GeezerVison

r?

There are a lot of commits here, but not all that much substance. Mostly just refactoring.

I started sketching out the beginnings of a very simple I/O API in `core::rt::io` that represents I/O streams as a single `Stream` trait instead of `Reader` / `Writer` pairs. This seems to be the more common pattern (at least this is how the .NET BCL does it) and it seems to me that separate readers and writers would make duplex streams very awkward. Regardless, I don't intend to go very far down the I/O API design road without some mailing list discussion.

I've also started on the uv bindings for file I/O but haven't gotten very far.

Also hooked up the new scheduler to `rust_start` and the compiletest driver. 70% of run-pass test cases already pass, but I wouldn't read too much into that.

I also split the direct, low-level uv bindings in two so that the scheduler can have its own set, leaving `std::net` on its own.

Downgrade let_unit_value to pedantic

Given that the false positive in rust-lang#1502 is marked E-hard and I don't have much hope of it getting fixed, I think it would be wise to disable this lint by default. I have had to suppress this lint in every substantial codebase (\>100k line) I have worked in. Any time this lint is being triggered, it's always the false positive case.

The motivation for this lint is documented as:

> A unit value cannot usefully be used anywhere. So binding one is kind of pointless.

with this example:

> ```rust
> let x = {
>     1;
> };
> ```

Sure, but the author would find this out via an unused_variable warning or from `x` not being the type that they need further down. If there ends up being a type error on `x`, clippy's advice isn't going to help get the code compiling because it can only run if the code already compiles.

changelog: Remove let_unit_value from default set of enabled lints

Rollup of 11 pull requests

Successful merges:

 - rust-lang#5406 (Fix update_lints)
 - rust-lang#5409 (Downgrade let_unit_value to pedantic)
 - rust-lang#5410 (Downgrade trivially_copy_pass_by_ref to pedantic)
 - rust-lang#5412 (Downgrade inefficient_to_string to pedantic)
 - rust-lang#5415 (Add new lint for `Result<T, E>.map_or(None, Some(T))`)
 - rust-lang#5417 (Update doc links and mentioned names in docs)
 - rust-lang#5419 (Downgrade unreadable_literal to pedantic)
 - rust-lang#5420 (Downgrade new_ret_no_self to pedantic)
 - rust-lang#5422 (CONTRIBUTING.md: fix broken triage link)
 - rust-lang#5424 (Incorrect suspicious_op_assign_impl)
 - rust-lang#5425 (Ehance opt_as_ref_deref lint.)

Failed merges:

 - rust-lang#5345 (Add lint for float in array comparison)
 - rust-lang#5411 (Downgrade implicit_hasher to pedantic)
 - rust-lang#5428 (Move cognitive_complexity to nursery)

r? @ghost

changelog: rollup

7535: Extract function assist r=cpud36 a=cpud36

This PR adds `extract function/method` assist. closes rust-lang#5409.

# Supported features
Assist should support extracting from expressions(`1`, `2 + 2`, `loop { }`) and from a series of statements, e.g.:
```rust
foo();
$0bar();
baz();$0
quix();
```
Assist also supports extracting parameters, like:
```rust
fn foo() -> i32 {
  let n = 1;
  $0n + 1$0
}

// -
fn foo() -> i32 {
  let n = 1;
  fun_name(n)
}

fn fun_name(n: i32) -> i32 {
  n + 1
}
```

Extracting methods also generally works.

Assist allows referencing outer variables, both mutably and immutably, and handles handles access to variables local to extracted function:
```rust
fn foo() {
  let mut n = 1;
  let mut m = 2;
  let mut moved_v = Vec::new();
  let mut ref_mut_v = Vec::new();
  $0
  n += 1;
  let k = 1;
  moved_v.push(n);
  let r = &mut m;
  ref_mut_v.push(*r);
  let h = 3;
  $0
  n = ref_mut_v.len() + k;
  n -= h + m;
}

// -
fn foo() {
  let mut n = 1;
  let mut m = 2;
  let mut moved_v = Vec::new();
  let mut ref_mut_v = Vec::new();
 
  let (k, h) =  fun_name(&mut n, moved_v, &mut m, &mut ref_mut_v);

  n = ref_mut_v.len() + k;
  n -= h + m;
}

fn fun_name(n: &mut i32, mut moved_v: Vec<i32>, m: &mut i32, ref_mut_v: &mut Vec<i32>) -> (i32, i32) {
  *n += 1;
  let k = 1;
  moved_v.push(*n);
  let r = m;
  ref_mut_v.push(*r);
  let h = 3;
  (k, h)
}
```

So we handle both input and output paramters

# Showcase

![extract_cursor_in_range_3](https://user-images.githubusercontent.com/4218373/106980190-c9870800-6770-11eb-83d9-3d36b2550ff6.gif)
![fill_match_arms_discard_wildcard](https://user-images.githubusercontent.com/4218373/106980197-cbe96200-6770-11eb-96b0-14c27894fac0.gif)
![ide_db_helpers_handle_kind](https://user-images.githubusercontent.com/4218373/106980201-cdb32580-6770-11eb-9e6e-6ac8155d65ac.gif)
![ide_db_imports_location_local_query](https://user-images.githubusercontent.com/4218373/106980205-cf7ce900-6770-11eb-8516-653c8fcca807.gif)

# Working with non-`Copy` types

Consider the following example:
```rust
fn foo() {
  let v = Vec::new();
  $0
  let n = v.len();
  $0
  let is_empty = v.is_empty();
}
```
`v` must be a parameter to extracted function. 
The question is, what type should it have.
It could be `v: Vec<i32>`, or `v: &Vec<i32>`. 
The former is incorrect for `Vec<i32>`, but the later is silly for `i32`.

To resolve this we need to know if the type implements `Copy` trait.

I didn't find any api available from assists to query this. 
`hir_ty::method_resolution::implements` seems relevant, but is isn't publicly re-exported from `hir`.

# Star(`*`) token and pointer dereference

If I understand correctly, in order to create expression like `*p`, one should use `ast::make::expr_prefix(T![*], ...)`, which
in turn calls `token(T![*])`.

`token` does not have star in `tokens::SOURCE_FILE`, so this panics.
I had to add `*` to `SOURCE_FILE` to make it work.

Correct me if this is not intended way to do this.

# Lowering access `value -> mut ref -> shared ref`

Consider the following example:
```rust
fn foo() {
  let v = Vec::new();
  $0 let n = v.len(); $0
}
```
`v` is not used after extracted function body, so both `v: &Vec<i32>` and `v: Vec<i32>` would work.
Currently the later would be chosen.

We can however check the body of extracted function and conclude that `v: &Vec<i32>` is sufficient.
Using `v: &Vec<i32>`(that is a minimal required access level) might be a better default.
I am unsure.

# Cleanup
The assist seems to be reasonably handling most of common cases.
If there are no concerns with code it produces(i.e. with test cases), I will start cleaning up

[edit]
added showcase


Co-authored-by: Vladyslav Katasonov <[email protected]>

miri: reimplement `pavg` intrinsics