Rollup merge of #97856 - compiler-errors:bad-let-suggestions, r=estebank

Don't suggest adding `let` in certain `if` conditions

Avoid being too eager to suggest `let` in an `if` condition with an `=`, namely when the LHS of the `=` isn't even valid as a pattern (to a first degree approximation).

This heustic I came up with kinda sucks. Let me know if it needs to be refined.

compiler/rustc_ast/src/ast.rs

@@ -1278,6 +1278,22 @@ impl Expr {
             },
         )
     }
+
+    // To a first-order approximation, is this a pattern
+    pub fn is_approximately_pattern(&self) -> bool {
+        match &self.peel_parens().kind {
+            ExprKind::Box(_)
+            | ExprKind::Array(_)
+            | ExprKind::Call(_, _)
+            | ExprKind::Tup(_)
+            | ExprKind::Lit(_)
+            | ExprKind::Range(_, _, _)
+            | ExprKind::Underscore
+            | ExprKind::Path(_, _)
+            | ExprKind::Struct(_) => true,
+            _ => false,
+        }
+    }
 }
 
 /// Limit types of a range (inclusive or exclusive)