Auto merge of #21278 - thchittenden:issue-21033-struct-var-pattern-fix, r=alexcrichton

Closes #21033. The new strategy for parsing a field pattern is to look 1 token ahead and if it's a colon, parse as "fieldname: pat", otherwise parse the shorthand form "(box) (ref) (mut) fieldname)". The previous strategy was to parse "(ref) (mut) fieldname" then if we encounter a colon, throw an error if either "ref" or "mut" were encountered.

diff --cc src/libsyntax/parse/parser.rs
Simple merge