v.gen.wasm: make wasm_relaxed_simd imply wasm_simd

Address the review on #27526: relaxed SIMD extends the SIMD/v128 feature, but
passing only -d wasm_relaxed_simd enabled .relaxed_simd without .simd. That made
wabt_validate_args() emit '--disable-simd --enable-relaxed-simd' and wasm-opt run
from -mvp without --enable-simd, so the advertised opt-in still failed under
-wasm-validate/-prod unless the user also guessed -d wasm_simd.

Add apply_feature_implications() so .relaxed_simd pulls in .simd, and cover it
with features_test.v (feature set + rendered wasm-opt/wasm-validate flags).

Author: medvednikov

vlib/v/gen/wasm/features.v

@@ -180,7 +180,20 @@ fn (g &Gen) enabled_wasm_features() []WasmFeature {
 			feats << feat
 		}
 	}
-	return feats
+	return apply_feature_implications(feats)
+}
+
+// apply_feature_implications expands `feats` with any feature implied by another.
+// Relaxed SIMD extends the SIMD/v128 feature, so requesting it must also enable
+// SIMD. Otherwise wabt_validate_args() emits `--disable-simd --enable-relaxed-simd`
+// and wasm-opt runs from `-mvp` without `--enable-simd`, so a `-d wasm_relaxed_simd`
+// build still fails under `-wasm-validate`/`-prod`.
+fn apply_feature_implications(feats []WasmFeature) []WasmFeature {
+	mut res := feats.clone()
+	if WasmFeature.relaxed_simd in res && WasmFeature.simd !in res {
+		res << .simd
+	}
+	return res
 }
 
 // binaryen_feature_flags renders the feature set into `wasm-opt` flags. It

vlib/v/gen/wasm/features_test.v

@@ -0,0 +1,32 @@
+module wasm
+
+fn test_relaxed_simd_implies_simd() {
+	// requesting relaxed SIMD on its own must also pull in the base SIMD feature
+	feats := apply_feature_implications([WasmFeature.relaxed_simd])
+	assert WasmFeature.simd in feats
+	assert WasmFeature.relaxed_simd in feats
+}
+
+fn test_relaxed_simd_does_not_duplicate_simd() {
+	// when SIMD is already present, the implication must not add a duplicate
+	feats := apply_feature_implications([WasmFeature.simd, .relaxed_simd])
+	assert feats.filter(it == WasmFeature.simd).len == 1
+}
+
+fn test_simd_without_relaxed_is_unchanged() {
+	feats := apply_feature_implications([WasmFeature.simd])
+	assert WasmFeature.simd in feats
+	assert WasmFeature.relaxed_simd !in feats
+}
+
+fn test_relaxed_simd_renders_simd_in_tool_flags() {
+	feats := apply_feature_implications([WasmFeature.relaxed_simd])
+	// wasm-opt must be told to enable SIMD as well as relaxed SIMD
+	binaryen := binaryen_feature_flags(feats)
+	assert binaryen.contains('--enable-simd')
+	assert binaryen.contains('--enable-relaxed-simd')
+	// wasm-validate must not disable SIMD while enabling relaxed SIMD
+	wabt := wabt_validate_args(feats)
+	assert '--disable-simd' !in wabt
+	assert '--enable-relaxed-simd' in wabt
+}