fix rust-lang#101880: suggest let for assignment, and some code refactor

src/test/ui/suggestions/suggest-let-for-assignment.fixed

@@ -0,0 +1,17 @@
+// run-rustfix
+
+fn main() {
+    let demo = 1; //~ ERROR cannot find value `demo` in this scope
+    dbg!(demo); //~ ERROR cannot find value `demo` in this scope
+
+    let x = "x"; //~ ERROR cannot find value `x` in this scope
+    println!("x: {}", x); //~ ERROR cannot find value `x` in this scope
+
+    if x == "x" {
+        //~^ ERROR cannot find value `x` in this scope
+        println!("x is 1");
+    }
+
+    let y = 1 + 2; //~ ERROR cannot find value `y` in this scope
+    println!("y: {}", y); //~ ERROR cannot find value `y` in this scope
+}

src/test/ui/suggestions/suggest-let-for-assignment.rs

@@ -0,0 +1,17 @@
+// run-rustfix
+
+fn main() {
+    demo = 1; //~ ERROR cannot find value `demo` in this scope
+    dbg!(demo); //~ ERROR cannot find value `demo` in this scope
+
+    x = "x"; //~ ERROR cannot find value `x` in this scope
+    println!("x: {}", x); //~ ERROR cannot find value `x` in this scope
+
+    if x == "x" {
+        //~^ ERROR cannot find value `x` in this scope
+        println!("x is 1");
+    }
+
+    y = 1 + 2; //~ ERROR cannot find value `y` in this scope
+    println!("y: {}", y); //~ ERROR cannot find value `y` in this scope
+}

src/test/ui/suggestions/suggest-let-for-assignment.stderr

@@ -0,0 +1,60 @@
+error[E0425]: cannot find value `demo` in this scope
+  --> $DIR/suggest-let-for-assignment.rs:4:5
+   |
+LL |     demo = 1;
+   |     ^^^^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let demo = 1;
+   |     +++
+
+error[E0425]: cannot find value `demo` in this scope
+  --> $DIR/suggest-let-for-assignment.rs:5:10
+   |
+LL |     dbg!(demo);
+   |          ^^^^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/suggest-let-for-assignment.rs:7:5
+   |
+LL |     x = "x";
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = "x";
+   |     +++
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/suggest-let-for-assignment.rs:8:23
+   |
+LL |     println!("x: {}", x);
+   |                       ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/suggest-let-for-assignment.rs:10:8
+   |
+LL |     if x == "x" {
+   |        ^ not found in this scope
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/suggest-let-for-assignment.rs:15:5
+   |
+LL |     y = 1 + 2;
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let y = 1 + 2;
+   |     +++
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/suggest-let-for-assignment.rs:16:23
+   |
+LL |     println!("y: {}", y);
+   |                       ^ not found in this scope
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0425`.