Show a better error when using --test with #[proc_macro_derive]

Fixes https://github.com/rust-lang/rust/issues/37480

Currently using `--test` with a crate that contains a `#[proc_macro_derive]` attribute causes an error. This PR doesn't attempt to fix the issue itself, or determine what tesing of a proc_macro_derive crate should be - just to provide a better error message.

rustbuild: allow dynamically linking LLVM

The makefiles and `mklldeps.py` called `llvm-config --shared-mode` to
find out if LLVM defaulted to shared or static libraries, and just went
with that.  But under rustbuild, `librustc_llvm/build.rs` was assuming
that LLVM should be static, and even forcing `--link-static` for 3.9+.

Now that build script also uses `--shared-mode` to learn the default,
which should work better for pre-3.9 configured for dynamic linking, as
it wasn't possible back then to choose differently via `llvm-config`.

Further, the configure script now has a new `--enable-llvm-link-shared`
option, which allows one to manually override `--link-shared` on 3.9+
instead of forcing static.

Update: There are now four static/shared scenarios that can happen
for the supported LLVM versions:

- 3.9+: By default use `llvm-config --link-static`
- 3.9+ and `--enable-llvm-link-shared`: Use `--link-shared` instead.
- 3.8: Use `llvm-config --shared-mode` and go with its answer.
- 3.7: Just assume static, maintaining the status quo.

fix `extern "aapcs" fn`

to actually use the AAPCS calling convention

closes #37810

This is technically a [breaking-change] because it changes the ABI of
`extern "aapcs"` functions that (a) involve `f32`/`f64` arguments/return
values and (b) are compiled for arm-eabihf targets from
"aapcs-vfp" (wrong) to "aapcs" (correct).

Appendix:

What these ABIs mean?

- In the "aapcs-vfp" ABI or "hard float" calling convention: Floating
point values are passed/returned through FPU registers (s0, s1, d0, etc.)

- Whereas, in the "aapcs" ABI or "soft float" calling convention:
Floating point values are passed/returned through general purpose
registers (r0, r1, etc.)

Mixing these ABIs can cause problems if the caller assumes that the
routine is using one of these ABIs but it's actually using the other
one.

---

r? @alexcrichton We are going this `extern "aapcs" fn` thing to implement some intrinsics (floatundidf) for the eabihf targets in order to comply with LLVM's calling convention of intrinsics.

Oh, and the value of the enum came from [here](http://llvm.org/docs/doxygen/html/namespacellvm_1_1CallingConv.html).

cc @TimNN @parched

instantiate closures on demand

this should fix compilation with `-C codegen-units=4` - tested locally
with `RUSTFLAGS='-C codegen-units=4' ../x.py test`

r? @michaelwoerister

fix travis: update Cargo.lock

ICH: Handle MacroDef HIR instances.

As of recently, `hir::MacroDef` instances are exported in crate metadata, which means we also store their ICH when doing incremental compilation. Even though exported macro definitions should not (yet) interact with incremental compilation, the ICH is also used for the general purpose crate hash, where macros should be included.

This PR implements ICH computation for `MacroDef`. In theory, the ICH of these MacroDefs is less stable than that of other HIR items, since I opted to just call the compiler-generated `Hash::hash()` for `Token::Interpolated` variants. `Token::Interpolated` contains AST data structures and it would have been a lot of effort to expand ICH computation to the AST too. Since quasi-quoting is rarely used *and* it would only make a difference if incremental compilation was extended to macros, the simpler implementation seemed like a good idea.

This fixes the problem reported in https://github.com/rust-lang/rust/issues/37756. The test still fails because of broken codegen-unit support though.

r? @nikomatsakis

Don't build the lexer verifier during tidy

Tidy is not the right place to do this. Tidy is for running lints.
We should instead be running the lexer/grammar tests as part of the test
suite.

This may fix nightly breakage, but I don't understand why.

https://buildbot.rust-lang.org/builders/nightly-dist-rustc-linux/builds/715/steps/distcheck/logs/stdio

r? @alexcrichton

cc @dns2utf8

Tidy is not the right place to do this. Tidy is for running lints.
We should instead be running the lexer/grammar tests as part of the test
suite.

