Currently, the `Debug` impl for `proc_macro::Span` just prints out
the byte range. This can make debugging proc macros (either as a crate
author or as a compiler developer) very frustrating, since neither the
actual filename nor the `SyntaxContext` is displayed.

This commit adds a perma-unstable flag `-Z span-debug`. When enabled,
the `Debug` impl for `proc_macro::Span` simply forwards directly to
`rustc_span::Span`. Once rust-lang#72618 is merged, this will start displaying
actual line numbers.

While `Debug` impls are not subject to Rust's normal stability
guarnatees, we probably shouldn't expose any additional information on
stable until `#![feature(proc_macro_span)]` is stabilized. Otherwise,
we would be providing a 'backdoor' way to access information that's
supposed be behind unstable APIs.

de-promote Duration::from_secs

In rust-lang#67531, we removed the `rustc_promotable` attribute from a bunch of `Duration` methods, but not from `Duration::from_secs`. This makes the current list of promotable functions the following (courtesy of @ecstatic-morse):

* `INT::min_value`, `INT::max_value`
* `std::mem::size_of`, `std::mem::align_of`
* `RangeInclusive::new` (backing `x..=y`)
* `std::ptr::null`, `std::ptr::null_mut`
* `RawWaker::new`, `RawWakerVTable::new` ???
* `Duration::from_secs`

I feel like the last one stands out a bit here -- the rest are all very core language primitives, and `RawWaker` has a strong motivation for getting a `'static` vtable. But a `&'static Duration`? That seems unlikely. So I propose we no longer promote calls to `Duration::from_secs`, which is what this PR does.

rust-lang#67531 saw zero regressions and I am not aware of anyone complaining that this broke their (non-cratered) code, so I consider it likely the same will be true here, but of course we'd do a crater run.

See [this document](https://github.com/rust-lang/const-eval/blob/master/promotion.md) for some more background on promotion and rust-lang/const-eval#19 for some of the concerns around promoting function calls.

…matsakis

Make `PolyTraitRef::self_ty` return `Binder<Ty>`

This came up during review of rust-lang#71618. The current implementation is the same as a call to `skip_binder` but harder to audit. Make it preserve binding levels and add a call to `skip_binder` at all use sites so they can be audited as part of rust-lang#72507.

…stebank

Be more careful around ty::Error in generators

cc rust-lang#72685

(doesn't close it because it's missing a reproduction to use as a test case)

r? @estebank

…rochenkov

Add `-Z span-debug` to allow for easier debugging of proc macros

Currently, the `Debug` impl for `proc_macro::Span` just prints out
the byte range. This can make debugging proc macros (either as a crate
author or as a compiler developer) very frustrating, since neither the
actual filename nor the `SyntaxContext` is displayed.

This commit adds a perma-unstable flag `-Z span-debug`. When enabled,
the `Debug` impl for `proc_macro::Span` simply forwards directly to
`rustc_span::Span`. Once rust-lang#72618 is merged, this will start displaying
actual line numbers.

While `Debug` impls are not subject to Rust's normal stability
guarnatees, we probably shouldn't expose any additional information on
stable until `#![feature(proc_macro_span)]` is stabilized. Otherwise,
we would be providing a 'backdoor' way to access information that's
supposed be behind unstable APIs.

…u16, r=sfackler

impl ToSocketAddrs for (String, u16)

This adds a convenience impl of `ToSocketAddrs for (String, u16)`. When authoring HTTP services it's common to take command line options for `host` and `port` and parse them into `String` and `u16` respectively. Consider the following program:
```rust
#[derive(Debug, StructOpt)]
struct Config {
    host: String,
    port: u16,
}

async fn main() -> io::Result<()> {
    let config = Config::from_args();
    let stream = TcpStream::connect((&*config.host, config.port))?; // &* is not ideal
    // ...
}
```

Networking is a pretty common starting point for people new to Rust, and seeing `&*` in basic examples can be confusing. Even as someone that has experience with networking in Rust I tend to forget that `String` can't be passed directly there. Instead with this patch we can omit the `&*` conversion and pass `host` directly:
```rust
#[derive(Debug, StructOpt)]
struct Config {
    host: String,
    port: u16,
}

async fn main() -> io::Result<()> {
    let config = Config::from_args();
    let stream = TcpStream::connect((config.host, config.port))?; // no more conversions!
    // ...
}
```

I think should be an easy and small ergonomics improvement for networking. Thanks!

… r=Xanewok

save_analysis: fix some ICEs

Fixes rust-lang#73020
Fixes rust-lang#73022
Fixes rust-lang#73041

remove outdated comment

r? @matthewjasper