librustc: Obsolete the old external crate renaming syntax.

Instead of `extern crate foo = bar`, write `extern crate bar as foo`.
Instead of `extern crate baz = "quux"`, write `extern crate "quux" as
baz`.

Closes #16461.

[breaking-change]

1 files changed, 1 insertions, 5 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 6aff1152f7e..073e88bc4a1 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -4783,11 +4783,7 @@ impl<'a> Parser<'a> {
                     self.bump();
                     let path = self.parse_str();
                     let span = self.span;
-                    self.span_warn(span,
-                            format!("this extern crate syntax is deprecated. \
-                            Use: extern crate \"{}\" as {};",
-                            path.ref0().get(), the_ident.as_str() ).as_slice()
-                    );
+                    self.obsolete(span, ObsoleteExternCrateRenaming);
                     Some(path)
                 } else {None};