Adjust assign-imm-local-twice.rs

- Document the test intention, including both the language semantics it
  is checking, as well as the borrowck diagnostics that this is
  exercising.
- Tag this test with `//@ run-rustfix` as this ui test exercises a
  suggestion diagnostics to make an immutable local var mutable.
- Minor error annotation reformatting.

Author: jieyouxu

tests/ui/assign-imm-local-twice.fixed

@@ -0,0 +1,21 @@
+//! Check that we do not allow assigning twice to an immutable variable. This test also checks a
+//! few pieces of borrowck diagnostics:
+//!
+//! - A multipart borrowck diagnostics that points out the first assignment to an immutable
+//!   variable, alongside violating assignments that follow subsequently.
+//! - A suggestion diagnostics to make the immutable binding mutable.
+
+//@ run-rustfix
+
+fn main() {
+    let mut v: isize;
+    //~^ HELP consider making this binding mutable
+    //~| SUGGESTION mut
+    v = 1;
+    //~^ NOTE first assignment
+    println!("v={}", v);
+    v = 2;
+    //~^ ERROR cannot assign twice to immutable variable
+    //~| NOTE cannot assign twice to immutable
+    println!("v={}", v);
+}

tests/ui/assign-imm-local-twice.rs

@@ -1,13 +1,21 @@
-fn test() {
+//! Check that we do not allow assigning twice to an immutable variable. This test also checks a
+//! few pieces of borrowck diagnostics:
+//!
+//! - A multipart borrowck diagnostics that points out the first assignment to an immutable
+//!   variable, alongside violating assignments that follow subsequently.
+//! - A suggestion diagnostics to make the immutable binding mutable.
+
+//@ run-rustfix
+
+fn main() {
     let v: isize;
     //~^ HELP consider making this binding mutable
     //~| SUGGESTION mut
-    v = 1; //~ NOTE first assignment
+    v = 1;
+    //~^ NOTE first assignment
     println!("v={}", v);
-    v = 2; //~ ERROR cannot assign twice to immutable variable
-           //~| NOTE cannot assign twice to immutable
+    v = 2;
+    //~^ ERROR cannot assign twice to immutable variable
+    //~| NOTE cannot assign twice to immutable
     println!("v={}", v);
 }
-
-fn main() {
-}

tests/ui/assign-imm-local-twice.stderr

@@ -1,9 +1,9 @@
 error[E0384]: cannot assign twice to immutable variable `v`
-  --> $DIR/assign-imm-local-twice.rs:7:5
+  --> $DIR/assign-imm-local-twice.rs:17:5
    |
 LL |     v = 1;
    |     ----- first assignment to `v`
-LL |     println!("v={}", v);
+...
 LL |     v = 2;
    |     ^^^^^ cannot assign twice to immutable variable
    |