diagnostics: add regression test for #82081

Author: notriddle

src/test/ui/suggestions/suggest-mut-method-for-loop-hashmap.fixed

@@ -0,0 +1,21 @@
+// run-rustfix
+// https://github.com/rust-lang/rust/issues/82081
+
+use std::collections::HashMap;
+
+struct Test {
+    v: u32,
+}
+
+fn main() {
+    let mut map = HashMap::new();
+    map.insert("a", Test { v: 0 });
+
+    for (_k, mut v) in map.iter_mut() {
+        //~^ HELP use mutable method
+        //~| NOTE this iterator yields `&` references
+        v.v += 1;
+        //~^ ERROR cannot assign to `v.v`
+        //~| NOTE `v` is a `&` reference
+    }
+}

src/test/ui/suggestions/suggest-mut-method-for-loop-hashmap.rs

@@ -0,0 +1,21 @@
+// run-rustfix
+// https://github.com/rust-lang/rust/issues/82081
+
+use std::collections::HashMap;
+
+struct Test {
+    v: u32,
+}
+
+fn main() {
+    let mut map = HashMap::new();
+    map.insert("a", Test { v: 0 });
+
+    for (_k, mut v) in map.iter() {
+        //~^ HELP use mutable method
+        //~| NOTE this iterator yields `&` references
+        v.v += 1;
+        //~^ ERROR cannot assign to `v.v`
+        //~| NOTE `v` is a `&` reference
+    }
+}

src/test/ui/suggestions/suggest-mut-method-for-loop-hashmap.stderr

@@ -0,0 +1,15 @@
+error[E0594]: cannot assign to `v.v`, which is behind a `&` reference
+  --> $DIR/suggest-mut-method-for-loop-hashmap.rs:17:9
+   |
+LL |     for (_k, mut v) in map.iter() {
+   |                        ----------
+   |                        |   |
+   |                        |   help: use mutable method: `iter_mut()`
+   |                        this iterator yields `&` references
+...
+LL |         v.v += 1;
+   |         ^^^^^^^^ `v` is a `&` reference, so the data it refers to cannot be written
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.