diff --git a/src/api/mod.rs b/src/api/mod.rs
index 7f128243..e9e03854 100644
--- a/src/api/mod.rs
+++ b/src/api/mod.rs
@@ -170,11 +170,8 @@ impl Engine {
         keyword: impl AsRef<str>,
         precedence: u8,
     ) -> Result<&mut Self, String> {
-        let precedence = Precedence::new(precedence);
-
-        if precedence.is_none() {
-            return Err("precedence cannot be zero".into());
-        }
+        let precedence =
+            Precedence::new(precedence).ok_or_else(|| "precedence cannot be zero".to_string())?;
 
         let keyword = keyword.as_ref();
 
@@ -213,7 +210,8 @@ impl Engine {
         }
 
         // Add to custom keywords
-        self.custom_keywords.insert(keyword.into(), precedence);
+        self.custom_keywords
+            .insert(keyword.into(), Some(precedence));
 
         Ok(self)
     }
diff --git a/src/api/type_names.rs b/src/api/type_names.rs
index 495916ed..f426154b 100644
--- a/src/api/type_names.rs
+++ b/src/api/type_names.rs
@@ -238,6 +238,7 @@ impl Engine {
     }
 
     /// Make a `Box<`[`EvalAltResult<ErrorMismatchDataType>`][ERR::ErrorMismatchDataType]`>`.
+    #[cold]
     #[inline(never)]
     #[must_use]
     pub(crate) fn make_type_mismatch_err<T>(&self, typ: &str, pos: Position) -> RhaiError {
diff --git a/src/eval/cache.rs b/src/eval/cache.rs
index 34a6941a..cf96dc24 100644
--- a/src/eval/cache.rs
+++ b/src/eval/cache.rs
@@ -79,7 +79,6 @@ impl Caches<'_> {
         self.stack.last_mut().unwrap()
     }
     /// Push an empty function resolution cache onto the stack and make it current.
-    #[allow(dead_code)]
     #[inline(always)]
     pub fn push_fn_resolution_cache(&mut self) {
         self.stack.push(Default::default());
diff --git a/src/func/script.rs b/src/func/script.rs
index 968b2309..8309c6ba 100644
--- a/src/func/script.rs
+++ b/src/func/script.rs
@@ -35,6 +35,7 @@ impl Engine {
         pos: Position,
         level: usize,
     ) -> RhaiResult {
+        #[cold]
         #[inline(never)]
         fn make_error(
             name: String,
diff --git a/src/packages/arithmetic.rs b/src/packages/arithmetic.rs
index d1181889..34615564 100644
--- a/src/packages/arithmetic.rs
+++ b/src/packages/arithmetic.rs
@@ -9,7 +9,8 @@ use std::prelude::v1::*;
 #[cfg(not(feature = "no_float"))]
 use num_traits::Float;
 
-#[inline]
+#[cold]
+#[inline(never)]
 pub fn make_err(msg: impl Into<String>) -> RhaiError {
     ERR::ErrorArithmetic(msg.into(), Position::NONE).into()
 }
diff --git a/src/packages/mod.rs b/src/packages/mod.rs
index 8a7e1dbc..6337a0b1 100644
--- a/src/packages/mod.rs
+++ b/src/packages/mod.rs
@@ -120,11 +120,9 @@ pub trait Package {
 ///
 /// def_package! {
 ///     /// My super-duper package.
-///     pub MyPackage(module @ 10) {
+///     pub MyPackage(module) {
 ///         // Load a native Rust function.
 ///         module.set_native_fn("my_add", add);
-///     } |> |engine| {
-///         engine.register_custom_operator("@", 160).unwrap();
 ///     }
 /// }
 /// ```
diff --git a/src/types/error.rs b/src/types/error.rs
index 6e9814cd..d426d5ed 100644
--- a/src/types/error.rs
+++ b/src/types/error.rs
@@ -249,6 +249,7 @@ impl fmt::Display for EvalAltResult {
 }
 
 impl<T: AsRef<str>> From<T> for EvalAltResult {
+    #[cold]
     #[inline(never)]
     fn from(err: T) -> Self {
         Self::ErrorRuntime(err.as_ref().to_string().into(), Position::NONE)
@@ -256,6 +257,7 @@ impl<T: AsRef<str>> From<T> for EvalAltResult {
 }
 
