Tracking issue for release notes of #149868: rustc: Stop passing `--allow-undefined` on wasm targets

[rustbot]

This issue tracks the release notes text for #149868.

cc @alexcrichton, @Mark-Simulacrum -- original issue/PR authors and assignees for drafting text

See the forge.rust-lang.org chapter about release notes for an overview of how the release team makes use of these tracking issues.

Release notes text

This section should be edited to specify the correct category(s) for the change, with succinct description(s) of what changed. Some things worth considering:

• Does this need an additional compat notes section?
• Was this a libs stabilization that should have additional headers to list new APIs under Stabilized APIs and Const Stabilized APIs?

# Compatibility Notes
- [rustc: Stop passing `--allow-undefined` on wasm targets](https://github.com/rust-lang/rust/pull/149868)

Tip

Use the previous releases for inspiration on how to write the release notes text and which categories to pick.

Release blog section

If this change is notable enough for inclusion in the blog post then this section should be edited to contain a draft for the blog post. Otherwise leave it empty.

WebAssembly targets no longer pass `--allow-undefined` to the linker which means that undefined symbols when linking are now a linker error instead of being converted to WebAssembly imports from the `"env"` module. This change prevents modules from linking unless all linking-related symbols are defined to catch bugs earlier and prevent accidental issues with symbol naming or similar.

Undefined linking-related symbols are often indicative of build-time related bugs or misconfiguration. If, however, the old behavior is intended then it can be re-enabled with `RUSTFLAGS=-Clink-arg=--allow-undefined` or by editing the source code and using `#[link(wasm_import_module = "env")]` on the block defining the symbol.

Note

If a blog post section is required the release-blog-post label should be added (@rustbot label +release-blog-post) to this issue as otherwise it may be missed by the release team.