Auto merge of #90460 - pietroalbini:bidi-stable, r=nikomatsakis,pietroalbini 1.56.1

[stable] Fix CVE-2021-42574 and prepare Rust 1.56.1

This PR implements new lints to mitigate the impact of [CVE-2021-42574], caused by the presence of bidirectional-override Unicode codepoints in the compiled source code. [See the advisory][advisory] for more information about the vulnerability.

The changes in this PR will be released later today as part of Rust 1.56.1.

[CVE-2021-42574]: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-42574
[advisory]: https://blog.rust-lang.org/2021/11/01/cve-2021-42574.html

2 files changed, 29 insertions, 0 deletions

diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 8fb678e2d20..e9cd30a058f 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -3416,3 +3416,31 @@ declare_lint! {
     Warn,
     "`break` expression with label and unlabeled loop as value expression"
 }
+
+declare_lint! {
+    /// The `text_direction_codepoint_in_comment` lint detects Unicode codepoints in comments that
+    /// change the visual representation of text on screen in a way that does not correspond to
+    /// their on memory representation.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(text_direction_codepoint_in_comment)]
+    /// fn main() {
+    ///     println!("{:?}"); // '‮');
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Unicode allows changing the visual flow of text on screen in order to support scripts that
+    /// are written right-to-left, but a specially crafted comment can make code that will be
+    /// compiled appear to be part of a comment, depending on the software used to read the code.
+    /// To avoid potential problems or confusion, such as in CVE-2021-42574, by default we deny
+    /// their use.
+    pub TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
+    Deny,
+    "invisible directionality-changing codepoints in comment"
+}
diff --git a/compiler/rustc_lint_defs/src/lib.rs b/compiler/rustc_lint_defs/src/lib.rs
index f89d531b5ef..feac2a7cfa4 100644
--- a/compiler/rustc_lint_defs/src/lib.rs
+++ b/compiler/rustc_lint_defs/src/lib.rs
@@ -306,6 +306,7 @@ pub enum BuiltinLintDiagnostics {
     TrailingMacro(bool, Ident),
     BreakWithLabelAndLoop(Span),
     NamedAsmLabel(String),
+    UnicodeTextFlow(Span, String),
 }
 
 /// Lints that are buffered up early on in the `Session` before the