diff --git a/pdl-compiler/src/analyzer.rs b/pdl-compiler/src/analyzer.rs
index 50140d4..90ed239 100644
--- a/pdl-compiler/src/analyzer.rs
+++ b/pdl-compiler/src/analyzer.rs
@@ -165,9 +165,10 @@ pub struct Scope<'d> {
 /// Gather size information about the full AST.
 #[derive(Debug)]
 pub struct Schema {
-    size: HashMap<usize, Size>,
-    padded_size: HashMap<usize, Option<usize>>,
-    payload_size: HashMap<usize, Size>,
+    decl_size: HashMap<DeclKey, Size>,
+    field_size: HashMap<FieldKey, Size>,
+    padded_size: HashMap<FieldKey, Option<usize>>,
+    payload_size: HashMap<DeclKey, Size>,
 }
 
 impl Diagnostics {
@@ -328,7 +329,7 @@ impl Schema {
     /// Check correct definition of packet sizes.
     /// Annotate fields and declarations with the size in bits.
     pub fn new(file: &File) -> Schema {
-        fn annotate_decl(schema: &mut Schema, scope: &HashMap<String, usize>, decl: &Decl) {
+        fn annotate_decl(schema: &mut Schema, scope: &HashMap<String, DeclKey>, decl: &Decl) {
             // Compute the padding size for each field.
             let mut padding = None;
             for field in decl.fields().rev() {
@@ -342,7 +343,7 @@ impl Schema {
             let mut size = decl
                 .parent_id()
                 .and_then(|parent_id| scope.get(parent_id))
-                .map(|key| schema.size(*key))
+                .map(|key| schema.decl_size(*key))
                 .unwrap_or(Size::Static(0));
             let mut payload_size = Size::Static(0);
 
@@ -378,13 +379,13 @@ impl Schema {
                 DeclDesc::Test { .. } => (Size::Static(0), Size::Static(0)),
             };
 
-            schema.size.insert(decl.key, size);
+            schema.decl_size.insert(decl.key, size);
             schema.payload_size.insert(decl.key, payload_size);
         }
 
         fn annotate_field(
             schema: &mut Schema,
-            scope: &HashMap<String, usize>,
+            scope: &HashMap<String, DeclKey>,
             decl: &Decl,
             field: &Field,
         ) -> Size {
@@ -445,7 +446,7 @@ impl Schema {
                 FieldDesc::Array { .. } => unreachable!(),
             };
 
-            schema.size.insert(field.key, size);
+            schema.field_size.insert(field.key, size);
             size
         }
 
@@ -457,7 +458,8 @@ impl Schema {
         }
 
         let mut schema = Schema {
-            size: Default::default(),
+            field_size: Default::default(),
+            decl_size: Default::default(),
             padded_size: Default::default(),
             payload_size: Default::default(),
         };
@@ -469,20 +471,24 @@ impl Schema {
         schema
     }
 
-    pub fn size(&self, key: usize) -> Size {
-        *self.size.get(&key).unwrap()
+    pub fn field_size(&self, key: FieldKey) -> Size {
+        *self.field_size.get(&key).unwrap()
     }
 
-    pub fn padded_size(&self, key: usize) -> Option<usize> {
+    pub fn decl_size(&self, key: DeclKey) -> Size {
+        *self.decl_size.get(&key).unwrap()
+    }
+
+    pub fn padded_size(&self, key: FieldKey) -> Option<usize> {
         *self.padded_size.get(&key).unwrap()
     }
 
-    pub fn payload_size(&self, key: usize) -> Size {
+    pub fn payload_size(&self, key: DeclKey) -> Size {
         *self.payload_size.get(&key).unwrap()
     }
 
-    pub fn total_size(&self, key: usize) -> Size {
-        self.size(key) + self.payload_size(key)
+    pub fn total_size(&self, key: DeclKey) -> Size {
+        self.decl_size(key) + self.payload_size(key)
     }
 }
 
