Fix fragment shader silent optimization (Issue #284)

Addresses issue #284 where fragment shaders were optimized to have no
output operations, causing silent rendering failures.

The real problem is not DCE eliminating entry points (they are always
rooted), but rather fragment shaders being optimized to have no
meaningful output writes to StorageClass::Output variables.

Implementation:
- Add validation after DCE to detect fragment shaders with no output
- Provide helpful error messages with examples
- Detect partial component writes that may have been optimized away
- Recursively check function calls for output operations

Test case:
- issue-284-dead-fragment.rs: Fragment shader with no output (fails with clear error)

The validation catches cases where developers write partial component
assignments that get optimized away, providing clear guidance on how
to fix the issue.

Author: zmr-233

crates/rustc_codegen_spirv/src/linker/mod.rs

@@ -23,7 +23,7 @@ use crate::custom_decorations::{CustomDecoration, SrcLocDecoration, ZombieDecora
 use crate::custom_insts;
 use either::Either;
 use rspirv::binary::{Assemble, Consumer};
-use rspirv::dr::{Block, Loader, Module, ModuleHeader, Operand};
+use rspirv::dr::{Block, Function, Loader, Module, ModuleHeader, Operand};
 use rspirv::spirv::{Op, StorageClass, Word};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::ErrorGuaranteed;
@@ -705,6 +705,9 @@ pub fn link(
             dce::dce(output);
         }
 
