Auto merge of #55275 - petrochenkov:extself, r=eddyb

experiment: Support aliasing local crate root in extern prelude

This PR provides some minimally invasive solution for the 2018 edition migration issue described in https://github.com/rust-lang/rust/issues/54647 and affecting proc macro crates.

`extern crate NAME as RENAME;` now accepts `NAME`=`self` and interprets it as referring to the local crate.
As with other `extern crate` items, `RENAME` in this case gets into extern prelude in accordance with https://github.com/rust-lang/rust/pull/54658, thus resolving https://github.com/rust-lang/rust/issues/54647.
```rust
extern crate self as serde; // Adds local crate to extern prelude as `serde`
```
This solution doesn't introduce any new syntax and has minimal maintenance cost, so it can be easily deprecated if something better is found in the future.

Closes https://github.com/rust-lang/rust/issues/54647

1 files changed, 5 insertions, 1 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index f13bb7df0b4..33715f206de 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -6783,7 +6783,11 @@ impl<'a> Parser<'a> {
         let error_msg = "crate name using dashes are not valid in `extern crate` statements";
         let suggestion_msg = "if the original crate name uses dashes you need to use underscores \
                               in the code";
-        let mut ident = self.parse_ident()?;
+        let mut ident = if self.token.is_keyword(keywords::SelfValue) {
+            self.parse_path_segment_ident()
+        } else {
+            self.parse_ident()
+        }?;
         let mut idents = vec![];
         let mut replacement = vec![];
         let mut fixed_crate_name = false;