Reserve guarded string literals (RFC 3593)

12 files changed, 1261 insertions, 0 deletions

diff --git a/tests/ui/rust-2024/auxiliary/reserved-guarded-strings-macro-2021.rs b/tests/ui/rust-2024/auxiliary/reserved-guarded-strings-macro-2021.rs
new file mode 100644
index 00000000000..81080fcdce3
--- /dev/null
+++ b/tests/ui/rust-2024/auxiliary/reserved-guarded-strings-macro-2021.rs
@@ -0,0 +1,20 @@
+//@ force-host
+//@ edition:2021
+//@ no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use std::str::FromStr;
+
+#[proc_macro]
+pub fn number_of_tokens_in_a_guarded_string_literal(_: TokenStream) -> TokenStream {
+    TokenStream::from_str("#\"abc\"#").unwrap().into_iter().count().to_string().parse().unwrap()
+}
+
+#[proc_macro]
+pub fn number_of_tokens_in_a_guarded_unterminated_string_literal(_: TokenStream) -> TokenStream {
+    TokenStream::from_str("#\"abc\"").unwrap().into_iter().count().to_string().parse().unwrap()
+}
diff --git a/tests/ui/rust-2024/auxiliary/reserved-guarded-strings-macro-2024.rs b/tests/ui/rust-2024/auxiliary/reserved-guarded-strings-macro-2024.rs
new file mode 100644
index 00000000000..2c3dc30f0ae
--- /dev/null
+++ b/tests/ui/rust-2024/auxiliary/reserved-guarded-strings-macro-2024.rs
@@ -0,0 +1,21 @@
+//@ force-host
+//@ compile-flags: -Zunstable-options
+//@ edition:2024
+//@ no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use std::str::FromStr;
+
+#[proc_macro]
+pub fn number_of_tokens_in_a_guarded_string_literal(_: TokenStream) -> TokenStream {
+    TokenStream::from_str("#\"abc\"#").unwrap().into_iter().count().to_string().parse().unwrap()
+}
+
+#[proc_macro]
+pub fn number_of_tokens_in_a_guarded_unterminated_string_literal(_: TokenStream) -> TokenStream {
+    TokenStream::from_str("#\"abc\"").unwrap().into_iter().count().to_string().parse().unwrap()
+}
diff --git a/tests/ui/rust-2024/reserved-guarded-strings-lexing.rs b/tests/ui/rust-2024/reserved-guarded-strings-lexing.rs
new file mode 100644
index 00000000000..83e0dcbb4be
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings-lexing.rs
@@ -0,0 +1,80 @@
+//@ edition:2021
+// ignore-tidy-linelength
+
+#![warn(rust_2024_guarded_string_incompatible_syntax)]
+
+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") };
+}
+
+macro_rules! demo5 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt ) => { println!("five tokens") };
+}
+
+macro_rules! demo7 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt $g:tt ) => { println!("seven tokens") };
+}
+
+
+fn main() {
+    demo3!(## "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(### "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(## "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo7!(### "foo"###);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo5!(###"foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo5!(#"foo"###);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!("foo"###);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    // Non-ascii identifiers
+    demo2!(Ñ"foo");
+    //~^ ERROR prefix `Ñ` is unknown
+    demo4!(Ñ#""#);
+    //~^ ERROR prefix `Ñ` is unknown
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(🙃#"");
+    //~^ ERROR identifiers cannot contain emoji
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+}
diff --git a/tests/ui/rust-2024/reserved-guarded-strings-lexing.stderr b/tests/ui/rust-2024/reserved-guarded-strings-lexing.stderr
new file mode 100644
index 00000000000..e2e1ac42f05
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings-lexing.stderr
@@ -0,0 +1,271 @@
+error: prefix `Ñ` is unknown
+  --> $DIR/reserved-guarded-strings-lexing.rs:70:12
+   |
+LL |     demo2!(Ñ"foo");
+   |            ^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: consider inserting whitespace here
+   |
+LL |     demo2!(Ñ "foo");
+   |             +
+
+error: prefix `Ñ` is unknown
+  --> $DIR/reserved-guarded-strings-lexing.rs:72:12
+   |
+LL |     demo4!(Ñ#""#);
+   |            ^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: consider inserting whitespace here
+   |
+LL |     demo4!(Ñ #""#);
+   |             +
+
+error: identifiers cannot contain emoji: `🙃`
+  --> $DIR/reserved-guarded-strings-lexing.rs:76:12
+   |
+LL |     demo3!(🙃#"");
+   |            ^^
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:28:12
+   |
+LL |     demo3!(## "foo");
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+note: the lint level is defined here
+  --> $DIR/reserved-guarded-strings-lexing.rs:4:9
+   |
+LL | #![warn(rust_2024_guarded_string_incompatible_syntax)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# # "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:31:12
+   |
+LL |     demo4!(### "foo");
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# ## "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:31:13
+   |
+LL |     demo4!(### "foo");
+   |             ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(## # "foo");
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:36:12
+   |
+LL |     demo4!(## "foo"#);
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# # "foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:39:12
+   |
+LL |     demo7!(### "foo"###);
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo7!(# ## "foo"###);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:39:13
