This commit knocks out a longstanding FIXME in rustbuild which should correctly
recompile stage0 compiletest and such whenever libstd itself changes. The
solution implemented here was to implement a notion of "order only" dependencies
and then add a new dependency stage for clearing out the tools dir, using
order-only deps to ensure that it happens correctly.

The dependency drawing for tools is a bit wonky now but I think this'll get the
job done.

Closes #39396

Improve the LLVM IR we generate for trivial functions, especially #[naked] ones.

These two small changes fix edef1c/libfringe#68:
* Don't emit ZST allocas, such as when returning `()`
* Don't emit a branch from LLVM's entry block to MIR's `START_BLOCK` unless needed
  * That is, if a loop branches back to it, although I'm not sure that's even valid MIR

travis: Enable rust-analysis package for more targets

This commit enables the `rust-analysis` package to be produced for all targets
that are part of the `dist-*` suite of docker images on Travis. Currently
these packages are showing up with `available = false` in the
`channel-rust-nightly.toml` manifest where we'd prefer to have them show up for
all targets.

Unfortunately rustup isn't handling the `available = false` section well right
now, so this should also inadvertently fix the nightly regression.

Add a resource-reusing method to `ToOwned`

`ToOwned::to_owned` generalizes `Clone::clone`, but `ToOwned` doesn't have an equivalent to `Clone::clone_from`.  This PR adds such a method as `clone_into` under a new unstable feature `toowned_clone_into`.

Analogous to `clone_from`, this has the obvious default implementation in terms of `to_owned`.  I've updated the `libcollections` impls: for `T:Clone` it uses `clone_from`, for `[T]` I moved the code from `Vec::clone_from` and implemented that in terms of this, and for `str` it's a predictable implementation in terms of `[u8]`.

Used it in `Cow::clone_from` to reuse resources when both are `Cow::Owned`, and added a test that `Cow<str>` thus keeps capacity in `clone_from` in that situation.

The obvious question: is this the right place for the method?
- It's here so it lives next to `to_owned`, making the default implementation reasonable, and avoiding another trait.  But allowing method syntax forces a name like `clone_into`, rather than something more consistent like `owned_from`.
- Another trait would allow `owned_from` and could support multiple owning types per borrow type.  But it'd be another single-method trait that generalizes `Clone`, and I don't know how to give it a default impl in terms of `ToOwned::to_owned`, since a blanket would mean overlapping impls problems.

I did it this way as it's simpler and many of the `Borrow`s/`AsRef`s don't make sense with `owned_from` anyway (`[T;1]:Borrow<[T]>`, `Arc<T>:Borrow<T>`, `String:AsRef<OsStr>`...).  I'd be happy to re-do it the other way, though, if someone has a good solution for the default handling.

(I can also update with `CStr`, `OsStr`, and `Path` once a direction is decided.)

This commit enables the `rust-analysis` package to be produced for all targets
that are part of the `dist-*` suite of docker images on Travis. Currently
these packages are showing up with `available = false` in the
`channel-rust-nightly.toml` manifest where we'd prefer to have them show up for
all targets.

Unfortunately rustup isn't handling the `available = false` section well right
now, so this should also inadvertently fix the nightly regression.

Handle subtyping in inference through obligations

We currently store subtyping relations in the `TypeVariables` structure as a kind of special case. This branch uses normal obligations to propagate subtyping, thus converting our inference variables into normal fallback. It also does a few other things:

- Removes the (unstable, outdated) support for custom type inference fallback.
    - It's not clear how we want this to work, but we know that we don't want it to work the way it currently does.
    - The existing support was also just getting in my way.
- Fixes #30225, which was caused by the trait caching code pretending type variables were normal unification variables, when indeed they were not (but now are).

