[naga wgsl-in] Handle automatic type conversions for switch selector and case expressions

This allows abstract-typed expressions to be used for some or all of
the switch selector and case selectors. If these are all not
convertible to the same concrete scalar integer type we return an
error. If all the selector expressions are abstract then they are
concretized to i32.

The note previously provided by the relevant error message, suggesting
adding or removing the `u` suffix from case values, has been
removed. While useful for simple literal values, it was comically
incorrect for more complex case expressions. The error message should
still be useful enough to allow the user to easily identify the
problem.

Author: jamienicol

CHANGELOG.md

@@ -200,6 +200,7 @@ By @Vecvec in [#6905](https://github.com/gfx-rs/wgpu/pull/6905), [#7086](https:/
 - Allow template lists to have a trailing comma. By @KentSlaney in [#7142](https://github.com/gfx-rs/wgpu/pull/7142).
 - Allow WGSL const declarations to have abstract types. By @jamienicol in [#7055](https://github.com/gfx-rs/wgpu/pull/7055) and [#7222](https://github.com/gfx-rs/wgpu/pull/7222).
 - Allows override-sized arrays to resolve to the same size without causing the type arena to panic. By @KentSlaney in [#7082](https://github.com/gfx-rs/wgpu/pull/7082).
+- Allow abstract types to be used for WGSL switch statement selector and case selector expressions. By @jamienicol in [#7250](https://github.com/gfx-rs/wgpu/pull/7250).
 
 #### General
 

naga/src/front/wgsl/error.rs

@@ -256,8 +256,13 @@ pub(crate) enum Error<'a> {
         /// the same identifier as `ident`, above.
         path: Box<[(Span, Span)]>,
     },
-    InvalidSwitchValue {
-        uint: bool,
+    InvalidSwitchSelector {
+        span: Span,
+    },
+    InvalidSwitchCase {
+        span: Span,
+    },
+    SwitchCaseTypeMismatch {
         span: Span,
     },
     CalledEntryPoint(Span),
@@ -772,26 +777,32 @@ impl<'a> Error<'a> {
                     .collect(),
                 notes: vec![],
             },
-            Error::InvalidSwitchValue { uint, span } => ParseError {
-                message: "invalid switch value".to_string(),
+            Error::InvalidSwitchSelector { span } => ParseError {
+                message: "invalid `switch` selector".to_string(),
                 labels: vec![(
                     span,
-                    if uint {
-                        "expected unsigned integer"
-                    } else {
-                        "expected signed integer"
-                    }
+                    "`switch` selector must be a scalar integer"
                     .into(),
                 )],
-                notes: vec![if uint {
-                    format!("suffix the integer with a `u`: `{}u`", &source[span])
-                } else {
-                    let span = span.to_range().unwrap();
-                    format!(
-                        "remove the `u` suffix: `{}`",
-                        &source[span.start..span.end - 1]
-                    )
-                }],
+                notes: vec![],
+            },
+            Error::InvalidSwitchCase { span } => ParseError {
+                message: "invalid `switch` case selector value".to_string(),
+                labels: vec![(
+                    span,
+                    "`switch` case selector must be a scalar integer const expression"
+                    .into(),
+                )],
+                notes: vec![],
+            },
+            Error::SwitchCaseTypeMismatch { span } => ParseError {
+                message: "invalid `switch` case selector value".to_string(),
+                labels: vec![(
+                    span,
+                    "`switch` case selector must have the same type as the `switch` selector expression"
+                    .into(),
+                )],
+                notes: vec![],
             },
             Error::CalledEntryPoint(span) => ParseError {
                 message: "entry point cannot be called".to_string(),

naga/src/front/wgsl/lower/mod.rs

@@ -1630,30 +1630,76 @@ impl<'source, 'temp> Lowerer<'source, 'temp> {
                 emitter.start(&ctx.function.expressions);
 
                 let mut ectx = ctx.as_expression(block, &mut emitter);
-                let selector = self.expression(selector, &mut ectx)?;
 
-                let uint =
-                    resolve_inner!(ectx, selector).scalar_kind() == Some(crate::ScalarKind::Uint);
+                // Determine the scalar type of the selector and case expressions, find the
+                // consensus type for automatic conversion, then convert them.
+                let (mut exprs, spans) = core::iter::once(selector)
+                    .chain(cases.iter().filter_map(|case| match case.value {
+                        ast::SwitchValue::Expr(expr) => Some(expr),
+                        ast::SwitchValue::Default => None,
+                    }))
+                    .enumerate()
+                    .map(|(i, expr)| {
+                        let span = ectx.ast_expressions.get_span(expr);
+                        let expr = self.expression_for_abstract(expr, &mut ectx)?;
+                        let ty = resolve_inner!(ectx, expr);
+                        match *ty {
+                            crate::TypeInner::Scalar(
+                                crate::Scalar::I32
+                                | crate::Scalar::U32
+                                | crate::Scalar::ABSTRACT_INT,
+                            ) => Ok((expr, span)),
+                            _ => match i {
+                                0 => Err(Error::InvalidSwitchSelector { span }),
+                                _ => Err(Error::InvalidSwitchCase { span }),
+                            },
+                        }
+                    })
+                    .collect::<Result<(Vec<_>, Vec<_>), _>>()?;
+
+                let mut consensus =
+                    ectx.automatic_conversion_consensus(&exprs)
+                        .map_err(|span_idx| Error::SwitchCaseTypeMismatch {
+                            span: spans[span_idx],
+                        })?;
+                // Concretize to I32 if the selector and all cases were abstract
+                if consensus == crate::Scalar::ABSTRACT_INT {
+                    consensus = crate::Scalar::I32;
+                }
+                for expr in &mut exprs {
+                    ectx.convert_to_leaf_scalar(expr, consensus)?;
+                }
+
                 block.extend(emitter.finish(&ctx.function.expressions));
 