+   |
+LL |     demo7!(### "foo"###);
+   |             ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo7!(## # "foo"###);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:39:21
+   |
+LL |     demo7!(### "foo"###);
+   |                     ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo7!(### "foo"# ##);
+   |                      +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:39:22
+   |
+LL |     demo7!(### "foo"###);
+   |                      ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo7!(### "foo"## #);
+   |                       +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:49:12
+   |
+LL |     demo5!(###"foo"#);
+   |            ^^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(# ##"foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:49:13
+   |
+LL |     demo5!(###"foo"#);
+   |             ^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(## #"foo"#);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:49:14
+   |
+LL |     demo5!(###"foo"#);
+   |              ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(### "foo"#);
+   |               +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:56:12
+   |
+LL |     demo5!(#"foo"###);
+   |            ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(# "foo"###);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:56:18
+   |
+LL |     demo5!(#"foo"###);
+   |                  ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(#"foo"# ##);
+   |                   +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:56:19
+   |
+LL |     demo5!(#"foo"###);
+   |                   ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(#"foo"## #);
+   |                    +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:63:17
+   |
+LL |     demo4!("foo"###);
+   |                 ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!("foo"# ##);
+   |                  +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:63:18
+   |
+LL |     demo4!("foo"###);
+   |                  ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!("foo"## #);
+   |                   +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:72:13
+   |
+LL |     demo4!(Ñ#""#);
+   |             ^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(Ñ# ""#);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-lexing.rs:76:13
+   |
+LL |     demo3!(🙃#"");
+   |              ^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(🙃# "");
+   |               +
+
+error: aborting due to 3 previous errors; 18 warnings emitted
+
diff --git a/tests/ui/rust-2024/reserved-guarded-strings-migration.fixed b/tests/ui/rust-2024/reserved-guarded-strings-migration.fixed
new file mode 100644
index 00000000000..d92df7b5375
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings-migration.fixed
@@ -0,0 +1,99 @@
+//@ check-pass
+//@ run-rustfix
+//@ edition:2021
+
+#![warn(rust_2024_guarded_string_incompatible_syntax)]
+
+macro_rules! demo1 {
+    ( $a:tt ) => { println!("one tokens") };
+}
+
+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") };
+}
+
+macro_rules! demo5 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt ) => { println!("five tokens") };
+}
+
+macro_rules! demo6 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt ) => { println!("six tokens") };
+}
+
+
+fn main() {
+    demo1!("");
+    demo2!(# "");
+    demo3!(# ""#);
+    demo2!(# "foo");
+    demo3!(# "foo"#);
+    demo2!("foo"#);
+
+    demo3!(# # "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(# # # "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(# # "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo6!(# # # "foo"# #);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo4!("foo"# # #);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo2!(# "");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(# ""#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(# # "");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo2!(# "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(# # "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(# "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(# # "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo5!(# # "foo"# #);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+}
diff --git a/tests/ui/rust-2024/reserved-guarded-strings-migration.rs b/tests/ui/rust-2024/reserved-guarded-strings-migration.rs
new file mode 100644
index 00000000000..5905f2abe32
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings-migration.rs
@@ -0,0 +1,99 @@
+//@ check-pass
+//@ run-rustfix
+//@ edition:2021
+
+#![warn(rust_2024_guarded_string_incompatible_syntax)]
+
+macro_rules! demo1 {
+    ( $a:tt ) => { println!("one tokens") };
+}
+
+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") };
+}
+
+macro_rules! demo5 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt ) => { println!("five tokens") };
+}
+
+macro_rules! demo6 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt ) => { println!("six tokens") };
+}
+
+
+fn main() {
+    demo1!("");
+    demo2!(# "");
+    demo3!(# ""#);
+    demo2!(# "foo");
+    demo3!(# "foo"#);
+    demo2!("foo"#);
+
