Avoid building as root with `sudo make install`

When running `sudo make install`, we only want to run the actual install
as root, the building of the documentation and the distribution folder
should happen as the non-root user.

Related to #13728.
Add help diagnostic messages

This adds ‘help’ diagnostic messages to rustc. This is used for anything that
provides help to the user, particularly the `--explain` messages that were
previously integrated into the relevant error message.
Fix several issues with the struct and enum representability-checking logic.
Tweak copy and formatting of Tasks guide
bench: Improve the spectralnorm shootout benchmark

This improves the spectralnorm shootout benchmark through a few vectors after
looking at the leading C implementation:

* The simd-based f64x2 is now used to parallelize a few computations
* RWLock usage has been removed. A custom `parallel` function was added as a
  form of stack-based fork-join parallelism. I found that the contention on the
  locks was high as well as hindering other optimizations.

This does, however, introduce one `unsafe` block into the benchmarks, which
previously had none.

In terms of timings, the before and after numbers are:

```
$ time ./shootout-spectralnorm-before
./shootout-spectralnorm-before  2.07s user 0.71s system 324% cpu 0.857 total
$ time ./shootout-spectralnorm-before 5500
./shootout-spectralnorm-before 5500  11.88s user 1.13s system 459% cpu 2.830 total
$ time ./shootout-spectralnorm-after
./shootout-spectralnorm-after  0.58s user 0.01s system 280% cpu 0.210 tota
$ time ./shootout-spectralnorm-after 5500
./shootout-spectralnorm-after 5500  3.55s user 0.01s system 455% cpu 0.783 total
```
Improve the error message for missing else clauses in if expressions
Only consider impliciy unboxed closure impl if the obligation is
actually for `Fn`, `FnMut`, or `FnOnce`.

Fixes #18019
Change _rhs to _index.

Because var is provided as an index not a right hand side.
Remove unnecessary ::core::ops::

It shows error: failed to resolve. Maybe a missing `extern crate core`
anyway.
Change `from` and `to` to `_from` and `_to`

This to avoid warning: unused variable: `from`,
#[warn(unused_variable)] on by default.
improve shootout-reverse-complement.rs using unsafe code
test: make the sort failure-safety test unsafeless and more obvious.

Previously it had some uninituitive conditionals due to the interaction
with the Rand construction and Clone reinitialisation to create
sequential identifying numbers. This replaces all that with just
constructing the DropCounters with the appropriate identifiers.
librustc: Improve method autoderef/deref/index behavior more, and enable
`IndexMut` on mutable vectors.

This fixes a bug whereby the mutability fixups for method behavior were
not kicking in after autoderef failed to happen at any level. It also
adds support for `Index` to the fixer-upper.

Closes #12825.
rustc: Rename lints per RFC 344

RFC 344 proposes a set of naming conventions for lints. This commit
renames existing lints to follow the conventions.

Use the following sed script to bring your code up to date:

```
s/unnecessary_typecast/unused_typecasts/g
s/unsigned_negate/unsigned_negation/g
s/type_limits/unused_comparisons/g
s/type_overflow/overflowing_literals/g
s/ctypes/improper_ctypes/g
s/owned_heap_memory/box_pointers/g
s/unused_attribute/unused_attributes/g
s/path_statement/path_statements/g
s/unused_must_use/unused_must_use/g
s/unused_result/unused_results/g
s/non_uppercase_statics/non_upper_case_globals/g
s/unnecessary_parens/unused_parens/g
s/unnecessary_import_braces/unused_import_braces/g
s/unused_unsafe/unused_unsafe/g
s/unsafe_block/unsafe_blocks/g
s/unused_mut/unused_mut/g
s/unnecessary_allocation/unused_allocation/g
s/missing_doc/missing_docs/g
s/unused_imports/unused_imports/g
s/unused_extern_crate/unused_extern_crates/g
s/unnecessary_qualification/unused_qualifications/g
s/unrecognized_lint/unknown_lints/g
s/unused_variable/unused_variables/g
s/dead_assignment/unused_assignments/g
s/unknown_crate_type/unknown_crate_types/g
s/variant_size_difference/variant_size_differences/g
s/transmute_fat_ptr/fat_ptr_transmutes/g
```

Closes #16545
Closes #17932

Due to deprecation, this is a:

[breaking-change]
rustc: Add missing lint registration

The pluggable lint changes apparently dropped the fat pointer transmute
lint by accident.

