Rollup merge of rust-lang#157577 - i-like-ICEs:fix-dyn-2015-parser, r=fmease

Fix parser error recovery treating 'dyn' as a strict keyword in Rust …

…2015 when used in `dyn + dyn`

Fixes: rust-lang#157565

### Description
A diagnostics improvement rust-lang#84896 in 2015 edition caused errors when `dyn + dyn` was used, even though `dyn` is a contextual keyword in the 2015 edition.

### Solution
A check was added in `rustc_parse/src/parser/ty.rs:1095`:
```rust
            if self.token.is_keyword(kw::Dyn) && self.token.span.edition().at_least_rust_2018() {
```
This checks that the version is not 2015 before throwing an error.
A regression test was also added, in `tests/ui/parser/dyn-2015-identifier.rs`

Author: JonathanBrouwer

compiler/rustc_parse/src/parser/ty.rs

@@ -1092,7 +1092,7 @@ impl<'a> Parser<'a> {
                 && (self.token.can_begin_type()
                     || (self.token.is_reserved_ident() && !self.token.is_keyword(kw::Where))))
         {
-            if self.token.is_keyword(kw::Dyn) {
+            if self.token.is_keyword(kw::Dyn) && self.token.span.edition().at_least_rust_2018() {
                 // Account for `&dyn Trait + dyn Other`.
                 self.bump();
                 self.dcx().emit_err(InvalidDynKeyword {

tests/ui/parser/dyn-2015-identifier.fail.stderr

@@ -0,0 +1,80 @@
+error[E0425]: cannot find type `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:7:11
+   |
+LL | type A0 = dyn;
+   |           ^^^ not found in this scope
+
+error[E0425]: cannot find type `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:13:11
+   |
+LL | type A2 = dyn<dyn, dyn>;
+   |           ^^^ not found in this scope
+
+error[E0425]: cannot find type `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:13:15
+   |
+LL | type A2 = dyn<dyn, dyn>;
+   |               ^^^ not found in this scope
+
+error[E0425]: cannot find type `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:13:20
+   |
+LL | type A2 = dyn<dyn, dyn>;
+   |                    ^^^ not found in this scope
+
+error[E0425]: cannot find type `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:18:11
+   |
+LL | type A3 = dyn<<dyn as dyn>::dyn>;
+   |           ^^^ not found in this scope
+
+error[E0405]: cannot find trait `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:18:23
+   |
+LL | type A3 = dyn<<dyn as dyn>::dyn>;
+   |                       ^^^ not found in this scope
+
+error[E0425]: cannot find type `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:18:16
+   |
+LL | type A3 = dyn<<dyn as dyn>::dyn>;
+   |                ^^^ not found in this scope
+
+error[E0405]: cannot find trait `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:24:11
+   |
+LL | type A4 = dyn + dyn;
+   |           ^^^ not found in this scope
+
+error[E0405]: cannot find trait `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:24:17
+   |
+LL | type A4 = dyn + dyn;
+   |                 ^^^ not found in this scope
+
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/dyn-2015-identifier.rs:24:11
+   |
+LL | type A4 = dyn + dyn;
+   |           ^^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2021/warnings-promoted-to-error.html>
+   = note: `#[warn(bare_trait_objects)]` (part of `#[warn(rust_2021_compatibility)]`) on by default
+help: if this is a dyn-compatible trait, use `dyn`
+   |
+LL | type A4 = dyn dyn + dyn;
+   |           +++
+
+error[E0433]: cannot find module or crate `dyn` in this scope
+  --> $DIR/dyn-2015-identifier.rs:10:11
+   |
+LL | type A1 = dyn::dyn;
+   |           ^^^ use of unresolved module or unlinked crate `dyn`
+   |
+   = help: you might be missing a crate named `dyn`
+
+error: aborting due to 10 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0405, E0425, E0433.
+For more information about an error, try `rustc --explain E0405`.

tests/ui/parser/dyn-2015-identifier.rs

@@ -0,0 +1,28 @@
+//@ edition: 2015
+//@ compile-flags: --crate-type=lib
+//@ revisions: pass fail
+//@[pass] check-pass
+#![cfg(fail)]
+
+type A0 = dyn;
+//[fail]~^ ERROR cannot find type `dyn` in this scope
+
+type A1 = dyn::dyn;
+//[fail]~^ ERROR cannot find module or crate `dyn` in this scope
+
+type A2 = dyn<dyn, dyn>;
+//[fail]~^ ERROR cannot find type `dyn` in this scope
+//[fail]~| ERROR cannot find type `dyn` in this scope
+//[fail]~| ERROR cannot find type `dyn` in this scope
+
+type A3 = dyn<<dyn as dyn>::dyn>;
+//[fail]~^ ERROR cannot find type `dyn` in this scope
+//[fail]~| ERROR cannot find type `dyn` in this scope
+//[fail]~| ERROR cannot find trait `dyn` in this scope
+
+// issue: <https://github.com/rust-lang/rust/issues/157565>
+type A4 = dyn + dyn;
+//[fail]~^ ERROR cannot find trait `dyn` in this scope
+//[fail]~| ERROR cannot find trait `dyn` in this scope
+//[fail]~| WARN trait objects without an explicit `dyn` are deprecated
+//[fail]~| WARN this is accepted in the current edition