switch mipsel-musl to soft float

Closes #34922.

r? @alexcrichton

Enable reuse of `.o` files if nothing has changed

This PR completes a first "spike" for incremental compilation by enabling us to reuse `.o` files when nothing has changed. When in incr. mode, we will save `.o` files into the temporary directory, then copy them back out again if they are still valid. The code is still a bit rough but it does seem to work. =)

r? @michaelwoerister

Fixes #34036
Fixes #34037
Fixes #34038

actually we shouldn't even hash nested items at all, but that is
addressed in a followup PR

Escape fewer Unicode codepoints in `Debug` impl of `str`

Use the same procedure as Python to determine whether a character is
printable, described in [PEP 3138]. In particular, this means that the
following character classes are escaped:

- Cc (Other, Control)
- Cf (Other, Format)
- Cs (Other, Surrogate), even though they can't appear in Rust strings
- Co (Other, Private Use)
- Cn (Other, Not Assigned)
- Zl (Separator, Line)
- Zp (Separator, Paragraph)
- Zs (Separator, Space), except for the ASCII space `' '` `0x20`

This allows for user-friendly inspection of strings that are not
English (e.g. compare `"\u{e9}\u{e8}\u{ea}"` to `"éèê"`).

Fixes #34318.
CC #34422.

[PEP 3138]: https://www.python.org/dev/peps/pep-3138/

In the older version, a `.o` and ` .bc` file were separate
work-products.  This newer version keeps, for each codegen-unit, a set
of files of different kinds. We assume that if any kinds are available
then all the kinds we need are available, since the precise set of
switches will depend on attributes and command-line switches.

Should probably test this: the effect of changing attributes in
particular might not be successfully tracked?

This checks the `previous_work_products` data from the dep-graph and
tries to simply copy a `.o` file if possible.  We also add new
work-products into the dep-graph, and create edges to/from the dep-node
for a work-product.

Work products are deleted if any of their inputs are dirty.

We used to use `Name`, but the session outlives the tokenizer, which
means that attempts to read this field after trans has complete
otherwise panic. All reads want an `InternedString` anyhow.

A work product right now is just a `.o` file. In the future it probably
includes other kinds of files, such as `.bc` files saving the
unoptimized LLVM IR.

However, because WorkProductIds must be independent of DefIds, so that
they don't need translation, this system may not be suitable *as is* for
storing fine-grained information (such as the MIR for individual defs),
as it was originally intended. We will want to refactor some for that.

Rollup of 7 pull requests

- Successful merges: #34951, #34963, #34969, #35013, #35037, #35040, #35058
- Failed merges:

Use &mut for `CombineFields` in inference relations

Because dropping obligations on the floor makes a mess.

r? @nikomatsakis

rustdoc: Fix tuple struct where clause rendering

For tuple structs the where clause comes after the definition.

Fixes #34928

move coerce-match{,-calls} into run-pass-valgrind

Closes #21696.

Avoid processing `feature`s on unconfigured crates

Fixes #34932, a regression caused by #34272.
r? @nrc

resolve: Fix ICE and extra diagnostics happening when unresolved imports are used in patterns

Closes https://github.com/rust-lang/rust/issues/34933

r? @jseyfried

Make vec::Drain and binary_heap::Drain covariant

I removed all mutable pointers/references, and added covariance tests similar to the ones in #32635. It builds and passes the tests, but I noticed that there weren't any tests of Drain's behaviour (at least not in libcollectionstest), so I'm not sure if my changes accidently broke Drain's behaviour. Should I add some tests for that (and if so, what should the tests include)?

Make vec::Drain and binary_heap::Drain covariant

I removed all mutable pointers/references, and added covariance tests similar to the ones in #32635. It builds and passes the tests, but I noticed that there weren't any tests of Drain's behaviour (at least not in libcollectionstest), so I'm not sure if my changes accidently broke Drain's behaviour. Should I add some tests for that (and if so, what should the tests include)?

macros: Improve `tt` matchers

Fixes #5846, fixes #22819.
r? @nrc

std: Fix usage of SOCK_CLOEXEC

This code path was intended to only get executed on Linux, but unfortunately the
`cfg!` was malformed so it actually never got executed.

Centralize and clean type error reporting

Refactors the code that handles type errors to be cleaner and fixes various edge cases.

This made the already-bad "type mismatch resolving" error message somewhat uglier. I want to fix that in another commit before this PR is merged.

Fixes #31173

r? @jonathandturner, cc @nikomatsakis

Avoid reseting the thread local interner at the beginning of `phase_1_parse_input`

The thread local interner is used before `phase_1_parse_input` to create `InternedString`s, which currently wrap `Rc<String>`s. Once `InternedString` is refactored to be an interned string id (like `Name`), resetting will invalidate everything that was interned before `phase_1_parse_input`.

The resets were only useful for the `rusti` project, which can now use `driver::reset_thread_local_state`.

r? @nrc

fix built-in target detection

previously the logic was accepting wrong triples (like
`x86_64_unknown-linux-musl`) as valid ones (like `x86_64-unknown-linux-musl`) if
they contained an underscore instead of a dash.

fixes #33329

---

r? @brson

I wanted to use a compile-fail test at first. But, you can't pass an extra `--target` flag to `rustc` for those because they already call `rustc --target $HOST` so you get a `error: Option 'target' given more than once.`. The run-make test used here works fine though.

DoubleEndedIterator for Args

This PR implements the DoubleEndedIterator trait for the `std::env::Args[Os]` structure, as well
as the internal implementations.

It is primarily motivated by me, as I happened to implement a simple `reversor` program many times
now, which so far had to use code like this:

```Rust
for arg in std::env::args().skip(1).collect::<Vec<_>>().iter().rev() {}
```

... even though I would have loved to do this instead:

```Rust
for arg in std::env::args().skip(1).rev() {}
```

The latter is more natural, and I did not find a reason for not implementing it.
After all, on every system, the number of arguments passed to the program are known
at runtime.

To my mind, it follows KISS, and does not try to be smart at all. Also, there are no unit-tests,
primarily as I did not find any existing tests for the `Args` struct either.

The windows implementation is basically a copy-pasted variant of the `next()` method implementation,
and I could imagine sharing most of the code instead. Actually I would be happy if the reviewer would
ask for it.

Rollup of 15 pull requests

- Successful merges: #34461, #34609, #34732, #34850, #34935, #34974, #34990, #34995, #35001, #35009, #35010, #35019, #35028, #35029, #35043
- Failed merges:

Fix Travis tests

r? @vadimcn

doc/book/trait-objects: remove empty lines at start of examples

I think it looks better without them.

Fixed missing comma in the csv dumper.

Added missing comma before `file_line_end`.

r? @nrc

Remove no_stack_check tests (#34915)

Part of fixes for #34915

Rewrite/expansion of `slice::split` doc examples.

None

Improve Open doc

Part of #29356.

r? @steveklabnik

Dir entry doc

Part of #29356.

r? @steveklabnik

Fix HashMap's values_mut example to use println!

Fix HashMap's values_mut example to use println!

Add DirBuilder doc examples

r? @steveklabnik

Part of #29329 and of #29356.

Update underscore usage (#34903)