This commit registers the lint.
rustc: Improve lint descriptions

Improves the description of `dead_code` and `unreachable_code` to
clarify the difference between them.
rustc: Add deprecation/renaming support for lints

Since a large number of lints are being renamed for RFC 344, this commit
adds some basic deprecation/renaming functionality to the pluggable lint
system. It allows a simple mapping of old to new names, and can warn
when old names are being used.

This change needs to be rolled out in stages. In this commit, the
deprecation warning is commented out, but the old name is forwarded to
the new one.

Once the commit lands and we have generated a new snapshot of the
compiler, we can add the deprecation warning and rename all uses of the
lints in the rust codebase.
Guide: develop the exposition of arrays, vectors, and slices.

The array is the fundamental concept; vectors are growable arrays, and
slices are views into either.  Show common array ops up front: length
and iteration.  Mention arrays are immutable by default.  Highlight
definite initialization and bounds-checking as safety features.  Show
that you only need a type suffix on one element of initializers.
Explain that vectors are a value-add library type over arrays, not a
fundamental type; show they have the same "interface." Motivate slices
as efficient views into arrays; explain you can slice vectors, Strings,
&str because they're backed by arrays.
auto merge of #18044 : aturon/rust/lint-conventions, r=pcwalton

[RFC 344](https://github.com/rust-lang/rfcs/pull/344) proposes a set of naming conventions for lints. This PR
renames existing lints to follow the conventions.

Use the following sed script to bring your code up to date:

```
s/unnecessary_typecast/unused_typecasts/g
s/unsigned_negate/unsigned_negation/g
s/type_limits/unused_comparisons/g
s/type_overflow/overflowing_literals/g
s/ctypes/improper_ctypes/g
s/owned_heap_memory/box_pointers/g
s/unused_attribute/unused_attributes/g
s/path_statement/path_statements/g
s/unused_must_use/unused_must_use/g
s/unused_result/unused_results/g
s/non_uppercase_statics/non_upper_case_globals/g
s/unnecessary_parens/unused_parens/g
s/unnecessary_import_braces/unused_import_braces/g
s/unused_unsafe/unused_unsafe/g
s/unsafe_block/unsafe_blocks/g
s/unused_mut/unused_mut/g
s/unnecessary_allocation/unused_allocation/g
s/missing_doc/missing_docs/g
s/unused_imports/unused_imports/g
s/unused_extern_crate/unused_extern_crates/g
s/unnecessary_qualification/unused_qualifications/g
s/unrecognized_lint/unknown_lints/g
s/unused_variable/unused_variables/g
s/dead_assignment/unused_assignments/g
s/unknown_crate_type/unknown_crate_types/g
s/variant_size_difference/variant_size_differences/g
s/transmute_fat_ptr/fat_ptr_transmutes/g
```

Since a large number of lints are being renamed for RFC 344, this PR
adds some basic deprecation/renaming functionality to the pluggable lint
system. It allows a simple mapping of old to new names, and can warn
when old names are being used.

This change needs to be rolled out in stages. In this PR, the
deprecation warning is commented out, but the old name is forwarded to
the new one.

Once the PR lands and we have generated a new snapshot of the
compiler, we can add the deprecation warning and rename all uses of the
lints in the rust codebase. I will file a PR to do so.

Closes #16545
Closes #17932

r? @pcwalton
Fix the description of raw byte string literals