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

[Willi0728]

…2015 when used in dyn + dyn

Fixes: #157565

Description

A diagnostics improvement #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:

            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

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

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[Willi0728]

@rustbot label +relnotes