diff --git a/src/alm/alm_optimizer.rs b/src/alm/alm_optimizer.rs
index 131df3f2..0fcb1fde 100644
--- a/src/alm/alm_optimizer.rs
+++ b/src/alm/alm_optimizer.rs
@@ -1415,7 +1415,7 @@ mod tests {
             "exists right away"
         );
 
-        let mut alm_optimizer = alm_optimizer
+        let alm_optimizer = alm_optimizer
             .with_initial_inner_tolerance(1e-3)
             .with_epsilon_tolerance(1e-3);
         assert!(!alm_optimizer.is_exit_criterion_satisfied());
@@ -1433,7 +1433,7 @@ mod tests {
         let panoc_cache = PANOCCache::new(nx, tolerance, lbfgs_mem);
         let mut alm_cache = AlmCache::new(panoc_cache, n1, n2);
         let alm_problem = make_dummy_alm_problem(n1, n2);
-        let mut alm_optimizer = AlmOptimizer::new(&mut alm_cache, alm_problem)
+        let alm_optimizer = AlmOptimizer::new(&mut alm_cache, alm_problem)
             .with_sufficient_decrease_coefficient(0.1);
 
         // should stall because iteration = 0
@@ -1458,7 +1458,7 @@ mod tests {
         let panoc_cache = PANOCCache::new(nx, tolerance, lbfgs_mem);
         let mut alm_cache = AlmCache::new(panoc_cache, n1, n2);
         let alm_problem = make_dummy_alm_problem(n1, n2);
-        let mut alm_optimizer = AlmOptimizer::new(&mut alm_cache, alm_problem)
+        let alm_optimizer = AlmOptimizer::new(&mut alm_cache, alm_problem)
             .with_sufficient_decrease_coefficient(0.1);
 
         // should stall because iteration = 0
@@ -1483,7 +1483,7 @@ mod tests {
         let panoc_cache = PANOCCache::new(nx, tolerance, lbfgs_mem);
         let mut alm_cache = AlmCache::new(panoc_cache, n1, n2);
         let alm_problem = make_dummy_alm_problem(n1, n2);
-        let mut alm_optimizer = AlmOptimizer::new(&mut alm_cache, alm_problem)
+        let alm_optimizer = AlmOptimizer::new(&mut alm_cache, alm_problem)
             .with_sufficient_decrease_coefficient(0.1);
 
         // should stall because iteration = 0
diff --git a/src/constraints/rectangle.rs b/src/constraints/rectangle.rs
index e7d86003..f8c8cf9f 100644
--- a/src/constraints/rectangle.rs
+++ b/src/constraints/rectangle.rs
@@ -35,7 +35,7 @@ impl<'a> Rectangle<'a> {
     ///   dimensions
     ///
     pub fn new(xmin: Option<&'a [f64]>, xmax: Option<&'a [f64]>) -> Self {
-        assert!(xmin != None || xmax != None); // xmin or xmax must be Some
+        assert!(xmin.is_some() || xmax.is_some()); // xmin or xmax must be Some
         assert!(
             xmin.is_none() || xmax.is_none() || xmin.unwrap().len() == xmax.unwrap().len(),
             "incompatible dimensions of xmin and xmax"
diff --git a/src/core/fbs/fbs_optimizer.rs b/src/core/fbs/fbs_optimizer.rs
index 2b536964..d714ab67 100644
--- a/src/core/fbs/fbs_optimizer.rs
+++ b/src/core/fbs/fbs_optimizer.rs
@@ -63,7 +63,7 @@ where
     ///
     /// The method panics if the specified tolerance is not positive
     pub fn with_tolerance(
-        mut self,
+        self,
         tolerance: f64,
     ) -> FBSOptimizer<'a, GradientType, ConstraintType, CostType> {
         assert!(tolerance > 0.0);
diff --git a/src/core/panoc/panoc_optimizer.rs b/src/core/panoc/panoc_optimizer.rs
index ae2c019c..602abd45 100644
--- a/src/core/panoc/panoc_optimizer.rs
+++ b/src/core/panoc/panoc_optimizer.rs
@@ -62,7 +62,7 @@ where
     /// ## Panics
     ///
     /// The method panics if the specified tolerance is not positive
-    pub fn with_tolerance(mut self, tolerance: f64) -> Self {
+    pub fn with_tolerance(self, tolerance: f64) -> Self {
         assert!(tolerance > 0.0, "tolerance must be larger than 0");
 
         self.panoc_engine.cache.tolerance = tolerance;