@@ -3062,9 +3068,9 @@ mod test {
         file.declarations
             .iter()
             .map(|decl| Annotations {
-                size: schema.size(decl.key),
+                size: schema.decl_size(decl.key),
                 payload_size: schema.payload_size(decl.key),
-                fields: decl.fields().map(|field| schema.size(field.key)).collect(),
+                fields: decl.fields().map(|field| schema.field_size(field.key)).collect(),
             })
             .collect()
     }
diff --git a/pdl-compiler/src/ast.rs b/pdl-compiler/src/ast.rs
index 6c6f154..e545c5e 100644
--- a/pdl-compiler/src/ast.rs
+++ b/pdl-compiler/src/ast.rs
@@ -105,6 +105,9 @@ pub struct Constraint {
     pub tag_id: Option<String>,
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct FieldKey(pub usize);
+
 #[derive(Debug, Serialize, Clone, PartialEq, Eq)]
 #[serde(tag = "kind")]
 pub enum FieldDesc {
@@ -154,7 +157,7 @@ pub struct Field {
     /// Unique identifier used to refer to the AST node in
     /// compilation environments.
     #[serde(skip_serializing)]
-    pub key: usize,
+    pub key: FieldKey,
     #[serde(flatten)]
     pub desc: FieldDesc,
     pub cond: Option<Constraint>,
@@ -167,6 +170,9 @@ pub struct TestCase {
     pub input: String,
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct DeclKey(pub usize);
+
 #[derive(Debug, Serialize, Clone, PartialEq, Eq)]
 #[serde(tag = "kind")]
 pub enum DeclDesc {
@@ -202,7 +208,7 @@ pub struct Decl {
     /// Unique identifier used to refer to the AST node in
     /// compilation environments.
     #[serde(skip_serializing)]
-    pub key: usize,
+    pub key: DeclKey,
     #[serde(flatten)]
     pub desc: DeclDesc,
 }
diff --git a/pdl-compiler/src/backends/rust.rs b/pdl-compiler/src/backends/rust.rs
index c51c242..855fdef 100644
--- a/pdl-compiler/src/backends/rust.rs
+++ b/pdl-compiler/src/backends/rust.rs
@@ -92,7 +92,9 @@ fn generate_packet_size_getter<'a>(
     let mut dynamic_widths = Vec::new();
 
     for field in fields {
-        if let Some(width) = schema.padded_size(field.key).or(schema.size(field.key).static_()) {
+        if let Some(width) =
+            schema.padded_size(field.key).or(schema.field_size(field.key).static_())
+        {
             constant_width += width;
             continue;
         }
diff --git a/pdl-compiler/src/backends/rust/parser.rs b/pdl-compiler/src/backends/rust/parser.rs
index 0bcb021..8ed47c4 100644
--- a/pdl-compiler/src/backends/rust/parser.rs
+++ b/pdl-compiler/src/backends/rust/parser.rs
@@ -143,7 +143,7 @@ impl<'a> FieldParser<'a> {
 
     fn add_bit_field(&mut self, field: &'a ast::Field) {
         self.chunk.push(BitField { shift: self.shift, field });
-        self.shift += self.schema.size(field.key).static_().unwrap();
+        self.shift += self.schema.field_size(field.key).static_().unwrap();
         if self.shift % 8 != 0 {
             return;
         }
@@ -184,7 +184,7 @@ impl<'a> FieldParser<'a> {
                 v = quote! { (#v >> #shift) }
             }
 
-            let width = self.schema.size(field.key).static_().unwrap();
+            let width = self.schema.field_size(field.key).static_().unwrap();
             let value_type = types::Integer::new(width);
             if !single_value && width < value_type.width {
                 // Mask value if we grabbed more than `width` and if
@@ -303,7 +303,7 @@ impl<'a> FieldParser<'a> {
         let mut offset = 0;
         for field in fields {
             if let Some(width) =
-                self.schema.padded_size(field.key).or(self.schema.size(field.key).static_())
+                self.schema.padded_size(field.key).or(self.schema.field_size(field.key).static_())
             {
                 offset += width;
             } else {
@@ -534,7 +534,7 @@ impl<'a> FieldParser<'a> {
         let id = id.to_ident();
         let type_id = type_id.to_ident();
 
-        self.code.push(match self.schema.size(decl.key) {
+        self.code.push(match self.schema.decl_size(decl.key) {
             analyzer::Size::Unknown | analyzer::Size::Dynamic => quote! {
                 let #id = #type_id::parse_inner(&mut #span)?;
             },
diff --git a/pdl-compiler/src/backends/rust/serializer.rs b/pdl-compiler/src/backends/rust/serializer.rs
index 00a980e..5c7a16f 100644
--- a/pdl-compiler/src/backends/rust/serializer.rs
+++ b/pdl-compiler/src/backends/rust/serializer.rs
@@ -139,7 +139,7 @@ impl<'a> FieldSerializer<'a> {
     }
 