+                let mut exprs = exprs.into_iter();
+                let selector = exprs
+                    .next()
+                    .expect("First element should be selector expression");
+
                 let cases = cases
                     .iter()
                     .map(|case| {
                         Ok(crate::SwitchCase {
                             value: match case.value {
                                 ast::SwitchValue::Expr(expr) => {
                                     let span = ctx.ast_expressions.get_span(expr);
-                                    let expr =
-                                        self.expression(expr, &mut ctx.as_global().as_const())?;
-                                    match ctx.module.to_ctx().eval_expr_to_literal(expr) {
-                                        Some(crate::Literal::I32(value)) if !uint => {
+                                    let expr = exprs.next().expect(
+                                        "Should yield expression for each SwitchValue::Expr case",
+                                    );
+                                    match ctx
+                                        .module
+                                        .to_ctx()
+                                        .eval_expr_to_literal_from(expr, &ctx.function.expressions)
+                                    {
+                                        Some(crate::Literal::I32(value)) => {
                                             crate::SwitchValue::I32(value)
                                         }
-                                        Some(crate::Literal::U32(value)) if uint => {
+                                        Some(crate::Literal::U32(value)) => {
                                             crate::SwitchValue::U32(value)
                                         }
                                         _ => {
-                                            return Err(Error::InvalidSwitchValue { uint, span });
+                                            return Err(Error::InvalidSwitchCase { span });
                                         }
                                     }
                                 }

naga/src/front/wgsl/tests.rs

@@ -322,9 +322,9 @@ fn parse_parentheses_switch() {
     parse_str(
         "
         fn main() {
-            var pos: f32;
-            switch pos > 1.0 {
-                default: { pos = 3.0; }
+            var pos: i32;
+            switch pos + 1 {
+                default: { pos = 3; }
             }
         }
     ",

naga/src/proc/mod.rs

@@ -456,7 +456,7 @@ impl GlobalCtx<'_> {
         self.eval_expr_to_literal_from(handle, self.global_expressions)
     }
 
-    fn eval_expr_to_literal_from(
+    pub(super) fn eval_expr_to_literal_from(
         &self,
         handle: crate::Handle<crate::Expression>,
         arena: &crate::Arena<crate::Expression>,

naga/tests/in/wgsl/control-flow.wgsl

@@ -79,6 +79,41 @@ fn switch_case_break() {
     return;
 }
 
+fn switch_selector_type_conversion() {
+    switch (0u) {
+        case 0: {
+        }
+        default: {
+        }
+    }
+
+    switch (0) {
+        case 0u: {
+        }
+        default: {
+        }
+    }
+}
+
+const ONE = 1;
+fn switch_const_expr_case_selectors() {
+    const TWO = 2;
+    switch (0) {
+        case i32(): {
+        }
+        case ONE: {
+        }
+        case TWO: {
+        }
+        case 1 + 2: {
+        }
+        case vec4(4).x: {
+        }
+        default: {
+        }
+    }
+}
+
 fn loop_switch_continue(x: i32) {
     loop {
         switch x {

naga/tests/out/glsl/control-flow.main.Compute.glsl

@@ -24,6 +24,48 @@ void switch_case_break() {
     return;
 }
 
+void switch_selector_type_conversion() {
+    switch(0u) {
+        case 0u: {
+            break;
+        }
+        default: {
+            break;
+        }
+    }
+    switch(0u) {
+        case 0u: {
+            return;
+        }
+        default: {
+            return;
+        }
+    }
+}
+
+void switch_const_expr_case_selectors() {
+    switch(0) {
+        case 0: {
+            return;
+        }
+        case 1: {
+            return;
+        }
+        case 2: {
+            return;
+        }
+        case 3: {
+            return;
+        }
+        case 4: {
+            return;
+        }
+        default: {
+            return;
+        }
+    }
+}
+
 void loop_switch_continue(int x) {
     while(true) {
         switch(x) {

naga/tests/out/hlsl/control-flow.hlsl

@@ -18,6 +18,50 @@ void switch_case_break()
     return;
 }
 
+void switch_selector_type_conversion()
+{
+    switch(0u) {
+        case 0u: {
+            break;
+        }
+        default: {
+            break;
+        }
+    }
+    switch(0u) {
+        case 0u: {
+            return;
+        }
+        default: {
+            return;
+        }
+    }
+}
+
+void switch_const_expr_case_selectors()
+{
+    switch(int(0)) {
+        case 0: {
+            return;
+        }
+        case 1: {
+            return;
+        }
+        case 2: {
+            return;
+        }
+        case 3: {
+            return;
+        }
+        case 4: {
+            return;
+        }
+        default: {
+            return;
+        }
+    }
+}
+
 void loop_switch_continue(int x)
 {
     uint2 loop_bound = uint2(0u, 0u);

naga/tests/out/msl/control-flow.msl

@@ -28,6 +28,50 @@ void switch_case_break(
     return;
 }
 
+void switch_selector_type_conversion(
+) {
+    switch(0u) {
+        case 0u: {
+            break;
+        }
+        default: {
+            break;
+        }
+    }
+    switch(0u) {
+        case 0u: {
+            return;
+        }
+        default: {
+            return;
+        }
+    }
+}
+
+void switch_const_expr_case_selectors(
+) {
+    switch(0) {
+        case 0: {
+            return;
+        }
+        case 1: {
+            return;
+        }
+        case 2: {
+            return;
+        }
+        case 3: {
+            return;
+        }
+        case 4: {
+            return;
+        }
+        default: {
+            return;
+        }
+    }
+}
+
 void loop_switch_continue(
     int x
 ) {