diff --git a/src/front/glsl/ast.rs b/src/front/glsl/ast.rs
index 5da25fbf57..c570fd79b3 100644
--- a/src/front/glsl/ast.rs
+++ b/src/front/glsl/ast.rs
@@ -200,6 +200,8 @@ impl<'function> Context<'function> {
             let var = VariableReference {
                 expr,
                 load: Some(self.expressions.append(Expression::Load { pointer: expr })),
+                // TODO: respect constant qualifier
+                mutable: true,
             };
 
             self.lookup_global_var_exps.insert(name.into(), var.clone());
@@ -218,7 +220,11 @@ impl<'function> Context<'function> {
                 .expressions
                 .append(Expression::Constant(*program.lookup_constants.get(name)?));
 
-            let var = VariableReference { expr, load: None };
+            let var = VariableReference {
+                expr,
+                load: None,
+                mutable: false,
+            };
 
             self.lookup_constant_exps.insert(name.into(), var.clone());
 
@@ -236,7 +242,7 @@ impl<'function> Context<'function> {
     }
 
     /// Add variable to current scope
-    pub fn add_local_var(&mut self, name: String, expr: Handle<Expression>) {
+    pub fn add_local_var(&mut self, name: String, expr: Handle<Expression>, mutable: bool) {
         if let Some(current) = self.scopes.last_mut() {
             let load = self.expressions.append(Expression::Load { pointer: expr });
 
@@ -245,13 +251,14 @@ impl<'function> Context<'function> {
                 VariableReference {
                     expr,
                     load: Some(load),
+                    mutable,
                 },
             );
         }
     }
 
     /// Add function argument to current scope
-    pub fn add_function_arg(&mut self, name: Option<String>, ty: Handle<Type>) {
+    pub fn add_function_arg(&mut self, name: Option<String>, ty: Handle<Type>, mutable: bool) {
         let index = self.arguments.len();
         self.arguments.push(FunctionArgument {
             name: name.clone(),
@@ -265,7 +272,14 @@ impl<'function> Context<'function> {
                     .expressions
                     .append(Expression::FunctionArgument(index as u32));
 
-                (*current).insert(name, VariableReference { expr, load: None });
+                (*current).insert(
+                    name,
+                    VariableReference {
+                        expr,
+                        load: None,
+                        mutable,
+                    },
+                );
             }
         }
     }
@@ -347,6 +361,13 @@ impl<'function> Context<'function> {
             }
             ExprKind::Variable(var) => {
                 if lhs {
+                    if !var.mutable {
+                        return Err(ErrorKind::SemanticError(
+                            expr.meta,
+                            "Variable cannot be used in LHS position".into(),
+                        ));
+                    }
+
                     var.expr
                 } else {
                     var.load.unwrap_or(var.expr)
@@ -392,6 +413,7 @@ impl<'function> Context<'function> {
 pub struct VariableReference {
     pub expr: Handle<Expression>,
     pub load: Option<Handle<Expression>>,
+    pub mutable: bool,
 }
 
 #[derive(Debug, Clone)]
@@ -469,7 +491,7 @@ pub enum StructLayout {
     Std140,
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum ParameterQualifier {
     In,
     Out,
diff --git a/src/front/glsl/parser.rs b/src/front/glsl/parser.rs
index bd90a66e3f..727d010b58 100644
--- a/src/front/glsl/parser.rs
+++ b/src/front/glsl/parser.rs
@@ -1518,13 +1518,15 @@ impl<'source, 'program, 'options> Parser<'source, 'program, 'options> {
     ) -> Result<()> {
         loop {
             if self.peek_type_name() || self.peek_parameter_qualifier() {
-                parameters.push(self.parse_parameter_qualifier());
+                let qualifier = self.parse_parameter_qualifier();
+                let mutable = qualifier != ParameterQualifier::Const;
+                parameters.push(qualifier);
                 let ty = self.parse_type_non_void()?.0;
 
                 match self.expect_peek()?.value {
                     TokenValue::Comma => {
                         self.bump()?;
-                        context.add_function_arg(None, ty);
+                        context.add_function_arg(None, ty, mutable);
                         continue;
                     }
                     TokenValue::Identifier(_) => {
@@ -1533,7 +1535,7 @@ impl<'source, 'program, 'options> Parser<'source, 'program, 'options> {
                         let size = self.parse_array_specifier()?;
                         let ty = self.maybe_array(ty, size);
 
-                        context.add_function_arg(Some(name), ty);
+                        context.add_function_arg(Some(name), ty, mutable);
 
                         if self.bump_if(TokenValue::Comma).is_some() {
                             continue;
diff --git a/src/front/glsl/variables.rs b/src/front/glsl/variables.rs
index 7985633c28..7aaff1d7aa 100644
--- a/src/front/glsl/variables.rs
+++ b/src/front/glsl/variables.rs
@@ -385,13 +385,19 @@ impl Program<'_> {
             return Err(ErrorKind::VariableAlreadyDeclared(name));
         }
 
-        // FIXME: this should be removed after
-        // fixing the lack of constant qualifier support
-        #[allow(clippy::never_loop)]
+        let mut mutable = true;
+
         for qualifier in qualifiers {
             match *qualifier {
                 TypeQualifier::StorageQualifier(StorageQualifier::Const) => {
-                    return Err(ErrorKind::NotImplemented("const qualifier"));
+                    if !mutable {
+                        return Err(ErrorKind::SemanticError(
+                            meta,
+                            "Cannot use more than one constant qualifier per declaration".into(),
+                        ));
+                    }
+
+                    mutable = false;
                 }
                 _ => {
                     return Err(ErrorKind::SemanticError(
@@ -409,7 +415,7 @@ impl Program<'_> {
         });
         let expr = ctx.expressions.append(Expression::LocalVariable(handle));
 
-        ctx.add_local_var(name, expr);
+        ctx.add_local_var(name, expr, mutable);
 
         Ok(expr)
     }