     fn add_bit_field(&mut self, field: &ast::Field) {
-        let width = self.schema.size(field.key).static_().unwrap();
+        let width = self.schema.field_size(field.key).static_().unwrap();
         let shift = self.shift;
 
         match &field.desc {
@@ -405,7 +405,7 @@ impl<'a> FieldSerializer<'a> {
             let padding_octets = padding_size / 8;
             let element_width = match &width {
                 Some(width) => Some(*width),
-                None => self.schema.size(decl.unwrap().key).static_(),
+                None => self.schema.decl_size(decl.unwrap().key).static_(),
             };
 
             let array_size = match element_width {
diff --git a/pdl-compiler/src/parser.rs b/pdl-compiler/src/parser.rs
index 6cdd465..57b42b4 100644
--- a/pdl-compiler/src/parser.rs
+++ b/pdl-compiler/src/parser.rs
@@ -190,8 +190,12 @@ trait Helpers<'i> {
 }
 
 impl<'a> Context<'a> {
-    fn key(&self) -> usize {
-        self.key.replace(self.key.get() + 1)
+    fn field_key(&self) -> ast::FieldKey {
+        ast::FieldKey(self.key.replace(self.key.get() + 1))
+    }
+
+    fn decl_key(&self) -> ast::DeclKey {
+        ast::DeclKey(self.key.replace(self.key.get() + 1))
     }
 }
 
@@ -418,7 +422,7 @@ fn parse_field(node: Node<'_>, context: &Context) -> Result<ast::Field, String>
     let mut children = desc.children();
     Ok(ast::Field {
         loc,
-        key: context.key(),
+        key: context.field_key(),
         cond: cond.map(|constraint| parse_constraint(constraint, context)).transpose()?,
         desc: match rule {
             Rule::checksum_field => {
@@ -559,7 +563,7 @@ fn parse_toplevel(root: Node<'_>, context: &Context) -> Result<ast::File, String
                 let function = parse_string(&mut children)?;
                 file.declarations.push(ast::Decl {
                     loc,
-                    key: context.key(),
+                    key: context.decl_key(),
                     desc: ast::DeclDesc::Checksum { id, function, width },
                 })
             }
@@ -571,7 +575,7 @@ fn parse_toplevel(root: Node<'_>, context: &Context) -> Result<ast::File, String
                 let function = parse_string(&mut children)?;
                 file.declarations.push(ast::Decl {
                     loc,
-                    key: context.key(),
+                    key: context.decl_key(),
                     desc: ast::DeclDesc::CustomField { id, function, width },
                 })
             }
@@ -583,7 +587,7 @@ fn parse_toplevel(root: Node<'_>, context: &Context) -> Result<ast::File, String
                 let tags = parse_enum_tag_list(&mut children, context)?;
                 file.declarations.push(ast::Decl {
                     loc,
-                    key: context.key(),
+                    key: context.decl_key(),
                     desc: ast::DeclDesc::Enum { id, width, tags },
                 })
             }
@@ -596,7 +600,7 @@ fn parse_toplevel(root: Node<'_>, context: &Context) -> Result<ast::File, String
                 let fields = parse_field_list_opt(&mut children, context)?;
                 file.declarations.push(ast::Decl {
                     loc,
-                    key: context.key(),
+                    key: context.decl_key(),
                     desc: ast::DeclDesc::Packet { id, parent_id, constraints, fields },
                 })
             }
@@ -609,7 +613,7 @@ fn parse_toplevel(root: Node<'_>, context: &Context) -> Result<ast::File, String
                 let fields = parse_field_list_opt(&mut children, context)?;
                 file.declarations.push(ast::Decl {
                     loc,
-                    key: context.key(),
+                    key: context.decl_key(),
                     desc: ast::DeclDesc::Struct { id, parent_id, constraints, fields },
                 })
             }
@@ -620,7 +624,7 @@ fn parse_toplevel(root: Node<'_>, context: &Context) -> Result<ast::File, String
                 let fields = parse_field_list(&mut children, context)?;
                 file.declarations.push(ast::Decl {
                     loc,
-                    key: context.key(),
+                    key: context.decl_key(),
                     desc: ast::DeclDesc::Group { id, fields },
                 })
             }
@@ -630,7 +634,7 @@ fn parse_toplevel(root: Node<'_>, context: &Context) -> Result<ast::File, String
         }
     }
     file.comments.append(&mut toplevel_comments);
-    file.max_key = context.key();
+    file.max_key = context.key.get();
     Ok(file)
 }