Use different numbers of `#`s when expanding documentation comments

Any documentation comments that contain raw-string-looking sequences may
pretty-print invalid code when expanding them, as the current logic
always uses the `r"literal"` form, without appending any `#`s.

This commit calculates the minimum number of `#`s required to wrap a
comment correctly and appends `#`s appropriately.

Fixes #27489.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index 0917346213f..b7afd12d8e5 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -365,6 +365,9 @@ pub mod builtin {
     /// stringification of all the tokens passed to the macro. No restrictions
     /// are placed on the syntax of the macro invocation itself.
     ///
+    /// Note that the expanded results of the input tokens may change in the
+    /// future. You should be careful if you rely on the output.
+    ///
     /// # Examples
     ///
     /// ```