Rollup merge of #66155 - GuillaumeGomez:long-err-explanation-E0594, r=Dylan-DPC

Add long error explanation for E0594

Part of #61137.

r? @Dylan-DPC

1 files changed, 23 insertions, 0 deletions

diff --git a/src/librustc_error_codes/error_codes/E0594.md b/src/librustc_error_codes/error_codes/E0594.md
new file mode 100644
index 00000000000..ad8eb631e63
--- /dev/null
+++ b/src/librustc_error_codes/error_codes/E0594.md
@@ -0,0 +1,23 @@
+A non-mutable value was assigned a value.
+
+Erroneous code example:
+
+```compile_fail,E0594
+struct SolarSystem {
+    earth: i32,
+}
+
+let ss = SolarSystem { earth: 3 };
+ss.earth = 2; // error!
+```
+
+To fix this error, declare `ss` as mutable by using the `mut` keyword:
+
+```
+struct SolarSystem {
+    earth: i32,
+}
+
+let mut ss = SolarSystem { earth: 3 }; // declaring `ss` as mutable
+ss.earth = 2; // ok!
+```