+        // Validate fragment shaders after DCE to catch silent optimization failures
+        validate_fragment_shader_outputs(sess, output)?;
+
         {
             let _timer = sess.timer("link_remove_duplicate_debuginfo");
             duplicates::remove_duplicate_debuginfo(output);
@@ -815,3 +818,157 @@ impl Drop for SpirtDumpGuard<'_> {
         }
     }
 }
+
+/// Validate that fragment shaders have meaningful output operations
+/// This catches the case described in issue #284 where fragment shaders
+/// are optimized to have no output, causing silent rendering failures
+fn validate_fragment_shader_outputs(sess: &Session, module: &Module) -> Result<()> {
+    use rspirv::spirv::{ExecutionModel, Op};
+
+    // Find all fragment entry points
+    for entry in &module.entry_points {
+        if entry.class.opcode == Op::EntryPoint {
+            if let Some(execution_model) = entry.operands.first() {
+                if execution_model.unwrap_execution_model() == ExecutionModel::Fragment {
+                    let function_id = entry.operands[1].unwrap_id_ref();
+                    let entry_name = entry.operands[2].unwrap_literal_string();
+
+                    // Check if this fragment shader has meaningful output operations
+                    if !fragment_shader_writes_output(module, function_id) {
+                        let mut err = sess.dcx().struct_err(format!(
+                            "fragment shader `{}` produces no output",
+                            entry_name
+                        ));
+
+                        err = err.with_help(
+                            "fragment shaders must write to output parameters to produce visible results"
+                        ).with_note(
+                            "use complete assignment like `*out_frag_color = vec4(r, g, b, a)` instead of partial component assignments"
+                        ).with_note(
+                            "partial component assignments may be optimized away if not all components are written"
+                        );
+
+                        // Look for the function to provide better diagnostics
+                        if let Some(func) = module
+                            .functions
+                            .iter()
+                            .find(|f| f.def_id() == Some(function_id))
+                        {
+                            if has_partial_output_writes(module, func) {
+                                err = err.with_note(
+                                    "detected partial component writes (e.g., `out.x = value`) which were optimized away"
+                                ).with_help(
+                                    "write all components at once: `*out_frag_color = vec4(r, g, b, 1.0)`"
+                                );
+                            }
+                        }
+
+                        return Err(err.emit());
+                    }
+                }
+            }
+        }
+    }
+
+    Ok(())
+}
+
+/// Check if a fragment shader function has any meaningful output writes
+fn fragment_shader_writes_output(module: &Module, function_id: Word) -> bool {
+    // Find the function definition
+    let function = match module
+        .functions
+        .iter()
+        .find(|f| f.def_id() == Some(function_id))
+    {
+        Some(func) => func,
+        None => return true, // If function not found, assume it's valid
+    };
+
+    // Check all instructions in the function for output writes
+    for block in &function.blocks {
+        for inst in &block.instructions {
+            if inst.class.opcode == Op::Store {
+                if let Some(target_id) = inst.operands.first().and_then(|op| op.id_ref_any()) {
+                    if is_output_storage_variable(module, target_id) {
+                        return true;
+                    }
+                }
+            }
+
+            // Check function calls recursively
+            if inst.class.opcode == Op::FunctionCall {
+                if let Some(callee_id) = inst.operands.first().and_then(|op| op.id_ref_any()) {
+                    if fragment_shader_writes_output(module, callee_id) {
+                        return true;
+                    }
+                }
+            }
+        }
+    }
+
+    false
+}
+
+/// Check if a fragment shader has partial output writes that might have been optimized away
+fn has_partial_output_writes(module: &Module, function: &Function) -> bool {
+    use rspirv::spirv::Op;
+
+    // Look for AccessChain operations on output variables
+    // This suggests the shader was trying to write individual components
+    for block in &function.blocks {
+        for inst in &block.instructions {
+            if matches!(inst.class.opcode, Op::AccessChain | Op::InBoundsAccessChain) {
+                if let Some(base_id) = inst.operands.first().and_then(|op| op.id_ref_any()) {
+                    if is_output_storage_variable(module, base_id) {
+                        return true;
+                    }
+                }
+            }
+        }
+    }
+
+    false
+}
+
+/// Check if a variable ID refers to an Output storage class variable
+fn is_output_storage_variable(module: &Module, var_id: Word) -> bool {
+    use rspirv::spirv::{Op, StorageClass};
+
+    // Check direct output variables
+    for inst in &module.types_global_values {
+        if inst.result_id == Some(var_id) && inst.class.opcode == Op::Variable {
+            if let Some(storage_class) = inst.operands.first() {
+                return storage_class.unwrap_storage_class() == StorageClass::Output;
+            }
+        }
+    }
+
+    // Check if this is a pointer derived from an output variable
+    for inst in &module.types_global_values {
+        if inst.result_id == Some(var_id)
+            && matches!(inst.class.opcode, Op::AccessChain | Op::InBoundsAccessChain)
+        {
+            if let Some(base_id) = inst.operands.first().and_then(|op| op.id_ref_any()) {
+                return is_output_storage_variable(module, base_id);
+            }
+        }
+    }
+
+    // Check in function bodies for derived pointers
+    for function in &module.functions {
+        for block in &function.blocks {
+            for inst in &block.instructions {
+                if inst.result_id == Some(var_id)
+                    && matches!(inst.class.opcode, Op::AccessChain | Op::InBoundsAccessChain)
+                {
+                    if let Some(base_id) = inst.operands.first().and_then(|op| op.id_ref_any()) {
+                        return is_output_storage_variable(module, base_id);
+                    }
+                }
+            }
+        }
+    }
+
+    false
+}

tests/compiletests/ui/dis/issue-284-dead-fragment.rs

@@ -0,0 +1,12 @@
+//@ compile-flags: --crate-type dylib --emit=metadata
+// Test for issue #284: A fragment shader that produces no output should fail
+
+use spirv_std::spirv;
+
+#[spirv(fragment)]
+pub fn main_fs(#[spirv(flat)] in_color: u32, out_frag_color: &mut spirv_std::glam::Vec4) {
+    // This fragment shader reads input but doesn't write output
+    // The assignment is optimized away, causing no visible output
+    let _temp = in_color;
+    // Note: No assignment to out_frag_color, so this shader produces no output
+}

tests/compiletests/ui/dis/issue-284-dead-fragment.stderr

@@ -0,0 +1,8 @@
+error: fragment shader `main_fs` produces no output
+  |
+  = help: fragment shaders must write to output parameters to produce visible results
+  = note: use complete assignment like `*out_frag_color = vec4(r, g, b, a)` instead of partial component assignments
+  = note: partial component assignments may be optimized away if not all components are written
+
+error: aborting due to 1 previous error
+