which requires a lot of unnecessary work to evaluate them further down the line.

This flag opts out of the min-const-fn checks entirely, which is usually
not what we want. The few cases where the flag is still necessary have
been annotated.

The only two uses of the associated methods are in librustc_mir and
librustdoc. Please tell me if there is a better choice.

- remove syntax::{help!, span_help!, span_note!}
- remove unused syntax::{struct_span_fatal, struct_span_err_or_warn!, span_err_or_warn!}
- lintify check_for_bindings_named_same_as_variants + conflicting_repr_hints
- inline syntax::{struct_span_warn!, diagnostic_used!}
- stringify_error_code! -> error_code! & use it more.
- find_plugin_registrar: de-fatalize an error
- de-fatalize metadata errors
- move type_error_struct! to rustc_typeck
- struct_span_err! -> rustc_errors

Fixes https://github.com/rust-lang/rust/issues/65721

FCP: https://github.com/rust-lang/rust/issues/65721#issuecomment-569118119

This reverts commit 15c30ddd.

This optimization depends on inlining for the identity
conversions introduced by the lowering of the `?`.
To take advantage of `SimplifyArmIdentity`, `-Z mir-opt-level=2`
is required because that triggers the inlining MIR optimization.

Tracking issue: https://github.com/rust-lang/rust/issues/61129

This iterator can be hot, and chained iterators are slow. The second
half of the chain is almost always empty, so this commit changes the
code to avoid the chained iteration.

This change reduces instruction counts for the `wg-grammar` benchmark by
up to 1.5%.

Remove some random unnecessary lint `allow`s