+    demo3!(## "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(### "foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(## "foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo6!(### "foo"##);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo4!("foo"###);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+
+    demo2!(#"");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(#""#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(##"");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo2!(#"foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(##"foo");
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo3!(#"foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo4!(##"foo"#);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    demo5!(##"foo"##);
+    //~^ WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+    //~| WARNING parsed as a guarded string in Rust 2024 [rust_2024_guarded_string_incompatible_syntax]
+    //~| WARNING hard error in Rust 2024
+}
diff --git a/tests/ui/rust-2024/reserved-guarded-strings-migration.stderr b/tests/ui/rust-2024/reserved-guarded-strings-migration.stderr
new file mode 100644
index 00000000000..d7f8e5c9b4b
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings-migration.stderr
@@ -0,0 +1,293 @@
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:40:12
+   |
+LL |     demo3!(## "foo");
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+note: the lint level is defined here
+  --> $DIR/reserved-guarded-strings-migration.rs:5:9
+   |
+LL | #![warn(rust_2024_guarded_string_incompatible_syntax)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# # "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:43:12
+   |
+LL |     demo4!(### "foo");
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# ## "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:43:13
+   |
+LL |     demo4!(### "foo");
+   |             ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(## # "foo");
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:48:12
+   |
+LL |     demo4!(## "foo"#);
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# # "foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:51:12
+   |
+LL |     demo6!(### "foo"##);
+   |            ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo6!(# ## "foo"##);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:51:13
+   |
+LL |     demo6!(### "foo"##);
+   |             ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo6!(## # "foo"##);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:51:21
+   |
+LL |     demo6!(### "foo"##);
+   |                     ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo6!(### "foo"# #);
+   |                      +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:59:17
+   |
+LL |     demo4!("foo"###);
+   |                 ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!("foo"# ##);
+   |                  +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:59:18
+   |
+LL |     demo4!("foo"###);
+   |                  ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!("foo"## #);
+   |                   +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:65:12
+   |
+LL |     demo2!(#"");
+   |            ^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo2!(# "");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:68:12
+   |
+LL |     demo3!(#""#);
+   |            ^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# ""#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:71:12
+   |
+LL |     demo3!(##"");
+   |            ^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# #"");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:71:13
+   |
+LL |     demo3!(##"");
+   |             ^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(## "");
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:76:12
+   |
+LL |     demo2!(#"foo");
+   |            ^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo2!(# "foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:79:12
+   |
+LL |     demo3!(##"foo");
+   |            ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# #"foo");
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:79:13
+   |
+LL |     demo3!(##"foo");
+   |             ^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(## "foo");
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:84:12
+   |
+LL |     demo3!(#"foo"#);
+   |            ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo3!(# "foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:87:12
+   |
+LL |     demo4!(##"foo"#);
+   |            ^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(# #"foo"#);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:87:13
+   |
+LL |     demo4!(##"foo"#);
+   |             ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo4!(## "foo"#);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:92:12
+   |
+LL |     demo5!(##"foo"##);
+   |            ^^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(# #"foo"##);
+   |             +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:92:13
+   |
+LL |     demo5!(##"foo"##);
+   |             ^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(## "foo"##);
+   |              +
+
+warning: will be parsed as a guarded string in Rust 2024
