Auto merge of #53542 - alexreg:impl-trait-in-bindings, r=cramertj

`impl trait` in bindings (feature: impl-trait-existential-types)

This PR enables `impl Trait` syntax (opaque types) to be used in bindings, e.g.

* `let foo: impl Clone = 1;`
* `static foo: impl Clone = 2;`
* `const foo: impl Clone = 3;`

This is part of [RFC 2071](https://github.com/rust-lang/rfcs/blob/master/text/2071-impl-trait-existential-types.md) ([tracking issue](#34511)), but exists behind the separate feature gate `impl_trait_in_bindings`.

CC @cramertj @oli-obk @eddyb @Centril @varkor

src/doc/unstable-book/src/language-features/impl-trait-in-bindings.md

@@ -0,0 +1,28 @@
+# `impl_trait_in_bindings`
+
+The tracking issue for this feature is: [#34511]
+
+[#34511]: https://github.com/rust-lang/rust/issues/34511
+
+------------------------
+
+The `impl_trait_in_bindings` feature gate lets you use `impl Trait` syntax in
+`let`, `static`, and `const` bindings.
+
+A simple example is:
+
+```rust
+#![feature(impl_trait_in_bindings)]
+
+use std::fmt::Debug;
+
+fn main() {
+    let a: impl Debug + Clone = 42;
+    let b = a.clone();
+    println!("{:?}", b); // prints `42`
+}
+```
+
+Note however that because the types of `a` and `b` are opaque in the above
+example, calling inherent methods or methods outside of the specified traits
+(e.g., `a.abs()` or `b.abs()`) is not allowed, and yields an error.

src/librustc/diagnostics.rs

@@ -1739,7 +1739,7 @@ specified exit code, use `std::process::exit`.
 
 E0562: r##"
 Abstract return types (written `impl Trait` for some trait `Trait`) are only
-allowed as function return types.
+allowed as function and inherent impl return types.
 
 Erroneous code example:
 

src/librustc/hir/def_id.rs

@@ -131,7 +131,6 @@ pub struct DefIndex(u32);
 /// thanks to `NodeCollector::new`.
 pub const CRATE_DEF_INDEX: DefIndex = DefIndex(0);
 
-
 impl fmt::Debug for DefIndex {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f,
@@ -216,7 +215,7 @@ impl DefIndexAddressSpace {
     }
 }
 
-/// A DefId identifies a particular *definition*, by combining a crate
+/// A `DefId` identifies a particular *definition*, by combining a crate
 /// index and a def index.
 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Copy)]
 pub struct DefId {