This commit is an implementation of [RFC 503][rfc] which is a stabilization
story for the prelude. Most of the RFC was directly applied, removing reexports.
Some reexports are kept around, however:
* `range` remains until range syntax has landed to reduce churn.
* `Path` and `GenericPath` remain until path reform lands. This is done to
prevent many imports of `GenericPath` which will soon be removed.
* All `io` traits remain until I/O reform lands so imports can be rewritten all
at once to `std::io::prelude::*`.
This is a breaking change because many prelude reexports have been removed, and
the RFC can be consulted for the exact list of removed reexports, as well as to
find the locations of where to import them.
[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0503-prelude-stabilization.md
[breaking-change]
Closes#20068
This takes building `librustc/lib.rs` from using 696 MB to 588 (`rustc --no-trans`), and 1.99 GB to 1.87 (`rustc -O`). It also reduces `sty` down to 32 bytes on platforms with 64-bit pointers, at the expense of some more side-tables in `ctxt`. I'm sure there's more gains to be had from reducing the size of the side tables (e.g. by making the actual things they're storing smaller).
r? @nikomatsakis
This cuts memory use dramatically from the previous commit, and reduces
use overall. E.g. the memory usage of `rustc -O librustc/lib.rs` seems
to drop 100MB from 1.98GB to 1.88GB (on one run anyway).
This extends the nullable enum opt to traverse beyond just the first level to find possible fields to use as the discriminant. So now, it'll work through structs, tuples, and fixed sized arrays. This also introduces a new lang item, NonZero, that you can use to wrap raw pointers or integral types to indicate to rustc that the underlying value is known to never be 0/NULL. We then use this in Vec, Rc and Arc to have them also benefit from the nullable enum opt.
As per https://github.com/rust-lang/rfcs/pull/499 NonZero is not exposed via the `libstd` facade.
```
x86_64 Linux:
T Option<T> (Before) Option<T> (After)
----------------------------------------------------------------------------------
Vec<int> 24 32 24
String 24 32 24
Rc<int> 8 16 8
Arc<int> 8 16 8
[Box<int>, ..2] 16 24 16
(String, uint) 32 40 32
```
Fixes#19419.
Fixes#13194.
Fixes#9378.
Fixes#7576.
Using the current directory may not always be appropriate, for example in
the case where it will unnecessarily trigger a backup to be made.
The only risk with this change is that systems might not have a mktemp.
I am not aware of such a system, but have not tested on Windows. It is
working on a basic Ubuntu and OS X installation.
[breaking-change]
This and the other commit in this PR change the rules for shadowing and globs to be
stricter. There were previously bugs where some glob imports would not be checked
for shadowing. Those are now fixed and you may have to adjust your imports to use
fewer globs.
