Auto merge of #51196 - Havvy:remove-keywords, r=petrochenkov

Implement RFC 2421, 'Keyword unreservations (pure, sizeof, alignof, offsetof)

On my local machine I was getting failures in `test/ui-fulldeps`, but I was also getting them again after reverting the change, so I'm hoping that this works.

I removed the test because that's what the other PR that unreserved a keyword did and I feel that it doesn't make sense/keep value to keep a test for something removed years ago.

src/libsyntax_pos/symbol.rs

@@ -384,34 +384,30 @@ declare_keywords! {
 
     // Keywords reserved for future use.
     (40, Abstract,           "abstract")
-    (41, Alignof,            "alignof")
-    (42, Become,             "become")
-    (43, Do,                 "do")
-    (44, Final,              "final")
-    (45, Macro,              "macro")
-    (46, Offsetof,           "offsetof")
-    (47, Override,           "override")
-    (48, Priv,               "priv")
-    (49, Pure,               "pure")
-    (50, Sizeof,             "sizeof")
-    (51, Typeof,             "typeof")
-    (52, Unsized,            "unsized")
-    (53, Virtual,            "virtual")
-    (54, Yield,              "yield")
+    (41, Become,             "become")
+    (42, Do,                 "do")
+    (43, Final,              "final")
+    (44, Macro,              "macro")
+    (45, Override,           "override")
+    (46, Priv,               "priv")
+    (47, Typeof,             "typeof")
+    (48, Unsized,            "unsized")
+    (49, Virtual,            "virtual")
+    (50, Yield,              "yield")
 
     // Edition-specific keywords reserved for future use.
-    (55, Async,              "async") // >= 2018 Edition Only
+    (51, Async,              "async") // >= 2018 Edition Only
 
     // Special lifetime names
-    (56, UnderscoreLifetime, "'_")
-    (57, StaticLifetime,     "'static")
+    (52, UnderscoreLifetime, "'_")
+    (53, StaticLifetime,     "'static")
 
     // Weak keywords, have special meaning only in specific contexts.
-    (58, Auto,               "auto")
-    (59, Catch,              "catch")
-    (60, Default,            "default")
-    (61, Dyn,                "dyn")
-    (62, Union,              "union")
+    (54, Auto,               "auto")
+    (55, Catch,              "catch")
+    (56, Default,            "default")
+    (57, Dyn,                "dyn")
+    (58, Union,              "union")
 }
 
 impl Symbol {

src/test/parse-fail/removed-syntax-fn-pure.rs → src/test/run-pass/rfc-2421-unreserve-pure-offsetof-sizeof-alignof.rs

@@ -1,4 +1,4 @@
-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -8,6 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z parse-only
+// Test that removed keywords are allowed as identifiers.
+fn main () {
+    let offsetof = ();
+    let alignof = ();
+    let sizeof = ();
+    let pure = ();
+}
 
-pure fn f() {} //~ ERROR expected item, found `pure`
+fn offsetof() {}
+fn alignof() {}
+fn sizeof() {}
+fn pure() {}