Reserve prefixed identifiers and string literals (RFC 3101)

This commit denies any identifiers immediately followed by
one of three tokens `"`, `'` or `#`, which is stricter than
the requirements of RFC 3101 but may be necessary according
to the discussion at [Zulip].

[Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/268952-edition-2021/topic/reserved.20prefixes/near/238470099

3 files changed, 129 insertions, 1 deletions

diff --git a/src/librustdoc/html/highlight.rs b/src/librustdoc/html/highlight.rs
index 51392ca1191..36e4a240f14 100644
--- a/src/librustdoc/html/highlight.rs
+++ b/src/librustdoc/html/highlight.rs
@@ -413,7 +413,7 @@ impl<'a> Classifier<'a> {
                 },
                 c => c,
             },
-            TokenKind::RawIdent => Class::Ident,
+            TokenKind::RawIdent | TokenKind::BadPrefix => Class::Ident,
             TokenKind::Lifetime { .. } => Class::Lifetime,
         };
         // Anything that didn't return above is the simple case where we the
diff --git a/src/test/ui/rust-2021/reserved-prefixes.rs b/src/test/ui/rust-2021/reserved-prefixes.rs
new file mode 100644
index 00000000000..5882c7d83d8
--- /dev/null
+++ b/src/test/ui/rust-2021/reserved-prefixes.rs
@@ -0,0 +1,36 @@
+// compile-flags: -Z unstable-options --edition 2021
+
+macro_rules! demo2 {
+    ( $a:tt $b:tt ) => { println!("two tokens") };
+}
+
+macro_rules! demo3 {
+    ( $a:tt $b:tt $c:tt ) => { println!("three tokens") };
+}
+
+macro_rules! demo4 {
+    ( $a:tt $b:tt $c:tt $d:tt ) => { println!("four tokens") };
+}
+
+fn main() {
+    demo3!(foo#bar);   //~ ERROR prefix `foo` is unknown
+    demo2!(foo"bar");  //~ ERROR prefix `foo` is unknown
+    demo2!(foo'b');    //~ ERROR prefix `foo` is unknown
+
+    demo2!(foo'b);     //~ ERROR prefix `foo` is unknown
+    demo3!(foo# bar);  //~ ERROR prefix `foo` is unknown
+    demo4!(foo#! bar); //~ ERROR prefix `foo` is unknown
+    demo4!(foo## bar); //~ ERROR prefix `foo` is unknown
+
+    demo4!(foo#bar#);
+    //~^ ERROR prefix `foo` is unknown
+    //~| ERROR prefix `bar` is unknown
+
+    demo3!(foo # bar);
+    demo3!(foo #bar);
+    demo4!(foo!#bar);
+    demo4!(foo ##bar);
+
+    demo3!(r"foo"#bar);
+    demo3!(r#foo#bar);
+}
diff --git a/src/test/ui/rust-2021/reserved-prefixes.stderr b/src/test/ui/rust-2021/reserved-prefixes.stderr
new file mode 100644
index 00000000000..ffaad315202
--- /dev/null
+++ b/src/test/ui/rust-2021/reserved-prefixes.stderr
@@ -0,0 +1,92 @@
+error: prefix `foo` is unknown
+  --> $DIR/reserved-prefixes.rs:16:12
+   |
+LL |     demo3!(foo#bar);
+   |            ^^^- help: consider inserting a whitespace before this `#`
+   |            |
+   |            unknown prefix
+   |
+   = note: prefixed identifiers and string literals are reserved since Rust 2021
+
+error: prefix `foo` is unknown
+  --> $DIR/reserved-prefixes.rs:17:12
+   |
+LL |     demo2!(foo"bar");
+   |            ^^^- help: consider inserting a whitespace before this `"`
+   |            |
+   |            unknown prefix
+   |
+   = note: prefixed identifiers and string literals are reserved since Rust 2021
+
+error: prefix `foo` is unknown
+  --> $DIR/reserved-prefixes.rs:18:12
+   |
+LL |     demo2!(foo'b');
+   |            ^^^- help: consider inserting a whitespace before this `'`
+   |            |
+   |            unknown prefix
+   |
+   = note: prefixed identifiers and string literals are reserved since Rust 2021
+
+error: prefix `foo` is unknown
+  --> $DIR/reserved-prefixes.rs:20:12
+   |
+LL |     demo2!(foo'b);
+   |            ^^^- help: consider inserting a whitespace before this `'`
+   |            |
+   |            unknown prefix
+   |
+   = note: prefixed identifiers and string literals are reserved since Rust 2021
+
+error: prefix `foo` is unknown
+  --> $DIR/reserved-prefixes.rs:21:12
+   |
+LL |     demo3!(foo# bar);
+   |            ^^^- help: consider inserting a whitespace before this `#`
+   |            |
+   |            unknown prefix
+   |
+   = note: prefixed identifiers and string literals are reserved since Rust 2021
+
+error: prefix `foo` is unknown
+  --> $DIR/reserved-prefixes.rs:22:12
+   |
+LL |     demo4!(foo#! bar);
+   |            ^^^- help: consider inserting a whitespace before this `#`
+   |            |
+   |            unknown prefix
+   |
+   = note: prefixed identifiers and string literals are reserved since Rust 2021
+
+error: prefix `foo` is unknown
+  --> $DIR/reserved-prefixes.rs:23:12
+   |
+LL |     demo4!(foo## bar);
+   |            ^^^- help: consider inserting a whitespace before this `#`
+   |            |
+   |            unknown prefix
+   |
+   = note: prefixed identifiers and string literals are reserved since Rust 2021
+
+error: prefix `foo` is unknown
+  --> $DIR/reserved-prefixes.rs:25:12
+   |
+LL |     demo4!(foo#bar#);
+   |            ^^^- help: consider inserting a whitespace before this `#`
+   |            |
+   |            unknown prefix
+   |
+   = note: prefixed identifiers and string literals are reserved since Rust 2021
+
+error: prefix `bar` is unknown
+  --> $DIR/reserved-prefixes.rs:25:16
+   |
+LL |     demo4!(foo#bar#);
+   |                ^^^- help: consider inserting a whitespace before this `#`
+   |                |
+   |                unknown prefix
+   |
+   = note: prefixed identifiers and string literals are reserved since Rust 2021
+
+error: aborting due to 9 previous errors
+