This may fix nightly breakage, but I don't know why.

save-analysis: handle << and >> operators inside [] in types

Fixes #37700

Improvements to the #[should_panic] feature

Add more error checking for the `#[should_panic]` attribute, and print the expected panic string when it does not match.

Fixes https://github.com/rust-lang/rust/issues/29000

Eg:
```running 3 tests
test test2 ... ok
test test1 ... FAILED
: Panic did not include expected string 'foo'
test test3 ... FAILED

failures:

---- test1 stdout ----
	thread 'test1' panicked at 'bar', test.rs:7
note: Run with `RUST_BACKTRACE=1` for a backtrace.

---- test3 stdout ----
	thread 'test3' panicked at 'bar', test.rs:18

```

rustbuild: Allow configuration of python interpreter

Add a configuration key to `config.toml`, read it from `./configure`, and add
auto-detection if none of those were specified.

Closes #35760

this should fix compilation with `-C codegen-units=4` - tested locally
with `RUSTFLAGS='-C codegen-units=4' ../x.py test`

rustdoc: add cli argument `--playground-url`

Add a new cli argument `--playground-url` for rustdoc:

`rustdoc lib.rs --playground-url="https://play.rust-lang.org/"`

`rustdoc book.md --playground-url="https://play.rust-lang.org/"`

This makes it possible for tools like https://docs.rs to generate crate docs that can submit samples code to run at https://play.rust-lang.org, even if the crate's author *forgot* putting `#![doc(html_playground_url = "https://play.rust-lang.org/")]` to crate root. By the way, I'd like to say, many crate authors are not aware of existing of `#![doc(html_playground_url = "https://play.rust-lang.org/")]`.

`--playground-url` may be reset by `--markdown-playground-url` or `#![doc(html_playground_url=...)]`, so it's backward compatible.

@alexcrichton since you implemented playground-url related features.

There are now four static/shared scenarios that can happen for the
supported LLVM versions:

- 3.9+: By default use `llvm-config --link-static`
- 3.9+ and `--enable-llvm-link-shared`: Use `--link-shared` instead.
- 3.8: Use `llvm-config --shared-mode` and go with its answer.
- 3.7: Just assume static, maintaining the status quo.

Fix bug in proc_macro dependency loading

Fixes #37839.
r? @alexcrichton

Separate impl items from the parent impl

This change separates impl item bodies out of the impl itself. This gives incremental more resolution. In so doing, it refactors how the visitors work, and cleans up a bit of the collect/check logic (mostly by moving things out of collect that didn't really belong there, because they were just checking conditions).

However, this is not as effective as I expected, for a kind of frustrating reason. In particular, when invoking `foo.bar()` you still wind up with dependencies on private items. The problem is that the method resolution code scans that list for methods with the name `bar` -- and this winds up touching *all* the methods, even private ones.

I can imagine two obvious ways to fix this:

- separating fn bodies from fn sigs (#35078, currently being pursued by @flodiebold)
- a more aggressive model of incremental that @michaelwoerister has been advocating, in which we hash the intermediate results (e.g., the outputs of collect) so that we can see that the intermediate result hasn't changed, even if a particular impl item has changed.

So all in all I'm not quite sure whether to land this or not. =) It still seems like it has to be a win in some cases, but not with the test cases we have just now. I can try to gin up some test cases, but I'm not sure if they will be totally realistic. On the other hand, some of the early refactorings to the visitor trait seem worthwhile to me regardless.

cc #36349 -- well, this is basically a fix for that issue, I guess

r? @michaelwoerister

NB: Based atop of @EddyB's PR https://github.com/rust-lang/rust/pull/37402; don't land until that lands.

Fixes #37700

Improved error reporting when target sysroot is missing.

Attempts to resolve #37131.
This is my first pull request on rust, so I would greatly appreciate any feedback you have on this.

Thanks!

Rollup of 8 pull requests

- Successful merges: #37752, #37757, #37759, #37766, #37772, #37799, #37806, #37821
- Failed merges: #37442

This sidesteps problems with long paths because the canonical path
includes the "magic long path prefix" on Windows.

(Unrelated to this PR series)

Also simplify some of the `ty::AssociatedItem` representation,
in particular by folding `has_value` into `hir::Defaultness`