During the sanity check, we keep track of the path we are below in a `Vec`.  We
avoid cloning that `Vec` unless we hit a pointer indirection.  The `String`
representation is only computed when validation actually fails.

This is still roughly 45ns slower than the old state, because it now works with
an MPlaceTy and uses the appropriate abstractions, instead of working with a
ptr-align pair directly.

- The logging in step.rs becomes debug! to make it stand out a bit more
- Dump value of operands (in `eval_operands`)
- Try to log a bit less verbose

* Value gets renamed to Operand, so that now interpret::{Place, Operand} are the
  "dynamic" versions of mir::{Place, Operand}.
* Operand and Place share the data for their "stuff is in memory"-base in a new
  type, MemPlace.  This also makes it possible to give some more precise types
  in other areas.  Both Operand and MemPlace have methods available to project
  into fields (and other kinds of projections) without causing further
  allocations.
* The type for "a Scalar or a ScalarPair" is called Value, and again used to
  give some more precise types.
* All of these have versions with an attached layout, so that we can more often
  drag the layout along instead of recomputing it.  This lets us get rid of
  `PlaceExtra::Downcast`.  MPlaceTy and PlaceTy can only be constructed
  in place.rs, making sure the layout is handled properly.
  (The same should eventually be done for ValTy and OpTy.)
* All the high-level functions to write typed memory take a Place, and live in
  place.rs.  All the high-level typed functions to read typed memory take an
  Operand, and live in operands.rs.

Exec gdb/lldb in rust-{gdb/lldb} wrapper scripts

This way, the process we get by executing `rust-gdb` or `rust-lldb` (eventually) is an actual `gdb` or `lldb` process and behaves accordingly. Previously (and at least to me unexpectedly) it was just a script waiting for the debugger to exit. Sending a signal (e.g. SIGINT) to the spawned process did therefore not affect the debugger process (which was just a child of the wrapper script).

In order to work around that we `exec` (according to [this](http://pubs.opengroup.org/onlinepubs/009695399/utilities/exec.html) part of the posix shell) and replace the script process with the debugger in the last line of the script. The lldb script had to be modified to not pass the configuration commands via a script file (which in my opinion is cleaner anyway).

Exhaustive integer matching

This adds a new feature flag `exhaustive_integer_patterns` that enables exhaustive matching of integer types by their values. For example, the following is now accepted:
```rust
#![feature(exhaustive_integer_patterns)]
#![feature(exclusive_range_pattern)]

fn matcher(x: u8) {
  match x { // ok
    0 .. 32 => { /* foo */ }
    32 => { /* bar */ }
    33 ..= 255 => { /* baz */ }
  }
}
```
This matching is permitted on all integer (signed/unsigned and char) types. Sensible error messages are also provided. For example:
```rust
fn matcher(x: u8) {
  match x { //~ ERROR
    0 .. 32 => { /* foo */ }
  }
}
```
results in:
```
error[E0004]: non-exhaustive patterns: `32u8...255u8` not covered
 --> matches.rs:3:9
  |
6 |   match x {
  |         ^ pattern `32u8...255u8` not covered
```

This implements https://github.com/rust-lang/rfcs/issues/1550 for https://github.com/rust-lang/rust/issues/50907. While there hasn't been a full RFC for this feature, it was suggested that this might be a feature that obviously complements the existing exhaustiveness checks (e.g. for `bool`) and so a feature gate would be sufficient for now.

Auto merge of #53439 - GuillaumeGomez:generate-blanket-impls-for-reexported-items, r=QuietMisdreavus

Generate blanket implementations for reexported items as well

Fixes #53374.

r? @QuietMisdreavus

resolve: Some macro resolution refactoring

Work towards completing https://github.com/rust-lang/rust/pull/50911#issuecomment-411605393

The last commit also fixes https://github.com/rust-lang/rust/issues/53269 by not using `def_id()` on `Def::Err` and also fixes https://github.com/rust-lang/rust/issues/53512.

Only fetch lib_features when there are unknown feature attributes

An attempt to win back some of the performance lost in https://github.com/rust-lang/rust/pull/52644#issuecomment-413761127.

cc @nnethercote

Rollup of 17 pull requests

Successful merges:

 - #53030 (Updated RELEASES.md for 1.29.0)
 - #53104 (expand the documentation on the `Unpin` trait)
 - #53213 (Stabilize IP associated constants)
 - #53296 (When closure with no arguments was expected, suggest wrapping)
 - #53329 (Replace usages of ptr::offset with ptr::{add,sub}.)
 - #53363 (add individual docs to `core::num::NonZero*`)
 - #53370 (Stabilize macro_vis_matcher)
 - #53393 (Mark libserialize functions as inline)
 - #53405 (restore the page title after escaping out of a search)
 - #53452 (Change target triple used to check for lldb in build-manifest)
 - #53462 (Document Box::into_raw returns non-null ptr)
 - #53465 (Remove LinkMeta struct)
 - #53492 (update lld submodule to include RISCV patch)
 - #53496 (Fix typos found by codespell.)
 - #53521 (syntax: Optimize some literal parsing)
 - #53540 (Moved issue-53157.rs into src/test/ui/consts/const-eval/)
 - #53551 (Avoid some Place clones.)

Failed merges:

r? @ghost

Avoid some Place clones.

This is a 0.5% speedup on ripgrep.

Replace usages of ptr::offset with ptr::{add,sub}.

Rust provides these helper methods – so let's use them!

Stabilise raw_identifiers feature

* [Reference PR](https://github.com/rust-lang-nursery/reference/pull/395)
* [Book PR](https://github.com/rust-lang/book/pull/1480)
* [Rust by Example PR](https://github.com/rust-lang/rust-by-example/pull/1095)

Closes #48589.

r? @cramertj
CC @cuviper @centril

Moved issue-53157.rs into src/test/ui/consts/const-eval/