1 files changed, 29 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0510.md b/compiler/rustc_error_codes/src/error_codes/E0510.md
new file mode 100644
index 000000000..e045e04bd
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0510.md
@@ -0,0 +1,29 @@
+The matched value was assigned in a match guard.
+
+Erroneous code example:
+
+```compile_fail,E0510
+let mut x = Some(0);
+match x {
+    None => {}
+    Some(_) if { x = None; false } => {} // error!
+    Some(_) => {}
+}
+```
+
+When matching on a variable it cannot be mutated in the match guards, as this
+could cause the match to be non-exhaustive.
+
+Here executing `x = None` would modify the value being matched and require us
+to go "back in time" to the `None` arm. To fix it, change the value in the match
+arm:
+
+```
+let mut x = Some(0);
+match x {
+    None => {}
+    Some(_) => {
+        x = None; // ok!
+    }
+}
+```