Do not emit layout(location) qualifiers for older GLSL targets.

ESSL 300 and GLSL <410 do not support this along with legacy targets.

README.md

@@ -236,9 +236,9 @@ This can be done with `Compiler::set_decoration(id, spv::DecorationDescriptorSet
 
 #### Linking by name for targets which do not support explicit locations (legacy GLSL/ESSL)
 
-Modern GLSL and HLSL sources will rely on explicit layout(location) qualifiers to guide the linking process,
-but legacy GLSL relies on symbol names to perform the linking. When emitting legacy shaders, these layout statements will be dropped,
-so it is important that the API user ensures that the names of I/O variables are sanitized to ensure that linking will work properly.
+Modern GLSL and HLSL sources (and SPIR-V) relies on explicit layout(location) qualifiers to guide the linking process between shader stages,
+but older GLSL relies on symbol names to perform the linking. When emitting shaders with older versions, these layout statements will be removed,
+so it is important that the API user ensures that the names of I/O variables are sanitized so that linking will work properly.
 The reflection API can rename variables, struct types and struct members to deal with these scenarios using `Compiler::set_name` and friends.
 
 ## Contributing

spirv_glsl.cpp

@@ -965,14 +965,30 @@ string CompilerGLSL::layout_for_variable(const SPIRVariable &var)
 
 	if (flags & (1ull << DecorationLocation))
 	{
-		uint64_t combined_decoration = 0;
-		for (uint32_t i = 0; i < meta[type.self].members.size(); i++)
-			combined_decoration |= combined_decoration_for_member(type, i);
+		bool can_use_varying_location = true;
 
-		// If our members have location decorations, we don't need to
-		// emit location decorations at the top as well (looks weird).
-		if ((combined_decoration & (1ull << DecorationLocation)) == 0)
-			attr.push_back(join("location = ", dec.location));
+		// Location specifiers are must have in SPIR-V, but they aren't really supported in earlier versions of GLSL.
+		// Be very explicit here about how to solve the issue.
+		if ((get_execution_model() != ExecutionModelVertex && var.storage == StorageClassInput) ||
+		    (get_execution_model() != ExecutionModelFragment && var.storage == StorageClassOutput))
+		{
+			if (!options.es && options.version < 410 && !options.separate_shader_objects)
+				can_use_varying_location = false;
+			else if (options.es && options.version < 310)
+				can_use_varying_location = false;
+		}
+
+		if (can_use_varying_location)
+		{
+			uint64_t combined_decoration = 0;
+			for (uint32_t i = 0; i < meta[type.self].members.size(); i++)
+				combined_decoration |= combined_decoration_for_member(type, i);
+
+			// If our members have location decorations, we don't need to
+			// emit location decorations at the top as well (looks weird).
+			if ((combined_decoration & (1ull << DecorationLocation)) == 0)
+				attr.push_back(join("location = ", dec.location));
+		}
 	}
 
 	// set = 0 is the default. Do not emit set = decoration in regular GLSL output, but