Validate.hs: Disallow variable shadowing in rules.

In a rule like this:

```
Bar(x+y) :- Foo(x, y), var y = x.
```

there are two legitimate choices for handling the re-introduction of
variable `y` in the second clause: (1) let the new declaration shadow
the existing one in the rest of then rule, and (2) fail.

The existing implementation did neither of those an generated invalid
Rust code instead.

This commit implements option 2.  Note that we already allow variable
shadowing inside expressions, but this does not seem like a good choice
in a rule scope.  Rules contain implicit variable declarations (e.g.,
variable `x` above), making shadowing extra confusing for users.  We
therefore err on the side of caution and simply disallow such
conflicting declarations.

src/Language/DifferentialDatalog/Validate.hs

@@ -359,8 +359,13 @@ ruleRHSValidate :: (MonadError String me) => DatalogProgram -> Rule -> RuleRHS -
 ruleRHSValidate d rl@Rule{..} (RHSLiteral _ atom) idx =
     atomValidate d (CtxRuleRAtom rl idx) atom
 
-ruleRHSValidate _ _ RHSCondition{} _ = return ()
-ruleRHSValidate _ _ RHSFlatMap{} _ = return ()
+ruleRHSValidate d rl RHSCondition{..} i = do
+    let ctx = CtxRuleRCond rl i
+    mapM_ (\v -> checkNoVar (pos v) d ctx (name v))
+          (exprVarDecls d ctx rhsExpr)
+ruleRHSValidate d rl RHSFlatMap{..} i = do
+    let ctx = CtxRuleRFlatMap rl i
+    checkNoVar (pos rhsMapExpr) d ctx rhsVar
 ruleRHSValidate _ _ RHSInspect{} _ = return ()
 
 ruleRHSValidate d rl RHSGroupBy{} idx = do

test/datalog_tests/rules.fail.ast.expected

@@ -77,3 +77,11 @@ in
 variable 'x'
     var x = FlatMap(v).
                     ^
+
+error: ./test/datalog_tests/rules.fail.dl:10:5-10:11: Variable y already defined in this scope
+    var y = x.
+    ^^^^^^
+
+error: ./test/datalog_tests/rules.fail.dl:10:21-10:22: Variable y already defined in this scope
+    var y = FlatMap(y).
+                    ^

test/datalog_tests/rules.fail.dl

@@ -153,3 +153,25 @@ relation Flat(x: usize)
 Flat(x) :-
     Vecs(v),
     var x = FlatMap(v).
+
+//---
+
+// Variable shadowing in a rule.
+
+input relation Foo(x: usize, y: usize)
+output relation Bar(z: usize)
+
+Bar(x+y) :-
+    Foo(x, y),
+    var y = x.
+
+//---
+
+// Variable shadowing in a rule.
+
+input relation Foo(x: usize, y: Vec<usize>)
+output relation Bar(z: usize)
+
+Bar(x+y) :-
+    Foo(x, y),
+    var y = FlatMap(y).