[naga] Ensure that FooResult expressions are correctly populated.

Make Naga module validation require that `CallResult` and
`AtomicResult` expressions are indeed visited by exactly one `Call` /
`Atomic` statement.

naga/src/valid/function.rs

@@ -22,6 +22,8 @@ pub enum CallError {
     },
     #[error("Result expression {0:?} has already been introduced earlier")]
     ResultAlreadyInScope(Handle<crate::Expression>),
+    #[error("Result expression {0:?} is populated by multiple `Call` statements")]
+    ResultAlreadyPopulated(Handle<crate::Expression>),
     #[error("Result value is invalid")]
     ResultValue(#[source] ExpressionError),
     #[error("Requires {required} arguments, but {seen} are provided")]
@@ -45,6 +47,8 @@ pub enum AtomicError {
     InvalidOperand(Handle<crate::Expression>),
     #[error("Result type for {0:?} doesn't match the statement")]
     ResultTypeMismatch(Handle<crate::Expression>),
+    #[error("Result expression {0:?} is populated by multiple `Atomic` statements")]
+    ResultAlreadyPopulated(Handle<crate::Expression>),
 }
 
 #[derive(Clone, Debug, thiserror::Error)]
@@ -174,6 +178,8 @@ pub enum FunctionError {
     InvalidSubgroup(#[from] SubgroupError),
     #[error("Emit statement should not cover \"result\" expressions like {0:?}")]
     EmitResult(Handle<crate::Expression>),
+    #[error("Expression not visited by the appropriate statement")]
+    UnvisitedExpression(Handle<crate::Expression>),
 }
 
 bitflags::bitflags! {
@@ -305,7 +311,13 @@ impl super::Validator {
             }
             match context.expressions[expr] {
                 crate::Expression::CallResult(callee)
-                    if fun.result.is_some() && callee == function => {}
+                    if fun.result.is_some() && callee == function =>
+                {
+                    if !self.needs_visit.remove(expr.index()) {
+                        return Err(CallError::ResultAlreadyPopulated(expr)
+                            .with_span_handle(expr, context.expressions));
+                    }
+                }
                 _ => {
                     return Err(CallError::ExpressionMismatch(result)
                         .with_span_handle(expr, context.expressions))
@@ -397,7 +409,14 @@ impl super::Validator {
                         }
                         _ => false,
                     }
-                } => {}
+                } =>
+            {
+                if !self.needs_visit.remove(result.index()) {
+                    return Err(AtomicError::ResultAlreadyPopulated(result)
+                        .with_span_handle(result, context.expressions)
+                        .into_other());
+                }
+            }
             _ => {
                 return Err(AtomicError::ResultTypeMismatch(result)
                     .with_span_handle(result, context.expressions)
@@ -1290,11 +1309,20 @@ impl super::Validator {
 
         self.valid_expression_set.clear();
         self.valid_expression_list.clear();
+        self.needs_visit.clear();
         for (handle, expr) in fun.expressions.iter() {
             if expr.needs_pre_emit() {
                 self.valid_expression_set.insert(handle.index());
             }
             if self.flags.contains(super::ValidationFlags::EXPRESSIONS) {
+                // Mark expressions that need to be visited by a particular kind of
+                // statement.
+                if let crate::Expression::CallResult(_) | crate::Expression::AtomicResult { .. } =
+                    *expr
+                {
+                    self.needs_visit.insert(handle.index());
+                }
+
                 match self.validate_expression(
                     handle,
                     expr,
@@ -1321,6 +1349,15 @@ impl super::Validator {
                 )?
                 .stages;
             info.available_stages &= stages;
+
+            if self.flags.contains(super::ValidationFlags::EXPRESSIONS) {
+                if let Some(unvisited) = self.needs_visit.iter().next() {
+                    let index = std::num::NonZeroU32::new(unvisited as u32 + 1).unwrap();
+                    let handle = Handle::new(index);
+                    return Err(FunctionError::UnvisitedExpression(handle)
+                        .with_span_handle(handle, &fun.expressions));
+                }
+            }
         }
         Ok(info)
     }

naga/src/valid/mod.rs

@@ -246,6 +246,26 @@ pub struct Validator {
     valid_expression_set: BitSet,
     override_ids: FastHashSet<u16>,
     allow_overrides: bool,
+
+    /// A checklist of expressions that must be visited by a specific kind of
+    /// statement.
+    ///
+    /// For example:
+    ///
+    /// - [`CallResult`] expressions must be visited by a [`Call`] statement.
+    /// - [`AtomicResult`] expressions must be visited by an [`Atomic`] statement.
+    ///
+    /// Be sure not to remove any [`Expression`] handle from this set unless
+    /// you've explicitly checked that it is the right kind of expression for
+    /// the visiting [`Statement`].
+    ///
+    /// [`CallResult`]: crate::Expression::CallResult
+    /// [`Call`]: crate::Statement::Call
+    /// [`AtomicResult`]: crate::Expression::AtomicResult
+    /// [`Atomic`]: crate::Statement::Atomic
+    /// [`Expression`]: crate::Expression
+    /// [`Statement`]: crate::Statement
+    needs_visit: BitSet,
 }
 
 #[derive(Clone, Debug, thiserror::Error)]
@@ -398,6 +418,7 @@ impl Validator {
             valid_expression_set: BitSet::new(),
             override_ids: FastHashSet::default(),
             allow_overrides: true,
+            needs_visit: BitSet::new(),
         }
     }
 

naga/tests/validation.rs

@@ -1,18 +1,30 @@
 use naga::{valid, Expression, Function, Scalar};
 
