`compiler_builtins` is currently injected as `extern crate
compiler_builtins as _`. This has made gating via diagnostics difficult
because it appears in the crate graph as a non-private dependency, and
there isn't an easy way to differentiate between the injected AST and
user-specified `extern crate compiler_builtins`.
Resolve this by injecting `compiler_builtins` during postprocessing
rather than early in the AST. Most of the time this isn't even needed
because it shows up in `std` or `core`'s crate graph, but injection is
still needed to ensure `#![no_core]` works correctly.
A similar change was attempted at [1] but this encountered errors
building `proc_macro` and `rustc-std-workspace-std`. Similar failures
showed up while working on this patch, which were traced back to
`compiler_builtins` showing up in the graph twice (once via dependency
and once via injection). This is resolved by not injecting if a
`#![compiler_builtins]` crate already exists.
[1]: https://github.com/rust-lang/rust/pull/113634
When writing a no_std binary, you'll be greeted with nonsensical errors
mentioning lang items like eh_personality and start. That's pretty bad
because it makes you think that you need to define them somewhere! But
oh no, now you're getting the `internal_features` lint telling you that
you shouldn't use them! But you need a no_std binary! What now?
No problem! Writing a no_std binary is super easy. Just use panic=abort
and supply your own platform specific entrypoint symbol (like `main`)
and you're good to go. Would be nice if the compiler told you that,
right?
This makes it so that it does do that.
