ruby: improve help file

This has improved autofixes
I hope it also helps humans

Author: yoff

ruby/ql/src/queries/variables/UninitializedLocal.md

@@ -0,0 +1,41 @@
+# Method call on `nil`
+
+## Description
+In Ruby, it is not necessary to explicitly initialize variables.
+If a local variable has not been explicitly initialized, it will have the value `nil`. If this happens unintended, though, the variable will not represent an object with the expected methods, and a method call on the variable will raise a `NoMethodError`.
+
+## Recommendation
+
+Ensure that the variable cannot be `nil` at the point hightligted by the alert.
+This can be achieved by using a safe navigation or adding a check for `nil`.
+
+Note: You do not need to explicitly initialize the variable, if you can make the program deal with the possible `nil` value. In particular, initializing the variable to `nil` will have no effect, as this is already the value of the variable. If `nil` is the only possibly default value, you need to handle the `nil` value instead of initializing the variable.
+
+## Examples
+
+In the following code, the call to `create_file` may fail and then the call `f.close` will raise a `NoMethodError` since `f` will be `nil` at that point.
+
+```ruby
+def dump(x)
+  f = create_file
+  f.puts(x)
+ensure
+  f.close
+end
+```
+
+We can fix this by using safe navigation:
+```ruby
+def dump(x)
+  f = create_file
+  f.puts(x)
+ensure
+  f&.close
+end
+```
+
+## References
+
+- https://www.rubyguides.com/: [Nil](https://www.rubyguides.com/2018/01/ruby-nil/)
+- https://ruby-doc.org/: [NoMethodError](https://ruby-doc.org/core-2.6.5/NoMethodError.html)
+

ruby/ql/src/queries/variables/UninitializedLocal.qhelp

@@ -72,13 +72,24 @@ private predicate isNilChecked(LocalVariableReadAccess read) {
   )
 }
 
+/**
+ * Holds if `name` is the name of a method defined on `nil`.
+ * See https://ruby-doc.org/core-2.5.8/NilClass.html
+ */
+private predicate isNilMethodName(string name) {
+  name in [
+      "inspect", "instance_of?", "is_a?", "kind_of?", "method", "nil?", "rationalize", "to_a",
+      "to_c", "to_f", "to_h", "to_i", "to_r", "to_s"
+    ]
+}
+
 class RelevantLocalVariableReadAccess extends LocalVariableReadAccess instanceof TVariableAccessReal
 {
   RelevantLocalVariableReadAccess() {
     not isInBooleanContext(this) and
     not isNilChecked(this) and
     not isGuarded(this) and
-    this = any(MethodCall m).getReceiver()
+    this = any(MethodCall m | not isNilMethodName(m.getMethodName())).getReceiver()
   }
 }