Because they traverse data structures and build up strings, which is
wasted effort if those strings aren't printed.

The patch also removes some now-unnecessary log_enabled! tests at call
sites.

Fixes rust-lang#49841

…ad-safe InternedString

…dreavus

Add "the Rustc book"

This PR introduces a new book into the documentation, "The rustc book". We already have books for Cargo, and for Rustdoc, rustc should have some too. This book is focused on *users* of rustc, and provides a nice place to write documentation for users.

I haven't put content here, but plan on scaffolding it out very soon, and wanted this PR open for a few discussions first. One of those is "what exactly should said TOC be?" I plan on having a proposed one up tomorrow, but figured I'd let people know to start thinking about it now.

The big one is that we also will want to put https://github.com/rust-lang-nursery/rustc-guide in-tree as well, and the naming is... tough. I'm proposing:

* doc.rust-lang.org/rustc is "The Rustc book", to mirror the other tools' books.
* doc.rust-lang.org/rustc-contribution is "The Rustc contribution guide", and contains that book

@nikomatsakis et al, any thoughts on this? I'm not attached to it in particular, but had to put something together to get this discussion going. I think mirroring the other tools is a good idea for this work, but am not sure where exactly that leaves yours.

Fixes rust-docs/team#11

…er-to-2018-04-24, r=alexcrichton

Bump bootstrap compiler to 2018-04-24

r? @alexcrichton

Fix ICE with erroneous `impl Trait` in a trait impl

Fixes rust-lang#49841.

…=QuietMisdreavus

Add setting to go to item if there is only one result

Fixes rust-lang#50216.

r? @QuietMisdreavus

…Misdreavus

Add more doc aliases

r? @QuietMisdreavus

…Simulacrum

Make dump_{alloc,allocs,local}() no-ops when tracing is disabled.

Because they traverse data structures and build up strings, which is
wasted effort if those strings aren't printed.

The patch also removes some now-unnecessary log_enabled! tests at call
sites.

This is a big win for the Debug and Opt runs of coercions, tuple-stress, html5ever, and encoding.
```
coercions-opt
        avg: -7.8%      min: -14.8%     max: 0.1%
coercions
        avg: -8.0%      min: -12.8%     max: 0.1%
tuple-stress
        avg: -7.2%      min: -10.8%     max: -0.7%
tuple-stress-opt
        avg: -6.9%      min: -10.7%     max: 0.6%
html5ever
        avg: -4.6%      min: -7.3%      max: -0.3%
encoding
        avg: -2.4%      min: -4.5%      max: 0.1%
html5ever-opt
        avg: -2.7%      min: -4.2%      max: -0.2%
encoding-opt
        avg: -1.4%      min: -2.4%      max: 0.0%
```

…woerister

Rename InternedString to LocalInternedString and introduce a new thread-safe InternedString

This is an allocation-free alternative to rust-lang#46972.