Auto merge of #101362 - compiler-errors:unnecessary-let, r=cjgillot

Suggest removing unnecessary prefix let in patterns

Helps with #101291, though I think `@estebank` probably wants this:

> Finally, I think it'd be nice if we could detect that we don't know for sure and "just" swallow the rest of the expression (find the next ; accounting for nested braces) or the end of the item (easier).

... to be implemented before we close that issue out completely.

src/test/ui/parser/unnecessary-let.rs

@@ -0,0 +1,11 @@
+fn main() {
+    for let x of [1, 2, 3] {}
+    //~^ ERROR expected pattern, found `let`
+    //~| ERROR missing `in` in `for` loop
+
+    match 1 {
+        let 1 => {}
+        //~^ ERROR expected pattern, found `let`
+        _ => {}
+    }
+}

src/test/ui/parser/unnecessary-let.stderr

@@ -0,0 +1,20 @@
+error: expected pattern, found `let`
+  --> $DIR/unnecessary-let.rs:2:9
+   |
+LL |     for let x of [1, 2, 3] {}
+   |         ^^^ help: remove the unnecessary `let` keyword
+
+error: missing `in` in `for` loop
+  --> $DIR/unnecessary-let.rs:2:15
+   |
+LL |     for let x of [1, 2, 3] {}
+   |               ^^ help: try using `in` here instead
+
+error: expected pattern, found `let`
+  --> $DIR/unnecessary-let.rs:7:9
+   |
+LL |         let 1 => {}
+   |         ^^^ help: remove the unnecessary `let` keyword
+
+error: aborting due to 3 previous errors
+