intravisit: Fold functionality of IdVisitor into the regular Visitor.

Rename _ to {integer} and {float} for unknown numeric types

This PR renames _ to {integer} or {float} for unknown numeric types, to help people parse error messages that have numeric types that haven't been nailed down.

Example:
```rust
fn main() {
    let x: String = 4;
}
```

Before:
```
error[E0308]: mismatched types
 --> quicktest.rs:2:21
  |
2 |     let x: String = 4;
  |                     ^ expected struct `std::string::String`, found integral variable
  |
  = note: expected type `std::string::String`
  = note:    found type `_`

error: aborting due to previous error
```

after:
```
error[E0308]: mismatched types
 --> quicktest.rs:2:21
  |
2 |     let x: String = 4;
  |                     ^ expected struct `std::string::String`, found integral variable
  |
  = note: expected type `std::string::String`
  = note:    found type `{integer}`

error: aborting due to previous error
```
```

resolve: Exclude inaccessible names from single imports

If a single import resolves to an inaccessible name in some but not all namespaces, avoid importing the name in the inaccessible namespaces.

Currently, the inaccessible namespaces are imported but cause a privacy error when used.

r? @nrc

Add non-panicking abs() functions to all signed integer types.

Currently, calling abs() on one of the signed integer types might panic (in
debug mode at least) because the absolute value of the largest negative value
can not be represented in that signed type. Unlike all other integer
operations, there is currently not a non-panicking version on this function.
This seems to just be an oversight in the design, therefore just adding it now.

Add a test for AddAssign on String

Fix #35047

Implement ARM personality routine in Rust.

Remove the `eh_personality_catch` lang item.
Use a simplified version of `cfg_if!` in libunwind.

Closes #34786

Better attribute and metaitem encapsulation throughout the compiler

This PR refactors most (hopefully all?) of the `MetaItem` interactions outside of `libsyntax` (and a few inside) to interact with MetaItems through the provided traits instead of directly creating / destruct / matching against them. This is a necessary first step to eventually converting `MetaItem`s to internally use `TokenStream` representations (which will make `MetaItem` interactions much nicer for macro writers once the new macro system is in place).

r? @nrc

Rollup of 7 pull requests

- Successful merges: #34258, #34894, #35050, #35062, #35066, #35072, #35087
- Failed merges:

Try to clear up some awkward wording

Fs docs

Fixes #29356.

r? @steveklabnik

Update docs for assert! and debug_assert!

Refer to #34455

Fix typos

r? @steveklabnik

Add documentation example for `str::Chars::as_str`.

None

More intuitive explantion of strings formatting

book/ffi: nullable pointer cleanup

Expand the "nullable pointer optimization" section with a code example. Fixes #34250.

I also noticed that many of the examples use the libc crate just for types such as `c_char` and `c_int`, which are now available through `std::os::raw`. I changed the ones that don't need to rely on libc. I'm glad to revert that part of the commit if it's unwanted churn.

Convert built-in targets to JSON

Convert the built-in targets to JSON to ensure that the JSON parser is always fully featured. This follows on #32988 and #32847. The PR includes a number of extra commits that are just intermediate changes necessary for bisectibility and the ability to prove correctness of the change.

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.

Currently, calling abs() on one of the signed integer types might panic (in
debug mode at least) because the absolute value of the largest negative value
can not be represented in that signed type. Unlike all other integer
operations, there is currently not a non-panicking version on this function.
This seems to just be an oversight in the design, therefore just adding it now.

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)?