( $( $f:ident -> $t:ty );* $(;)* ) => {
$(
#[link(wasm_import_module = "the-world")]
extern "C" {
fn $f() -> $t;
}
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5 changes: 5 additions & 0 deletions tests/run-make/wasm-unexpected-features/foo.rs
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Expand Up @@ -14,6 +14,11 @@ unsafe extern "Rust" {
fn __rust_alloc_error_handler(size: usize, align: usize) -> !;
}

#[rustc_std_internal_symbol]
pub unsafe fn __rdl_alloc_error_handler(_size: usize, _align: usize) -> ! {
loop {}
}

#[used]
static mut BUF: [u8; 1024] = [0; 1024];

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1 change: 1 addition & 0 deletions tests/ui/linking/executable-no-mangle-strip.rs
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@@ -1,5 +1,6 @@
//@ run-pass
//@ ignore-windows-gnu: only statics marked with used can be GC-ed on windows-gnu
//@ ignore-wasm: wasm, for better or worse, exports all #[no_mangle]
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@mati865 mati865 Apr 2, 2026 •
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Pardon me if this was already discussed, but I couldn't find it in this thread.

I don't see how this comment relates to removal of --allow-undefined. I'm not familiar with WASM linkers (yet), but I don't see how forbidding undefined symbols should affect exports.

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@alexcrichton alexcrichton Apr 3, 2026

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This test, with this PR, fails on wasm with:

running 1 tests

[ui] tests/ui/linking/executable-no-mangle-strip.rs ... F


failures:

---- [ui] tests/ui/linking/executable-no-mangle-strip.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/home/alex/code/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc" "/home/alex/code/rust/tests/ui/linking/executable-no-mangle-strip.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/home/alex/.cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/home/alex/code/rust/vendor" "--sysroot" "/home/alex/code/rust/build/x86_64-unknown-linux-gnu/stage1" "--target=wasm32-wasip1" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/home/alex/code/rust/build/x86_64-unknown-linux-gnu/test/ui/linking/executable-no-mangle-strip/a.wasm" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/home/alex/code/rust/build/wasm32-wasip1/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error: linking with `rust-lld` failed: exit status: 1
   |
   = note:  "rust-lld" "-flavor" "wasm" "--export" "__main_void" "--export" "function_marked_with_no_mangle" "-z" "stack-size=1048576" "--stack-first" "--no-demangle" "<sysroot>/lib/rustlib/wasm32-wasip1/lib/self-contained/crt1-command.o" "<2 object files omitted>" "<sysroot>/lib/rustlib/wasm32-wasip1/lib/{libpanic_abort-*,libstd-*,libwasi-*,libcfg_if-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,liblibc-*}.rlib" "-l" "c" "<sysroot>/lib/rustlib/wasm32-wasip1/lib/{librustc_std_workspace_core-*,liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "-L" "/home/alex/code/rust/build/wasm32-wasip1/native/rust-test-helpers" "-L" "<sysroot>/lib/rustlib/wasm32-wasip1/lib/self-contained" "-o" "/home/alex/code/rust/build/x86_64-unknown-linux-gnu/test/ui/linking/executable-no-mangle-strip/a.wasm" "--gc-sections" "-O3" "--strip-debug"
   = note: some arguments are omitted. use `--verbose` to show all linker arguments
   = note: rust-lld: error: /home/alex/code/rust/build/x86_64-unknown-linux-gnu/test/ui/linking/executable-no-mangle-strip/a.executable_no_mangle_strip.4e0277abecc73a6b-cgu.0.rcgu.o: undefined symbol: THIS_SYMBOL_SHOULD_BE_UNDEFINED


error: aborting due to 1 previous error
------------------------------------------

---- [ui] tests/ui/linking/executable-no-mangle-strip.rs stdout end ----

failures:
    [ui] tests/ui/linking/executable-no-mangle-strip.rs

Notably function_marked_with_no_mangle is exported from the binary (via --export function_marked_with_no_mangle) which causes the undefined symbol here to be an error.

My understanding for the cause of this is the limit_rdylib_exports target spec option. That's false on wasm but true on most other targets. I forget the reason why it's different, however, and would have to dig in to understand better. That's at least why it's here though.

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@mati865 mati865 Apr 5, 2026

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Fair enough, thanks for the explanation!


// Regression test for <https://github.com/rust-lang/rust/issues/139744>.
// Functions in the binary marked with no_mangle should be GC-ed if they
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