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diff --git a/src/librustc_error_codes/error_codes/E0631.md b/src/librustc_error_codes/error_codes/E0631.md
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+This error indicates a type mismatch in closure arguments.
+
+Erroneous code example:
+
+```compile_fail,E0631
+fn test_strings(string_vec: Vec<String>) -> Vec<bool> {
+  string_vec
+    .iter()
+    .map(|arg: &i32| arg.eq("Test String"))
+    .collect()
+}
+```
+
+The closure passed to `map` expects a `&String` argument, since `some_vec`
+has the type `Vec<String>`.
+However, the closure argument is annotated as an `&i32`, which does not match
+the type of the iterable.
+
+This can be resolved by changing the type annotation or removing it entirely
+if it can be inferred.
+
+```
+fn test_strings(string_vec: Vec<String>) -> Vec<bool> {
+  string_vec
+    .iter()
+    .map(|arg| arg.eq("Test String"))
+    .collect()
+}
+```