Add --allow-fragment-no-output compiler flag to bypass fragment shader validation

Implements a new compiler flag that allows fragment shaders with no output
operations to compile without errors. This addresses issue #284 where
fragment shaders optimized by DCE (Dead Code Elimination) would fail
validation even when the lack of output was intentional.

Changes:
- Add `allow_fragment_no_output` field to linker Options struct
- Add command-line flag parsing for `--allow-fragment-no-output`
- Skip fragment shader output validation when flag is enabled
- Update error message to include usage hint for the new flag

This flag is intended for testing scenarios and special use cases where
fragment shaders legitimately produce no output. The validation remains
active by default to catch unintentional issues.

Author: zmr-233

crates/rustc_codegen_spirv/src/linker/mod.rs

@@ -55,6 +55,9 @@ pub struct Options {
     /// **Note**: currently only used for unit testing, and not exposed elsewhere.
     pub keep_link_exports: bool,
 
+    /// Allow fragment shaders with no output operations (disables Issue #284 validation)
+    pub allow_fragment_no_output: bool,
+
     // NOTE(eddyb) these are debugging options that used to be env vars
     // (for more information see `docs/src/codegen-args.md`).
     pub dump_post_merge: Option<PathBuf>,
@@ -711,7 +714,9 @@ pub fn link(
         }
 
         // Validate fragment shaders after DCE to catch silent optimization failures
-        validate_fragment_shader_outputs(sess, output)?;
+        if !opts.allow_fragment_no_output {
+            validate_fragment_shader_outputs(sess, output)?;
+        }
 
         {
             let _timer = sess.timer("link_remove_duplicate_debuginfo");
@@ -851,6 +856,8 @@ fn validate_fragment_shader_outputs(sess: &Session, module: &Module) -> Result<(
                             "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"
+                        ).with_note(
+                            "to disable this validation (e.g. for testing), use `--allow-fragment-no-output`"
                         );
 
                         // Look for the function to provide better diagnostics
@@ -864,6 +871,8 @@ fn validate_fragment_shader_outputs(sess: &Session, module: &Module) -> Result<(
                                     "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)`"
+                                ).with_note(
+                                    "alternatively, use `--allow-fragment-no-output` to disable this validation"
                                 );
                             }
                         }

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

@@ -3,6 +3,7 @@ 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
+  = note: to disable this validation (e.g. for testing), use `--allow-fragment-no-output`
 
 error: aborting due to 1 previous error