`no_late_bound_regions().unwrap()`, which allows us to write code that
doesn't necessarily *fail* when there are higher-ranked trait bounds.

This is super black magic internals at the moment, but having it
somewhere semi-public seems good. The current versions weren't being
rendered, and they'll be useful for some people.

Fixes #21281

upgrade the inference based on expected type so that it is able to
infer the fn kind in isolation even if the full signature is not
available (and we could perhaps do better still in some cases, such as
extracting just the types of the arguments but not the return value).

sed -i 's/in range(\([^,]*\), *\([^()]*\))/in \1\.\.\2/g' **/*.rs

Fix the issue-20055-box-trait.rs test to actually test `Box<Trait>`.

Fix #21695.

This fixes a subtle issue where temporaries were being allocated (but
not necessarily initialized) to the (parent) terminating scope of a
match expression; in particular, the code to zero out the temporary
emitted by `datum.store_to` is only attached to the particular
match-arm for that temporary, but when going down other arms of the
match expression, the temporary may falsely appear to have been
initialized, depending on what the stack held at that location, and
thus may have its destructor erroneously run at the end of the
terminating scope.

Test cases to appear in a follow-up commit.

Fix #20055

So far, the source location an LLVM instruction was linked to was controlled by
`debuginfo::set_source_location()` and `debuginfo::clear_source_location()`.
This interface mimicked how LLVM's `IRBuilder` handles debug location
assignment. While this interface has some theoretical performance benefits, it
also makes things terribly unstable: One sets some quasi-global state and then
hopes that it is still correct when a given instruction is emitted---an
assumption that has been proven to not hold a bit too often.

This patch requires the debug source location to be passed to the actual
instruction emitting function. This makes source location assignment explicit
and will prevent future changes to `trans` from accidentally breaking things in
the majority of cases.

This patch does not yet implement the new principle for all instruction kinds
but the stepping experience should have improved significantly nonetheless
already.

There are two places left where we used to only know the byte
size of/offset into an array and had to cast to i8 and back to get the
right addresses. But by now, we always know the sizes in terms of the
number of elements in the array. In fact we have to add an extra Mul
instruction so we can use the weird cast-to-u8 code. So we should really
just embrace our new knowledge and use simple GEPs to do the address
calculations.

Additionally, the pointer calculations in bind_subslice_pat don't handle
zero-sized types correctly, producing slices that point outside the
array that is being matched against. Using GEP fixes that as well.

Fixes #3729

fmt::Show is for debugging, and can and should be implemented for
all public types. This trait is used with `{:?}` syntax. There still
exists #[derive(Show)].

fmt::String is for types that faithfully be represented as a String.
Because of this, there is no way to derive fmt::String, all
implementations must be purposeful. It is used by the default format
syntax, `{}`.

This will break most instances of `{}`, since that now requires the type
to impl fmt::String. In most cases, replacing `{}` with `{:?}` is the
correct fix. Types that were being printed specifically for users should
receive a fmt::String implementation to fix this.

Part of #20013

[breaking-change]

Fix ICE that @steveklabnik encountered in rust-ice. The problems turned out to be that were being very loose with bound regions in trans (we were basically just ignoring and flattening binders). Since binders are significant to subtyping and hence to trait selection, this can cause a lot of problems. So this patch makes us treat them more strictly -- for example, we propagate binders, and avoid skipping past the `Binder` by writing `foo.0`.

Fixes #20644.