+  --> $DIR/reserved-guarded-strings-migration.rs:92:19
+   |
+LL |     demo5!(##"foo"##);
+   |                   ^^
+   |
+   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
+   = note: for more information, see issue #123735 <https://github.com/rust-lang/rust/issues/123735>
+help: insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
+   |
+LL |     demo5!(##"foo"# #);
+   |                    +
+
+warning: 22 warnings emitted
+
diff --git a/tests/ui/rust-2024/reserved-guarded-strings-via-macro-2.rs b/tests/ui/rust-2024/reserved-guarded-strings-via-macro-2.rs
new file mode 100644
index 00000000000..3f9f373ba22
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings-via-macro-2.rs
@@ -0,0 +1,18 @@
+//@ edition:2021
+//@ aux-build:reserved-guarded-strings-macro-2021.rs
+//@ aux-build:reserved-guarded-strings-macro-2024.rs
+
+extern crate reserved_guarded_strings_macro_2021 as m2021;
+extern crate reserved_guarded_strings_macro_2024 as m2024;
+
+fn main() {
+    // Ok:
+    m2021::number_of_tokens_in_a_guarded_string_literal!();
+    m2021::number_of_tokens_in_a_guarded_unterminated_string_literal!();
+
+    // Error, even though *this* crate is 2021:
+    m2024::number_of_tokens_in_a_guarded_string_literal!();
+    //~^ ERROR invalid string literal
+    m2024::number_of_tokens_in_a_guarded_unterminated_string_literal!();
+    //~^ ERROR invalid string literal
+}
diff --git a/tests/ui/rust-2024/reserved-guarded-strings-via-macro-2.stderr b/tests/ui/rust-2024/reserved-guarded-strings-via-macro-2.stderr
new file mode 100644
index 00000000000..1074c8a682b
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings-via-macro-2.stderr
@@ -0,0 +1,20 @@
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings-via-macro-2.rs:14:5
+   |
+LL |     m2024::number_of_tokens_in_a_guarded_string_literal!();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+   = note: this error originates in the macro `m2024::number_of_tokens_in_a_guarded_string_literal` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings-via-macro-2.rs:16:5
+   |
+LL |     m2024::number_of_tokens_in_a_guarded_unterminated_string_literal!();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+   = note: this error originates in the macro `m2024::number_of_tokens_in_a_guarded_unterminated_string_literal` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+
diff --git a/tests/ui/rust-2024/reserved-guarded-strings-via-macro.rs b/tests/ui/rust-2024/reserved-guarded-strings-via-macro.rs
new file mode 100644
index 00000000000..f9e3c1e3c51
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings-via-macro.rs
@@ -0,0 +1,12 @@
+//@ run-pass
+//@ compile-flags: -Zunstable-options
+//@ edition:2024
+//@ aux-build:reserved-guarded-strings-macro-2021.rs
+
+extern crate reserved_guarded_strings_macro_2021 as m2021;
+
+fn main() {
+    // Ok, even though *this* crate is 2024:
+    assert_eq!(m2021::number_of_tokens_in_a_guarded_string_literal!(), 3);
+    assert_eq!(m2021::number_of_tokens_in_a_guarded_unterminated_string_literal!(), 2);
+}
diff --git a/tests/ui/rust-2024/reserved-guarded-strings.rs b/tests/ui/rust-2024/reserved-guarded-strings.rs
new file mode 100644
index 00000000000..dab97039be0
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings.rs
@@ -0,0 +1,74 @@
+//@ compile-flags: -Zunstable-options
+//@ edition:2024
+// ignore-tidy-linelength
+
+macro_rules! demo1 {
+    ( $a:tt ) => { println!("one tokens") };
+}
+
+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") };
+}
+
+macro_rules! demo5 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt ) => { println!("five tokens") };
+}
+
+macro_rules! demo6 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt ) => { println!("six tokens") };
+}
+
+macro_rules! demo7 {
+    ( $a:tt $b:tt $c:tt $d:tt $e:tt $f:tt $g:tt ) => { println!("seven tokens") };
+}
+
+macro_rules! demon {
+    ( $($n:tt)* ) => { println!("unknown number of tokens") };
+}
+
+fn main() {
+    demo1!("");
+    demo2!(# "");
+    demo3!(# ""#);
+    demo2!(# "foo");
+    demo3!(# "foo"#);
+    demo2!("foo"#);
+
+    demo2!(blah"xx"); //~ ERROR prefix `blah` is unknown
+    demo2!(blah#"xx"#);
+    //~^ ERROR prefix `blah` is unknown
+    //~| ERROR invalid string literal
+
+    demo2!(## "foo"); //~ ERROR invalid string literal
+    demo3!("foo"###); //~ ERROR invalid string literal
+    demo3!(### "foo"); //~ ERROR invalid string literal
+    demo3!(## "foo"#); //~ ERROR invalid string literal
+    demo5!(### "foo"###);
+    //~^ ERROR invalid string literal
+    //~| ERROR invalid string literal
+
+    demo1!(#""); //~ ERROR invalid string literal
+    demo1!(#""#); //~ ERROR invalid string literal
+    demo1!(####""); //~ ERROR invalid string literal
+    demo1!(#"foo"); //~ ERROR invalid string literal
+    demo1!(###"foo"); //~ ERROR invalid string literal
+    demo1!(#"foo"#); //~ ERROR invalid string literal
+    demo1!(###"foo"#); //~ ERROR invalid string literal
+    demo1!(###"foo"##); //~ ERROR invalid string literal
+    demo1!(###"foo"###); //~ ERROR invalid string literal
+    demo2!(#"foo"###);
+    //~^ ERROR invalid string literal
+    //~| ERROR invalid string literal
+
+    // More than 255 hashes
