reserving-syntax.md: Expand and add detail

Author: joshtriplett

src/rust-2021/reserving-syntax.md

@@ -2,44 +2,47 @@
 
 ## Summary
 
-- `any_prefix#..`, `any_prefix".."`, and `any_prefix'..'` are reserved syntax, and no longer tokenize.
+- `any_prefix#`, `any_prefix"..."`, and `any_prefix'...'` are now reserved
+  syntax, and no longer tokenize.
 - This is mostly relevant to macros. E.g. `quote!{ #a#b }` is no longer accepted.
 - It doesn't treat keywords specially, so e.g. `match".." {}` is no longer accepted.
 - Insert whitespace to avoid errors.
 
 ## Details
 
-To make space for some new syntax in the future,
+To make space for new syntax in the future,
 we've decided to reserve syntax for prefixed identifiers and literals:
 `prefix#identifier`, `prefix"string"`, `prefix'c'`, and `prefix#123`,
 where `prefix` can be any identifier.
 (Except those that already have a meaning, such as `b'…'` and `r"…"`.)
+This provides syntax we can expand into in the future without requiring an
+edition boundary. We may use that for temporary syntax until the next edition,
+or for permanent syntax if appropriate.
 
-This is a breaking change, since macros can currently accept `hello"world"`,
-which they will see as two separate tokens: `hello` and `"world"`.
-The (automatic) fix is simple though. Just insert a space: `hello "world"`.
-
-<!--
-The original plan was to reserve only `k#` and `f""` for future use,
-but reserving *all* possible prefixes did not have many downsides.
-It leaves more space for new syntax which would otherwise need to wait for another edition.
--->
+Without an edition, this would be a breaking change, since macros can currently
+accept syntax such as `hello"world"`, which they will see as two separate
+tokens: `hello` and `"world"`. The (automatic) fix is simple though: just
+insert a space: `hello "world"`. Likewise, `prefix#ident` should become
+`prefix #ident`.
 
 Other than turning these into a tokenization error,
 [the RFC][10] does not attach a meaning to any prefix yet.
 Assigning meaning to specific prefixes is left to future proposals,
-which will&mdash;thanks to reserving these prefixes now&mdash;not be breaking changes.
-
-These are some new prefixes you might see in the future:
+which will now&mdash;thanks to reserving these prefixes&mdash;not be breaking changes.
 
-- `f""` as a short-hand for a format string.
-  For example, `f"hello {name}"` as a short-hand for the equivalent `format_args!()` invocation.
-
-- `c""` or `z""` for null-terminated C strings.
+Some new prefixes you might potentially see in the future (though we haven't
+committed to any of them yet):
 
 - `k#keyword` to allow writing keywords that don't exist yet in the current edition.
   For example, while `async` is not a keyword in edition 2015,
   this prefix would've allowed us to accept `k#async` in edition 2015
   without having to wait for edition 2018 to reserve `async` as a keyword.
 
+- `f""` as a short-hand for a format string.
+  For example, `f"hello {name}"` as a short-hand for the equivalent `format!()` invocation.
+
+- `s""` for `String` literals.
+
+- `c""` or `z""` for null-terminated C strings.
+
 [10]: https://github.com/rust-lang/rfcs/pull/3101