This commit disallows acquiring a function pointer to functions tagged as
`#[rustc_args_required_const]`. This is intended to be used as future-proofing
for the stdsimd crate to avoid taking a function pointer to any intrinsic which
has a hard requirement that one of the arguments is a constant value.

This turns out to not be backwards compatible.

Namely, the mutable borrows also carries a flag indicating whether
they should support two-phase borrows.

This allows us to thread down, from the point of the borrow's
introduction, whether the particular adjustment that created it is one
that yields two-phase mutable borrows.

not skipping any wfchecks on default impls

    a `default impl` need not include all items from the trait
    a `default impl` alone does not mean that a type implements the trait

Previously, when the type of a method receiver could not be determined,
the error message would, potentially confusingly, highlight the span of
the entire method call.

Resolves #36598, resolves #42234.

Move `check_casts` and `resolve_generator_interiors` to before fallback.

Rename `apply_fallback_if_possible` to `fallback_if_possible`.

Refactor `select_all_obligations_or_error`.

Put all fallback in `apply_fallback_if_possible`.

It had only one caller.

This refactoring tries to make numeric fallback easier to reason about.
Instead of applying all fallbacks at an arbitrary point in the middle
of inference, we apply the fallback only when necessary and only for
the variable that requires it, which for numeric fallback turns out to
be just casts.

The only visible consequence seems to be some error messages where
instead of getting `i32` we get `{integer}` because we are less eager
about fallback.

The bigger goal is to make it easier to integrate user fallbacks into
inference, if we ever figure that out.

the `or_else` part was dead code.

Create the concept of an `StructuredDiagnostic` that is self-contained
with enough knowledge of all variables to create a `DiagnosticBuilder`,
including different possible versions (one line output and expanded
explanations).

Adds support for immovable generators. Move checking of invalid borrows across suspension points to borrowck. Fixes #44197, #45259 and #45093.

It is also intended for use with unions.

Use of builtin indexing is now only checked after types are fully
resolved.
Types are not correctly checked in case of autoderefs.

This is the first part of the RFC 1937 that supports new
`Termination` trait in the rust `main` function.

When a parse error occurs on a block, the parser will recover and create
a block with the statements collected until that point. Now a flag
stating that a recovery has been performed in this block is propagated
so that the type checker knows that the type of the block (which will be
identified as `()`) shouldn't be checked against the expectation to
reduce the amount of irrelevant diagnostic errors shown to the user.

The Generics now contain one Vec of an enum for the generic parameters,
rather than two separate Vec's for lifetime and type parameters.

Additionally, places that previously used Vec<LifetimeDef> now use
Vec<GenericParam> instead.

We now add the suitable `impl Trait` constraints.

No functional change.