There is one fishy part of these changes: when computing the LUB/GLB of a "bivariant" type parameter, I currently return the `a` value. Bivariant type parameters are only allowed in a very particular situation, where the type parameter is only used as an associated type output, like this:

```rust
pub struct Foo<A, B>
    where A: Fn() -> B
{
    data: A
}
```

In principle, if one had `T=Foo<A, &'a u32>` and `U=Foo<A, &'b u32>` and (e.g.) `A: for<'a> Fn() -> &'a u32`, then I think that computing the LUB of `T` and `U` might do the wrong thing. Probably the right behavior is just to create a fresh type variable. However, that particular example would not compile (because the where-clause is illegal; `'a` does not appear in any input type). I was not able to make an example that *would* compile and demonstrate this shortcoming, and handling the LUB/GLB was mildly inconvenient, so I left it as is. I am considering whether to revisit this or what.

I have started a crater run to test the impact of these changes.

to_owned generalizes clone; this generalizes clone_from.  Use to_owned to
give it a default impl.  Customize the impl for [T], str, and T:Clone.

Use it in Cow::clone_from to reuse resources when cloning Owned into Owned.

Derive Hash for ThreadId + better example

Derive `Hash` for `ThreadId` (see comments in #21507). Useful for making maps based on thread, e.g. `HashMap<ThreadId, ?>`. Also update example code for thread IDs to be more useful.

Rollup of 9 pull requests

- Successful merges: #41063, #41087, #41141, #41166, #41183, #41205, #41206, #41232, #41243
- Failed merges:

Minor nits in primitive str

Some minor updates to linking, added some links, doc format, etc.

r? @GuillaumeGomez

move rvalue checking to MIR

Fix pairs of doubles using an illegal <8 x i8> vector.

Accidentally introduced in #40658 and discovered in some Objective-C bindings (returning `NSPoint`).
Turns out LLVM will widen element types of illegal vectors instead of increasing element count, i.e. it will zero-extend `<8 x i8>` to `<8 x i16>`, interleaving the bytes, instead of using the first 8 of `<16 x i8>`.

Highlight and simplify mismatched types

Shorten mismatched types errors by replacing subtypes that are not
different with `_`, and highlighting only the subtypes that are
different.

Given a file

```rust
struct X<T1, T2> {
    x: T1,
    y: T2,
}

fn foo() -> X<X<String, String>, String> {
    X { x: X {x: "".to_string(), y: 2}, y: "".to_string()}
}

fn bar() -> Option<String> {
    "".to_string()
}
```

provide the following output

```rust
error[E0308]: mismatched types
  --> file.rs:6:5
   |
 6 |     X { x: X {x: "".to_string(), y: 2}, y: "".to_string()}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::string::String`, found {integer}
   |
   = note: expected type `X<X<_, std::string::String>, _>`
                                 ^^^^^^^^^^^^^^^^^^^   // < highlighted
              found type `X<X<_, {integer}>, _>`
                                 ^^^^^^^^^             // < highlighted

error[E0308]: mismatched types
  --> file.rs:6:5
   |
10 |     "".to_string()
   |     ^^^^^^^^^^^^^^ expected struct `std::option::Option`, found `std::string::String`
   |
   = note: expected type `Option<std::string::String>`
                          ^^^^^^^                   ^  // < highlighted
              found type `std::string::String`
```

Fix #21025. Re: #40186. Follow up to #39906.

I'm looking to change how this output is accomplished so that it doesn't create list of strings to pass around, but rather add an elided `Ty` placeholder, and use the same string formatting for normal types. I'll be doing that soonish.

r? @nikomatsakis

COPYRIGHT: remove hoedown license

Hoedown was removed in b96fef84

Also cleanup src/tools/tidy/src/main.rs

Update cargo submodules

Brings in a fix for #40955 through rust-lang/cargo#3898.

Closes #40955

ICH: Replace old, transitive metadata hashing with direct hashing approach.

This PR replaces the old crate metadata hashing strategy with a new one that directly (but stably) hashes all values we encode into the metadata. Previously we would track what data got accessed during metadata encoding and then hash the input nodes (HIR and upstream metadata) that were transitively reachable from the accessed data. While this strategy was sound, it had two major downsides:

1. It was susceptible to generating false positives, i.e. some input node might have changed without actually affecting the content of the metadata. That metadata entry would still show up as changed.
2. It was susceptible to quadratic blow-up when many metadata nodes shared the same input nodes, which would then get hashed over and over again.

