Add long error for E0631 and update ui tests.

src/librustc_error_codes/error_codes.rs

@@ -347,6 +347,7 @@ E0622: include_str!("./error_codes/E0622.md"),
 E0623: include_str!("./error_codes/E0623.md"),
 E0624: include_str!("./error_codes/E0624.md"),
 E0626: include_str!("./error_codes/E0626.md"),
+E0631: include_str!("./error_codes/E0631.md"),
 E0633: include_str!("./error_codes/E0633.md"),
 E0635: include_str!("./error_codes/E0635.md"),
 E0636: include_str!("./error_codes/E0636.md"),
@@ -580,7 +581,6 @@ E0745: include_str!("./error_codes/E0745.md"),
     // rustc_const_unstable attribute must be paired with stable/unstable
     // attribute
     E0630,
-    E0631, // type mismatch in closure arguments
     E0632, // cannot provide explicit generic arguments when `impl Trait` is
            // used in argument position
     E0634, // type has conflicting packed representaton hints

src/librustc_error_codes/error_codes/E0631.md

@@ -0,0 +1,29 @@
+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()
+}
+```