 impl<T: AsRef<str>> From<T> for Box<EvalAltResult> {
+    #[cold]
     #[inline(never)]
     fn from(err: T) -> Self {
         EvalAltResult::ErrorRuntime(err.as_ref().to_string().into(), Position::NONE).into()
@@ -266,6 +268,8 @@ impl EvalAltResult {
     /// Is this a pseudo error?  A pseudo error is one that does not occur naturally.
     ///
     /// [`LoopBreak`][EvalAltResult::LoopBreak] and [`Return`][EvalAltResult::Return] are pseudo errors.
+    #[cold]
+    #[inline(never)]
     #[must_use]
     pub const fn is_pseudo_error(&self) -> bool {
         match self {
@@ -274,6 +278,8 @@ impl EvalAltResult {
         }
     }
     /// Can this error be caught?
+    #[cold]
+    #[inline(never)]
     #[must_use]
     pub const fn is_catchable(&self) -> bool {
         match self {
@@ -319,6 +325,8 @@ impl EvalAltResult {
         }
     }
     /// Is this error a system exception?
+    #[cold]
+    #[inline(never)]
     #[must_use]
     pub const fn is_system_exception(&self) -> bool {
         match self {
@@ -338,6 +346,8 @@ impl EvalAltResult {
     }
     /// Get the [position][Position] of this error.
     #[cfg(not(feature = "no_object"))]
+    #[cold]
+    #[inline(never)]
     pub(crate) fn dump_fields(&self, map: &mut crate::Map) {
         map.insert(
             "error".into(),
@@ -419,6 +429,8 @@ impl EvalAltResult {
         };
     }
     /// Unwrap this error and get the very base error.
+    #[cold]
+    #[inline(never)]
     #[must_use]
     pub fn unwrap_inner(&self) -> &Self {
         match self {
@@ -429,6 +441,8 @@ impl EvalAltResult {
         }
     }
     /// Get the [position][Position] of this error.
+    #[cold]
+    #[inline(never)]
     #[must_use]
     pub const fn position(&self) -> Position {
         match self {
@@ -470,18 +484,24 @@ impl EvalAltResult {
     /// Remove the [position][Position] information from this error.
     ///
     /// The [position][Position] of this error is set to [`NONE`][Position::NONE] afterwards.
+    #[cold]
+    #[inline(never)]
     pub fn clear_position(&mut self) -> &mut Self {
         self.set_position(Position::NONE)
     }
     /// Remove the [position][Position] information from this error and return it.
     ///
     /// The [position][Position] of this error is set to [`NONE`][Position::NONE] afterwards.
+    #[cold]
+    #[inline(never)]
     pub fn take_position(&mut self) -> Position {
         let pos = self.position();
         self.set_position(Position::NONE);
         pos
     }
     /// Override the [position][Position] of this error.
+    #[cold]
+    #[inline(never)]
     pub fn set_position(&mut self, new_position: Position) -> &mut Self {
         match self {
             Self::ErrorSystem(..) => (),
@@ -522,6 +542,7 @@ impl EvalAltResult {
     }
     /// Consume the current [`EvalAltResult`] and return a new one with the specified [`Position`]
     /// if the current position is [`Position::NONE`].
+    #[cold]
     #[inline(never)]
     #[must_use]
     pub(crate) fn fill_position(mut self: Box<Self>, new_position: Position) -> Box<Self> {
diff --git a/src/types/parse_error.rs b/src/types/parse_error.rs
index 47e3589a..5cbdcb5d 100644
--- a/src/types/parse_error.rs
+++ b/src/types/parse_error.rs
@@ -263,6 +263,7 @@ impl fmt::Display for ParseErrorType {
 }
 
 impl From<LexError> for ParseErrorType {
+    #[cold]
     #[inline(never)]
     fn from(err: LexError) -> Self {
         match err {
@@ -300,13 +301,15 @@ impl fmt::Display for ParseError {
 
 impl ParseError {
     /// Get the [type][ParseErrorType] of this parse error.
-    #[inline(always)]
+    #[cold]
+    #[inline(never)]
     #[must_use]
     pub const fn err_type(&self) -> &ParseErrorType {
         &self.0
     }
     /// Get the [position][Position] of this parse error.
-    #[inline(always)]
+    #[cold]
+    #[inline(never)]
     #[must_use]
     pub const fn position(&self) -> Position {
         self.1
@@ -314,28 +317,32 @@ impl ParseError {
 }
 
 impl From<ParseErrorType> for RhaiError {
-    #[inline(always)]
+    #[cold]
+    #[inline(never)]
     fn from(err: ParseErrorType) -> Self {
         Box::new(err.into())
     }
 }
 
 impl From<ParseErrorType> for ERR {
-    #[inline(always)]
+    #[cold]
+    #[inline(never)]
     fn from(err: ParseErrorType) -> Self {
         Self::ErrorParsing(err, Position::NONE)
     }
 }
 
 impl From<ParseError> for RhaiError {
-    #[inline(always)]
+    #[cold]
+    #[inline(never)]
     fn from(err: ParseError) -> Self {
         Box::new(err.into())
     }
 }
 
 impl From<ParseError> for ERR {
-    #[inline(always)]
+    #[cold]
+    #[inline(never)]
     fn from(err: ParseError) -> Self {
         Self::ErrorParsing(*err.0, err.1)
     }