FreeBSD and using system limit instead for others.

Do not re-parse function signatures to suggest generics

This PR uses the existing resolution rib infrastructure to channel the correct span information to suggest generic parameters.  This allows to avoid re-parsing a function's source code.

Drive-by cleanup: this removes useless `FnItemRibKind` from late resolution ribs.  All the use cases are already covered by `ItemRibKind` and `AssocItemRibKind` which have more precise semantics.

Move Error trait into core

This PR moves the error trait from the standard library into a new unstable `error` module within the core library. The goal of this PR is to help unify error reporting across the std and no_std ecosystems, as well as open the door to integrating the error trait into the panic reporting system when reporting panics whose source is an errors (such as via `expect`).

This PR is a rewrite of rust-lang#90328 using new compiler features that have been added to support error in core.

InferCtxt tainted_by_errors_flag should be Option<ErrorGuaranteed>

Fixes rust-lang#100321.
Use Cell<Option<ErrorGuaranteed>> to guarantee that we emit an error when that flag is set.

… r=davidtwco

Migrate `rustc_plugin_impl` to `SessionDiagnostic`

Migration of the `rustc_plugin_impl` crate.
~Draft PR because it is blocked on rust-lang#100694 for `#[fatal(...)]` support~ (this has been merged, and I've changed over to `#[diag(...)]` now too), but I would also like to know if what I did with `LoadPluginError` is okay, because all it does is display the error message from `libloading` ([See conversation on zulip](https://rust-lang.zulipchat.com/#narrow/stream/147480-t-compiler.2Fwg-diagnostics/topic/.23100717.20diagnostic.20translation/near/294327843)). This crate is apparently for a deprecated feature which is used by servo, so I don't know how much this matters anyway.

…nexen

net listen backlog update, follow-up from rust-lang#97963.

FreeBSD and using system limit instead for others.

…fee1-dead

Fix rustc_parse_format precision & width spans

When a `precision`/`width` was `CountIsName - {:name$}` or `CountIs - {:10}` the `precision_span`/`width_span` was set to `None`

For `width` the name span in `CountIsName(_, name_span)` had its `.start` off by one

r? `@fee1-dead` / cc `@PrestonFrom` since this is similar to rust-lang#99987

…ylan-DPC

Fix typo in UnreachableProp

r? `@Dylan-DPC`