librustc: Change the syntax of subslice matching to use postfix `..`

instead of prefix `..`.

This breaks code that looked like:

    match foo {
        [ first, ..middle, last ] => { ... }
    }

Change this code to:

    match foo {
        [ first, middle.., last ] => { ... }
    }

RFC #55.

Closes #16967.

[breaking-change]

1 files changed, 5 insertions, 0 deletions

diff --git a/src/libsyntax/parse/obsolete.rs b/src/libsyntax/parse/obsolete.rs
index 5273addf4f5..ec6fd013d08 100644
--- a/src/libsyntax/parse/obsolete.rs
+++ b/src/libsyntax/parse/obsolete.rs
@@ -35,6 +35,7 @@ pub enum ObsoleteSyntax {
     ObsoleteManagedType,
     ObsoleteManagedExpr,
     ObsoleteImportRenaming,
+    ObsoleteSubsliceMatch,
 }
 
 pub trait ParserObsoleteMethods {
@@ -87,6 +88,10 @@ impl<'a> ParserObsoleteMethods for parser::Parser<'a> {
             ObsoleteImportRenaming => (
                 "`use foo = bar` syntax",
                 "write `use bar as foo` instead"
+            ),
+            ObsoleteSubsliceMatch => (
+                "subslice match syntax",
+                "instead of `..xs`, write `xs..` in a pattern"
             )
         };