Rollup of 19 pull requests

- Successful merges: #40317, #40516, #40524, #40606, #40683, #40751, #40778, #40813, #40818, #40819, #40824, #40828, #40832, #40833, #40837, #40849, #40852, #40853, #40865
- Failed merges:

appveyor: Downgrade MinGW to 6.2.0

It looks like the 6.3.0 MinGW comes with a gdb which has issues (#40184) that an
attempted workaround (#40777) does not actually fix (#40835). The original
motivation for upgradin MinGW was to fix build flakiness (#40546) due to newer
builds not exhibiting the same bug, so let's hope that 6.2.0 isn't too far back
in time and still contains the fix we need.

Closes #40835

Fix broken Markdown and bad links in the error index

This makes sure RFC links point to the RFC text not the pull request.

r? @steveklabnik

rustbuild: Fix compiler docs again

The docs need to be built with the rustbuild feature so the correct
stability attributes (rustc_private) get applied.

r? @alexcrichton

bugfix: finalize resolutions of macros in trait positions

Fixes #40845.
r? @nrc

change string references in asciiext

Fix typo in char::to_uppercase documentation

Original documentation appears to have been copied from `char::to_lowercase` in a manner that made it imply that `char::to_uppercase` actually mapped unicode characters to their **lowercase** equivalent.

libcore: fix compilation on 16bit target (MSP430).

Since PR #40601 has been merged, libcore no longer compiles on MSP430.
The reason is this code in `break_patterns`:
```rust
 let mut random = len;
 random ^= random << 13;
 random ^= random >> 17;
 random ^= random << 5;
 random &= modulus - 1;
```
It assumes that `len` is at least a 32 bit integer.
As a workaround replace `break_patterns` with an empty function for 16bit targets.

cc @stjepang
cc @alexcrichton

rustdoc to accept `#` at the start of a markdown file #40560

This may be a bit odd if `#` and `%` lines are mixed up, but that's not something I've found while doing my search and replace.

FromStr implementation example

Referencing #29375. Added example implementation of FromStr trait to API Documentation

Link ParseBoolError to from_str method of bool

Referencing task in #29375. Sorry for not opening another branch on my fork for this. Was working on this early this morning and forgot to branch off master

Don't stutter in operator trait descriptions

Fixes first item on #29365.

r? @steveklabnik

macros: fix ICE on some nested macro definitions

Fixes #40770.
r? @nrc

Update cargo submodule

I'm not really sure what we want the cadence here to be. We'll at the very least
update the Cargo submodule right before all releases, but otherwise I figured we
could just do it whenever needed or otherwise weekly (or something like that).

In any case, I don't have a super strong particular reason to do this, it's just
been a week or so since the release!

save-analysis: allow clients to get data directly without writing to a file.

on-demand-ify `custom_coerce_unsized_kind` and `inherent-impls`

This "on-demand" task both checks for errors and computes the custom unsized kind, if any. This task is only defined on impls of `CoerceUnsized`; invoking it on any other kind of impl results in a bug. This is just to avoid having an `Option`, could easily be changed.

r? @EddyB

Improve `InternedString` usability

std: Don't cache stdio handles on Windows

This alters the stdio code on Windows to always call `GetStdHandle` whenever the
stdio read/write functions are called as this allows us to track changes to the
value over time (such as if a process calls `SetStdHandle` while it's running).

Closes #40490

Update libc to 0.2.21

Update to include android aarch64 and x86 improvements.

It looks like the 6.3.0 MinGW comes with a gdb which has issues (#40184) that an
attempted workaround (#40777) does not actually fix (#40835). The original
motivation for upgradin MinGW was to fix build flakiness (#40546) due to newer
builds not exhibiting the same bug, so let's hope that 6.2.0 isn't too far back
in time and still contains the fix we need.

Closes #40835

This makes sure RFC links point to the RFC text not the pull request.

The docs need to be built with the rustbuild feature so the correct
stability attributes (rustc_private) get applied.

keep the AST node-id when lowering ExprKind::Range

When the Range expression is the root of a constant, its node-id is
used for the def-id of the body, so it has to be preserved in the AST ->
HIR lowering.

Fixes #40749.

r? @EddyB
beta-nominating because regression

Select 3 random points instead of just 1.
Also the code now compiles on 16bit architectures.

Rollup of 7 pull requests

- Successful merges: #40642, #40734, #40740, #40771, #40807, #40820, #40821
- Failed merges:

Allow `use` macro imports to shadow global macros

Terminology:
 - global scope: builtin macros, macros from the prelude, `#[macro_use]`, or `#![plugin(..)]`.
 - legacy scope: crate-local `macro_rules!`.
 - modern scope: `use` macro imports, `macro` (once implemented).

Today, the legacy scope can shadow the global scope (modulo RFC 1560 expanded shadowing restrictions). However, the modern scope cannot shadow or be shadowed by either the global or legacy scopes, leading to ambiguity errors.

This PR allows the modern scope to shadow the global scope (subject to some restrictions).
More specifically, a name in the global scope is as shadowable as a glob import in the module `self`. In other words, we imagine a special, implicit glob import in each module item:
```rust
mod foo {
    #[lexical_only] // Not accessible via `foo::<name>`, like pre-RFC 1560 `use` imports.
    #[shadowable_by_legacy_scope] // for back-compat
    use <global_macros>::*;
}
```

r? @nrc

Remove internal liblog

This commit deletes the internal liblog in favor of the implementation that
lives on crates.io. Similarly it's also setting a convention for adding crates
to the compiler. The main restriction right now is that we want compiler
implementation details to be unreachable from normal Rust code (e.g. requires a
feature), and by default everything in the sysroot is reachable via `extern
crate`.

The proposal here is to require that crates pulled in have these lines in their
`src/lib.rs`:

    #![cfg_attr(rustbuild, feature(staged_api, rustc_private))]
    #![cfg_attr(rustbuild, unstable(feature = "rustc_private", issue = "27812"))]

This'll mean that by default they're not using these attributes but when
compiled as part of the compiler they do a few things:

* Mark themselves as entirely unstable via the `staged_api` feature and the
  `#![unstable]` attribute.
* Allow usage of other unstable crates via `feature(rustc_private)` which is
  required if the crate relies on any other crates to compile (other than std).

travis: Update sccache again

Looks like the last version was built with mio 0.6.5 which now has known bugs
against it. This build includes mio 0.6.6

try to fix the build on emscripten

The "upstream" emscripten tar.gz now extracts to `emsdk-portable` instead of `emsdk_portable`, breaking our CI. It might be better to vendor a specific version of emscripten instead of using the latest, but I could not find a good way of doing that.

r? @alexcrichton