The new method does not have these disadvantages and it's also a first step towards caching more intermediate results in the compiler.

Metadata hashing/cross-crate incremental compilation is still kept behind the `-Zincremental-cc` flag even after this PR. Once the new method has proven itself with more tests, we can remove the flag and enable cross-crate support by default again.

r? @nikomatsakis
cc @rust-lang/compiler

Use proper span for tuple index parsed as float

Fix diagnostic suggestion from:

```rust
help: try parenthesizing the first index
  |     (1, (2, 3)).((1, (2, 3)).1).1;
```

to the correct:

```rust
help: try parenthesizing the first index
  |     ((1, (2, 3)).1).1;
```

Fix #41081.

remove unnecessary tasks

Remove various unnecessary tasks. All of these are "always execute" tasks that don't do any writes to tracked state (or else an assert would trigger, anyhow). In some cases, they issue lints or errors, but we''ll deal with that -- and anyway side-effects outside of a task don't cause problems for anything that I can see.

The one non-trivial refactoring here is the borrowck conversion, which adds the requirement to go from a `DefId` to a `BodyId`. I tried to make a useful helper here.

r? @EddyB

cc #40746
cc @cramertj @michaelwoerister

Instead of collecting all potential inputs to some metadata entry and
hashing those, we directly hash the values we are storing in metadata.
This is more accurate and doesn't suffer from quadratic blow-up when
many entries have the same dependencies.

Add contribution instructions to stdlib docs

Generally programming language docs have instructions on how to contribute changes.

I couldn't find any in the rust docs, so I figured I'd add an instructions section, let me know if this belongs somewhere else!

Add the RLS as a submodule and build a package out of it

r? @brson (and cc @alexcrichton) Please review closely, I am not at all convinced I've done the right things here. I did run `x.py dist` and it makes an rls package which looks right to my eyes, but I haven't tested on non-linux platforms nor am I really sure what it should look like.

This does not attempt to run tests for the RLS yet.

Hoedown was removed in b96fef84

Also cleanup src/tools/tidy/src/main.rs
Signed-off-by: NNODA, Kai <nodakai@gmail.com>

Rollup of 8 pull requests

- Successful merges: #40377, #40559, #41173, #41202, #41204, #41209, #41216, #41231
- Failed merges:

These are not user expressible anyhow.

For the most part, it seems to be better, but one side-effect is that I
cannot seem to reproduce E0102 anymore.

When we are generalizing a super/sub-type, we have to replace type
variables with a fresh variable (and not just region variables).  So if
we know that `Box<?T> <: ?U`, for example, we instantiate `?U` with
`Box<?V>` and then relate `Box<?T>` to `Box<?V>` (and hence require that
`?T <: ?V`).

This change has some complex interactions, however:

First, the occurs check must be updated to detect constraints like `?T
<: ?U` and `?U <: Box<?T>`. If we're not careful, we'll create a
never-ending sequence of new variables. To address this, we add a second
unification set into `type_variables` that tracks type variables related
through **either** equality **or** subtyping, and use that during the
occurs-check.

Second, the "fudge regions if ok" code was expecting no new type
variables to be created. It must be updated to create new type variables
outside of the probe. This is relatively straight-forward under the new
scheme, since type variables are now independent from one another, and
any relations are moderated by pending subtype obliations and so forth.
This part would be tricky to backport though.

cc #18653
cc #40951

In some specific cases, the new scheme was failing to learn as much from
a LUB/GLB operaiton as the old code, which caused coercion to go awry. A
slight ordering hack fixes this.

In some cases, we give multiple primary spans, in which case we would
report one `//~` annotation per primary span. That was very confusing
because these things are reported to the user as a single error.

UI tests would be better here.