+/// Validation should fail if `AtomicResult` expressions are not
+/// populated by `Atomic` statements.
 #[test]
-fn emit_atomic_result() {
+fn populate_atomic_result() {
     use naga::{Module, Type, TypeInner};
 
-    // We want to ensure that the *only* problem with the code is the
-    // use of an `Emit` statement instead of an `Atomic` statement. So
-    // validate two versions of the module varying only in that
-    // aspect.
-    //
-    // Looking at uses of the `atomic` makes it easy to identify the
-    // differences between the two variants.
-    fn variant(
-        atomic: bool,
+    /// Different variants of the test case that we want to exercise.
+    enum Variant {
+        /// An `AtomicResult` expression with an `Atomic` statement
+        /// that populates it: valid.
+        Atomic,
+
+        /// An `AtomicResult` expression visited by an `Emit`
+        /// statement: invalid.
+        Emit,
+
+        /// An `AtomicResult` expression visited by no statement at
+        /// all: invalid
+        None,
+    }
+
+    // Looking at uses of `variant` should make it easy to identify
+    // the differences between the test cases.
+    fn try_variant(
+        variant: Variant,
     ) -> Result<naga::valid::ModuleInfo, naga::WithSpan<naga::valid::ValidationError>> {
         let span = naga::Span::default();
         let mut module = Module::default();
@@ -56,21 +68,25 @@ fn emit_atomic_result() {
             span,
         );
 
-        if atomic {
-            fun.body.push(
-                naga::Statement::Atomic {
-                    pointer: ex_global,
-                    fun: naga::AtomicFunction::Add,
-                    value: ex_42,
-                    result: ex_result,
-                },
-                span,
-            );
-        } else {
-            fun.body.push(
-                naga::Statement::Emit(naga::Range::new_from_bounds(ex_result, ex_result)),
-                span,
-            );
+        match variant {
+            Variant::Atomic => {
+                fun.body.push(
+                    naga::Statement::Atomic {
+                        pointer: ex_global,
+                        fun: naga::AtomicFunction::Add,
+                        value: ex_42,
+                        result: ex_result,
+                    },
+                    span,
+                );
+            }
+            Variant::Emit => {
+                fun.body.push(
+                    naga::Statement::Emit(naga::Range::new_from_bounds(ex_result, ex_result)),
+                    span,
+                );
+            }
+            Variant::None => {}
         }
 
         module.functions.append(fun, span);
@@ -82,23 +98,34 @@ fn emit_atomic_result() {
         .validate(&module)
     }
 
-    variant(true).expect("module should validate");
-    assert!(variant(false).is_err());
+    try_variant(Variant::Atomic).expect("module should validate");
+    assert!(try_variant(Variant::Emit).is_err());
+    assert!(try_variant(Variant::None).is_err());
 }
 
 #[test]
-fn emit_call_result() {
+fn populate_call_result() {
     use naga::{Module, Type, TypeInner};
 
-    // We want to ensure that the *only* problem with the code is the
-    // use of an `Emit` statement instead of a `Call` statement. So
-    // validate two versions of the module varying only in that
-    // aspect.
-    //
-    // Looking at uses of the `call` makes it easy to identify the
-    // differences between the two variants.
-    fn variant(
-        call: bool,
+    /// Different variants of the test case that we want to exercise.
+    enum Variant {
+        /// A `CallResult` expression with an `Call` statement that
+        /// populates it: valid.
+        Call,
+
+        /// A `CallResult` expression visited by an `Emit` statement:
+        /// invalid.
+        Emit,
+
+        /// A `CallResult` expression visited by no statement at all:
+        /// invalid
+        None,
+    }
+
+    // Looking at uses of `variant` should make it easy to identify
+    // the differences between the test cases.
+    fn try_variant(
+        variant: Variant,
     ) -> Result<naga::valid::ModuleInfo, naga::WithSpan<naga::valid::ValidationError>> {
         let span = naga::Span::default();
         let mut module = Module::default();
@@ -130,20 +157,24 @@ fn emit_call_result() {
             .expressions
             .append(Expression::CallResult(fun_callee), span);
 
-        if call {
-            fun_caller.body.push(
-                naga::Statement::Call {
-                    function: fun_callee,
-                    arguments: vec![],
-                    result: Some(ex_result),
-                },
-                span,
-            );
-        } else {
-            fun_caller.body.push(
-                naga::Statement::Emit(naga::Range::new_from_bounds(ex_result, ex_result)),
-                span,
-            );
+        match variant {
+            Variant::Call => {
+                fun_caller.body.push(
+                    naga::Statement::Call {
+                        function: fun_callee,
+                        arguments: vec![],
+                        result: Some(ex_result),
+                    },
+                    span,
+                );
+            }
+            Variant::Emit => {
+                fun_caller.body.push(
+                    naga::Statement::Emit(naga::Range::new_from_bounds(ex_result, ex_result)),
+                    span,
+                );
+            }
+            Variant::None => {}
         }
 
         module.functions.append(fun_caller, span);
@@ -155,8 +186,9 @@ fn emit_call_result() {
         .validate(&module)
     }
 
-    variant(true).expect("should validate");
-    assert!(variant(false).is_err());
+    try_variant(Variant::Call).expect("should validate");
+    assert!(try_variant(Variant::Emit).is_err());
+    assert!(try_variant(Variant::None).is_err());
 }
 
 #[test]