ruby: only report on method calls

Interviewing a Ruby developer, I learned that
dealing with nil is common practice.
So alerts are mostly useful, if we can point to a place where this has gone wrong.

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

@@ -12,6 +12,8 @@ Consider the following example:
 
 <p>
 This will generate an alert on the last access to <code>m</code>, where it is not clear that the programmer intended to read from the local variable.
+In fact, the last access to <code>m</code> is a method call, and the value of the local variable is <code>nil</code>,
+so this will raise a <code>NoMethodError</code>.
 </p>
 
 </overview>
@@ -19,8 +21,7 @@ This will generate an alert on the last access to <code>m</code>, where it is no
 
 <p>
 Ensure that you check the control and data flow in the method carefully.
-Check that the variable reference is spelled correctly, perhaps the variable has been renamed and the reference needs to be updated.
-Another possibility is that an exception may be raised before the variable is assigned, in which case the read should be protected by a check for <code>nil</code>.
+Add a check for <code>nil</code> before the read, or rewrite the code to ensure that the variable is always initialized before being read.
 </p>
 
 </recommendation>
@@ -28,8 +29,8 @@ Another possibility is that an exception may be raised before the variable is as
 <references>
 
 
-<li>Wikipedia: <a href="http://en.wikipedia.org/wiki/Dead_store">Dead store</a>.</li>
-
+<li>https://www.rubyguides.com/: <a href="https://www.rubyguides.com/2018/01/ruby-nil/">Nil</a>.</li>
+<li>https://ruby-doc.org/: <a href="https://ruby-doc.org/core-2.6.5/NoMethodError.html">NoMethodError</a>.</li>
 
 
 </references>

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

@@ -77,7 +77,8 @@ class RelevantLocalVariableReadAccess extends LocalVariableReadAccess instanceof
   RelevantLocalVariableReadAccess() {
     not isInBooleanContext(this) and
     not isNilChecked(this) and
-    not isGuarded(this)
+    not isGuarded(this) and
+    this = any(MethodCall m).getReceiver()
   }
 }
 

ruby/ql/src/queries/variables/examples/UninitializedLocal.rb

@@ -5,10 +5,10 @@ end
 def foo
   m # calls m above
   if false
-      m = 0
+      m = "0"
       m # reads local variable m
   else
   end
-  m # reads uninitialized local variable m, `nil`
+  m.strip # reads uninitialized local variable m, `nil`, and crashes
   m2 # undefined local variable or method 'm2' for main (NameError)
 end

ruby/ql/test/query-tests/variables/UninitializedLocal/UninitializedLocal.rb

@@ -5,57 +5,57 @@ end
 def foo
   m # calls m above
   if false
-      m = 0
+      m = "0"
       m # reads local variable m
   else
   end
-  m  #$ Alert
+  m.strip  #$ Alert
   m2 # undefined local variable or method 'm2' for main (NameError)
 end
 
 def test_guards
-    if (a = 3 && a)  # OK - a is in a Boolean context
-        a
+    if (a = "3" && a)  # OK - a is in a Boolean context
+        a.strip
     end
-    if (a = 3) && a  # OK - a is assigned in the previous conjunct
-        a
+    if (a = "3") && a  # OK - a is assigned in the previous conjunct
+        a.strip
     end
-    if !(a = 3) or a  # OK - a is assigned in the previous conjunct
-        a
+    if !(a = "3") or a  # OK - a is assigned in the previous conjunct
+        a.strip
     end
     if false
-        b = 0
+        b = "0"
     end
     b.nil?
     b || 0  # OK
-    b&.m  # OK - safe navigation
-    b if b  # OK
+    b&.strip  # OK - safe navigation
+    b.strip if b  # OK
     b.close if b && !b.closed # OK
     b.blowup if b || !b.blownup #$ Alert
 
     if false
-        c = 0
+        c = "0"
     end
     unless c
         return
     end
-    c  # OK - given above unless
+    c.strip  # OK - given above unless
 
     if false
-        d = 0
+        d = "0"
     end
     if (d.nil?)
         return
     end
-    d # OK - given above check
+    d.strip # OK - given above check
 
     if false
-        e = 0
+        e = "0"
     end
     unless (!e.nil?)
         return
     end
-    e  # OK - given above unless
+    e.strip  # OK - given above unless
 end
 
 def test_loop
@@ -66,12 +66,12 @@ def test_loop
             set_a
         end   
     end until a  # OK
-    a  # OK - given previous until
+    a.strip  # OK - given previous until
 end
 
 def test_for
-    for i in 0..10  # OK - since 0..10 cannot raise
-        i
+    for i in ["foo", "bar"]  # OK - since 0..10 cannot raise
+        puts i.strip
     end
-    i  #$ SPURIOUS: Alert
+    i.strip  #$ SPURIOUS: Alert
 end