Remove the quad_precision_float feature gate

The f128 type has very little support in the compiler and the feature is
basically unusable today. Supporting half-baked features in the compiler can be
detrimental to the long-term development of the compiler, and hence this feature
is being removed.
Optimize out exhortations about being careful.

Yes, it is important to be careful, but repeated emphasis about it is probably
not helpful — it starts to sound like you came for a tutorial but found a
finger-wagging lecture.

Even after I removed a few of these comments, there are still several left in
the text. That's probably fine! A couple of mentions of how this is dangerous
and you ought to be careful may be a good reminder to the reader.

After making the edits, I reflowed the paragraphs that I had touched, using
emacs's "M-x fill-paragraph", with fill-column equal to 70.
auto merge of #15160 : alexcrichton/rust/remove-f128, r=brson

The f128 type has very little support in the compiler and the feature is
basically unusable today. Supporting half-baked features in the compiler can be
detrimental to the long-term development of the compiler, and hence this feature
is being removed.
compiletest: Remove superfluous to_string calls
librustc: Remove cross borrowing from mutable `Box`es to `&mut`.

This will break code like:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(a);

Change it to:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(&mut *a);

RFC 33; issue #10504.

[breaking-change]
auto merge of #15165 : zookoatleastauthoritycom/rust/14148-Optimize-out-exhortations-about-being-careful-2, r=huonw

Yes, it is important to be careful, but repeated emphasis about it is probably
not helpful — it starts to sound like you came for a tutorial but found a
finger-wagging lecture.

Even after I removed a few of these comments, there are still several left in
the text. That's probably fine! A couple of mentions of how this is dangerous
and you ought to be careful may be a good reminder to the reader.

After making the edits, I reflowed the paragraphs that I had touched, using
emacs's "M-x fill-paragraph", with fill-column equal to 70.
librustc: Don't try to perform the magical
vector-reference-to-unsafe-pointer-to-element cast if the type to be
casted to is not fully specified.

This is a conservative change to fix the user-visible symptoms of the
issue. A more flexible treatment would delay cast checks to after
function typechecking.

This can break code that did:

    let x: *u8 = &([0, 0]) as *_;

Change this code to:

    let x: *u8 = &([0, 0]) as *u8;

Closes #14893.

[breaking-change]
auto merge of #15164 : pcwalton/rust/vector-cast-weirdness, r=alexcrichton

vector-reference-to-unsafe-pointer-to-element cast if the type to be
casted to is not fully specified.

This is a conservative change to fix the user-visible symptoms of the
issue. A more flexible treatment would delay cast checks to after
function typechecking.

This can break code that did:

    let x: *u8 = &([0, 0]) as *_;

Change this code to:

    let x: *u8 = &([0, 0]) as *u8;

Closes #14893.

[breaking-change]

r? @alexcrichton
Fix a typo in fmt.rs
Fix wrong type name: IoError<()> -> IoResult<()>
auto merge of #15174 : alco/rust/patch-1, r=alexcrichton
Make an example more clear with sample code.

Fixes #11113.
Guide: install instructions
fix feedback from @huonw
thanks @sfackler
auto merge of #15121 : steveklabnik/rust/matching_docs, r=alexcrichton

Fixes #11113.
auto merge of #15173 : Sawyer47/rust/de-to-string, r=alexcrichton
Guide: Hello, world!

Let's write our first Rust program!
auto merge of #15171 : pcwalton/rust/remove-cross-borrowing, r=brson

This will break code like:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(a);

Change it to:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(&mut *a);

RFC 33; issue #10504.

[breaking-change]

r? @brson
Add extra ignore directives.