rustc assumes that regular `extern "Rust"` functions unwind only if the
`unwind` panic runtime is linked. `throw` was annotated as such, but
unwound unconditionally. This could cause UB when a crate built with `-C
panic=abort` called `throw` from `core` built with `-C panic=unwind`,
since no terminator was added to handle the panic arising from calling an
allegedly non-unwinding `extern "Rust"` function.
rustc was taught to recognize this condition since
https://github.com/rust-lang/rust/pull/144225 and prevented such
linkage, but this caused regressions in
https://github.com/rust-lang/rust/issues/148246, since this meant that
Emscripten projects could not be built with `-C panic=abort` without
recompiling std.
The most straightforward solution would be to move `throw` into the
`panic_unwind` crate, so that it's only compiled if the panic runtime is
guaranteed to be `unwind`, but this is messy due to our architecture.
Instead, move it into `unwind::wasm`, which is only compiled for
bare-metal targets that default to `panic = "abort"`, rendering the
issue moot.
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