The codegen implementation already works for this, so we're:

* propagating track_caller attr from trait def to impl
* relaxing errors
* adding tests

Approved in a recent lang team meeting:
https://github.com/rust-lang/lang-team/blob/master/minutes/2020-01-09.md

We can do this now that opt_associated_item doesn't have any panicking paths.

#[track_caller] in traits

Per rust-lang#47809 (comment), this allows the `#[track_caller]` attribute on trait methods.

Includes tests for `#[track_caller]` with:

* "regular" trait impls
* default trait impls
* "blanket-tracked" trait impls, where the annotation is in the trait definition and is inherited by "regular" impls of the trait

…r=oli-obk

miri engine: turn error sanity checks into assertions

We had these as debug assertions so far to make sure our test suite is clean, but really these are conditions that should never arise and also @eddyb told me to turn non-performance-critical debug assertions into full assertions so here we go. ;)

I propose that we do a check-only crater run to make sure this does not actually happen in practice.

r? @oli-obk

Use getentropy(2) on macos

resolves rust-lang#70179

Only display definition when suggesting a typo

Closes rust-lang#70206
r? @Centril

…-morse

resolve: Avoid "self-confirming" import resolutions in one more case

So the idea behind "blacklisted bindings" is that we must ignore some name definitions during resolution because otherwise they cause infinite cycles.
E.g. import
```rust
use my_crate;
```
would refer to itself (on 2018 edition) without this blacklisting, because `use my_crate;` is the first name in scope when we are resolving `my_crate` here.

In this PR we are doing this blacklisting for the case
```rust
use same::same;
```
, namely blacklisting the second `same` when resolving the first `same`.
This was previously forgotten.

Fixes rust-lang#62767

parser: simplify & remove unused field

r? @petrochenkov

handle ConstKind::Unresolved after monomorphizing

fixes rust-lang#70125

r? @bjorn3

…an-DPC

remove redundant closures (clippy::redundant_closure)

…an-DPC

Clean up E0449 explanation

r? @Dylan-DPC