fix: Write gl_PointSize = 1.0; in case of vertex shader and `allow_…

…point_size` == true

According to https://docs.gl/el3/gl_PointSize
```
The variable gl_PointSize is intended for a vertex shader to write the size of the point to be rasterized. It is measured in pixels. If gl_PointSize is not written to, its value is undefined in subsequent pipeline stages.
```

benches/criterion.rs

@@ -250,6 +250,7 @@ fn backends(c: &mut Criterion) {
                         shader_stage: ep.stage,
                         entry_point: ep.name.clone(),
                         multiview: None,
+                        allow_point_size: true,
                     };
 
                     // might be `Err` if missing features

cli/src/bin/naga.rs

@@ -443,18 +443,20 @@ fn run() -> Result<(), Box<dyn std::error::Error>> {
             stage @ ("vert" | "frag" | "comp") => {
                 use naga::back::glsl;
 
+                let shader_stage = match stage {
+                    "vert" => naga::ShaderStage::Vertex,
+                    "frag" => naga::ShaderStage::Fragment,
+                    "comp" => naga::ShaderStage::Compute,
+                    _ => unreachable!(),
+                };
                 let pipeline_options = glsl::PipelineOptions {
                     entry_point: match params.entry_point {
                         Some(ref name) => name.clone(),
                         None => "main".to_string(),
                     },
-                    shader_stage: match stage {
-                        "vert" => naga::ShaderStage::Vertex,
-                        "frag" => naga::ShaderStage::Fragment,
-                        "comp" => naga::ShaderStage::Compute,
-                        _ => unreachable!(),
-                    },
+                    shader_stage: shader_stage,
                     multiview: None,
+                    allow_point_size: shader_stage == naga::ShaderStage::Vertex,
                 };
 
                 let mut buffer = String::new();

src/back/glsl/mod.rs

@@ -260,6 +260,8 @@ pub struct PipelineOptions {
     pub entry_point: String,
     /// How many views to render to, if doing multiview rendering.
     pub multiview: Option<std::num::NonZeroU32>,
+    /// Allow `BuiltIn::PointSize` in the vertex shader.
+    pub allow_point_size: bool,
 }
 
 /// Reflection info for texture mappings and uniforms.
@@ -434,6 +436,7 @@ pub struct Writer<'a, W> {
     need_bake_expressions: back::NeedBakeExpressions,
     /// How many views to render to, if doing multiview rendering.
     multiview: Option<std::num::NonZeroU32>,
+    pipeline_options: &'a PipelineOptions,
 }
 
 impl<'a, W: Write> Writer<'a, W> {
@@ -489,6 +492,7 @@ impl<'a, W: Write> Writer<'a, W> {
             block_id: IdGenerator::default(),
             named_expressions: Default::default(),
             need_bake_expressions: Default::default(),
+            pipeline_options,
         };
 
         // Find all features required to print this module
@@ -2072,6 +2076,16 @@ impl<'a, W: Write> Writer<'a, W> {
                                 write!(self.out, "{}", level)?;
                             }
                         }
+
+                        if let back::FunctionType::EntryPoint(ep_index) = ctx.ty {
+                            if self.module.entry_points[ep_index as usize].stage
+                                == ShaderStage::Vertex
+                                && self.pipeline_options.allow_point_size
+                            {
+                                writeln!(self.out, "gl_PointSize = 1.0;",)?;
+                                write!(self.out, "{}", level)?;
+                            }
+                        }
                         writeln!(self.out, "return;")?;
                     }
                 }

tests/snapshots.rs

@@ -301,6 +301,7 @@ fn write_output_glsl(
         shader_stage: stage,
         entry_point: ep_name.to_string(),
         multiview,
+        allow_point_size: true,
     };
 
     let mut buffer = String::new();