From 698f8c2f01ea549d77d7dc3338a12e04c11057b9 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Wed, 17 Apr 2024 14:02:58 +0200
Subject: Adding upstream version 1.64.0+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../rustc_error_codes/src/error_codes/E0389.md     | 63 ++++++++++++++++++++++
 1 file changed, 63 insertions(+)
 create mode 100644 compiler/rustc_error_codes/src/error_codes/E0389.md

(limited to 'compiler/rustc_error_codes/src/error_codes/E0389.md')

diff --git a/compiler/rustc_error_codes/src/error_codes/E0389.md b/compiler/rustc_error_codes/src/error_codes/E0389.md
new file mode 100644
index 000000000..9f064e44c
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0389.md
@@ -0,0 +1,63 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+An attempt was made to mutate data using a non-mutable reference. This
+commonly occurs when attempting to assign to a non-mutable reference of a
+mutable reference (`&(&mut T)`).
+
+Erroneous code example:
+
+```compile_fail
+struct FancyNum {
+    num: u8,
+}
+
+fn main() {
+    let mut fancy = FancyNum{ num: 5 };
+    let fancy_ref = &(&mut fancy);
+    fancy_ref.num = 6; // error: cannot assign to data in a `&` reference
+    println!("{}", fancy_ref.num);
+}
+```
+
+Here, `&mut fancy` is mutable, but `&(&mut fancy)` is not. Creating an
+immutable reference to a value borrows it immutably. There can be multiple
+references of type `&(&mut T)` that point to the same value, so they must be
+immutable to prevent multiple mutable references to the same value.
+
+To fix this, either remove the outer reference:
+
+```
+struct FancyNum {
+    num: u8,
+}
+
+fn main() {
+    let mut fancy = FancyNum{ num: 5 };
+
+    let fancy_ref = &mut fancy;
+    // `fancy_ref` is now &mut FancyNum, rather than &(&mut FancyNum)
+
+    fancy_ref.num = 6; // No error!
+
+    println!("{}", fancy_ref.num);
+}
+```
+
+Or make the outer reference mutable:
+
+```
+struct FancyNum {
+    num: u8
+}
+
+fn main() {
+    let mut fancy = FancyNum{ num: 5 };
+
+    let fancy_ref = &mut (&mut fancy);
+    // `fancy_ref` is now &mut(&mut FancyNum), rather than &(&mut FancyNum)
+
+    fancy_ref.num = 6; // No error!
+
+    println!("{}", fancy_ref.num);
+}
+```
-- 
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