Add rustup.sh to the repo

Just to have it somewhere to point to. Updating it will not
automatically update the one on static.rust-lang.org.
improve clone_from_slice performance
add intmax
Add assume intrinsic

Adds an `assume` intrinsic that gets translated to llvm.assume. It is
used on a boolean expression and allows the optimizer to assume that
the expression is true.

This implements #18051.
Improve documentation of assume intrinsic.
Parser: Fix spans of explicit self arg idents
Fix crazy-pants match in Option docs
Use strlen in CString::len
Guide: articulate the advantages of ownership over garbage collection.

Explain the primary disadvantage of garbage collection is runtime
overhead and unpredictable pauses.  Elucidate where the name "race
condition" comes from.  Emphasize that Rust can guarantee your code is
free of race conditions and other memory errors, with no runtime
overhead.
auto merge of #18201 : Gankro/rust/option-what, r=alexcrichton

:whale:
Prune unused parameters.
Implement "method matching". Similar to trait matching, but with the twist that we
have to take the transformed self type into account, and allow for coercion between
the receiver and the actual type declared on the impl.
Permit DST types to unify like other types.

Also:
1. stop eagerly coercing from `[T, ..n]` to `[T]` only do so if requested.
2. permit error to be interact more freely.

Fixes #17178.
Generalize the replace-late-bound-regions function to operate
over anything that is foldable, not just fn signatures.
rustdoc: hide private traits in strip-private pass

Fix #16563
Change method lookup infrastructure to use the trait methods. Instead
of tracking individual candidates per impl, we just track one
candidate for the extension trait itself, and let the trait resolution
handle walking the individual impls and so forth. Also change the
interface to report back a richer notion of error.
Correct case where the old version of method lookup was incorrectly matching,
as far as I can tell.
The new method lookup mechanism typechecks calls against the method type declared in the trait, not in the impl. In some cases that results in tighter rules, and in some cases looser. Correct for that.
Test where the old infrastructure failed to detect the (applicable) impl of `FnMut` for
some reason.
Various minor cases where errors are reported in slightly different ways.