+    demon!(####################################################################################################################################################################################################################################################################"foo");
+    //~^ ERROR invalid string literal
+}
diff --git a/tests/ui/rust-2024/reserved-guarded-strings.stderr b/tests/ui/rust-2024/reserved-guarded-strings.stderr
new file mode 100644
index 00000000000..f465ba7944a
--- /dev/null
+++ b/tests/ui/rust-2024/reserved-guarded-strings.stderr
@@ -0,0 +1,254 @@
+error: prefix `blah` is unknown
+  --> $DIR/reserved-guarded-strings.rs:45:12
+   |
+LL |     demo2!(blah"xx");
+   |            ^^^^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: consider inserting whitespace here
+   |
+LL |     demo2!(blah "xx");
+   |                +
+
+error: prefix `blah` is unknown
+  --> $DIR/reserved-guarded-strings.rs:46:12
+   |
+LL |     demo2!(blah#"xx"#);
+   |            ^^^^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: consider inserting whitespace here
+   |
+LL |     demo2!(blah #"xx"#);
+   |                +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:46:16
+   |
+LL |     demo2!(blah#"xx"#);
+   |                ^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo2!(blah# "xx"#);
+   |                 +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:50:12
+   |
+LL |     demo2!(## "foo");
+   |            ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo2!(# # "foo");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:51:17
+   |
+LL |     demo3!("foo"###);
+   |                 ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo3!("foo"# ##);
+   |                  +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:52:12
+   |
+LL |     demo3!(### "foo");
+   |            ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo3!(# ## "foo");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:53:12
+   |
+LL |     demo3!(## "foo"#);
+   |            ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo3!(# # "foo"#);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:54:12
+   |
+LL |     demo5!(### "foo"###);
+   |            ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo5!(# ## "foo"###);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:54:21
+   |
+LL |     demo5!(### "foo"###);
+   |                     ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo5!(### "foo"# ##);
+   |                      +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:58:12
+   |
+LL |     demo1!(#"");
+   |            ^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# "");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:59:12
+   |
+LL |     demo1!(#""#);
+   |            ^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ""#);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:60:12
+   |
+LL |     demo1!(####"");
+   |            ^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ###"");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:61:12
+   |
+LL |     demo1!(#"foo");
+   |            ^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# "foo");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:62:12
+   |
+LL |     demo1!(###"foo");
+   |            ^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ##"foo");
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:63:12
+   |
+LL |     demo1!(#"foo"#);
+   |            ^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# "foo"#);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:64:12
+   |
+LL |     demo1!(###"foo"#);
+   |            ^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ##"foo"#);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:65:12
+   |
+LL |     demo1!(###"foo"##);
+   |            ^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ##"foo"##);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:66:12
+   |
+LL |     demo1!(###"foo"###);
+   |            ^^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo1!(# ##"foo"###);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:67:12
+   |
+LL |     demo2!(#"foo"###);
+   |            ^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo2!(# "foo"###);
+   |             +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:67:19
+   |
+LL |     demo2!(#"foo"###);
+   |                   ^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demo2!(#"foo"## #);
+   |                    +
+
+error: invalid string literal
+  --> $DIR/reserved-guarded-strings.rs:72:12
+   |
+LL | ...n!(####################################################################################################################################################################################################################################################################"foo...
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: unprefixed guarded string literals are reserved for future use since Rust 2024
+help: consider inserting whitespace here
+   |
+LL |     demon!(# ###################################################################################################################################################################################################################################################################"foo");
+   |             +
+
+error: aborting due to 21 previous errors
+