diff --git a/crates/apollo-compiler/src/ast/impls.rs b/crates/apollo-compiler/src/ast/impls.rs
index db80d284a..7767c17ea 100644
--- a/crates/apollo-compiler/src/ast/impls.rs
+++ b/crates/apollo-compiler/src/ast/impls.rs
@@ -39,7 +39,7 @@ impl Document {
     ///
     /// Does not perform any validation beyond this syntactic level.
     pub fn check_parse_errors(&self) -> Result<(), DiagnosticList> {
-        let mut errors = DiagnosticList::new(None, self.sources.clone());
+        let mut errors = DiagnosticList::new(self.sources.clone());
         for (file_id, source) in self.sources.iter() {
             source.validate_parse_errors(&mut errors, *file_id)
         }
@@ -54,7 +54,7 @@ impl Document {
             self,
             type_system_definitions_are_errors,
         );
-        let mut errors = DiagnosticList::new(None, self.sources.clone());
+        let mut errors = DiagnosticList::new(self.sources.clone());
         crate::executable::validation::validate_standalone_executable(&mut errors, &executable);
         errors.into_result()
     }
diff --git a/crates/apollo-compiler/src/executable/mod.rs b/crates/apollo-compiler/src/executable/mod.rs
index 8ccbb8e29..76c3cee52 100644
--- a/crates/apollo-compiler/src/executable/mod.rs
+++ b/crates/apollo-compiler/src/executable/mod.rs
@@ -22,6 +22,7 @@ pub use crate::ast::{
 use crate::validation::DiagnosticList;
 use crate::validation::NodeLocation;
 use std::fmt;
+use std::sync::Arc;
 
 /// Executable definitions, annotated with type information
 #[derive(Debug, Clone, Default)]
@@ -228,7 +229,9 @@ impl ExecutableDocument {
     }
 
     pub fn validate(&self, schema: &Schema) -> Result<(), DiagnosticList> {
-        let mut errors = DiagnosticList::new(Some(schema.sources.clone()), self.sources.clone());
+        let mut sources = IndexMap::clone(&schema.sources);
+        sources.extend(self.sources.iter().map(|(k, v)| (*k, v.clone())));
+        let mut errors = DiagnosticList::new(Arc::new(sources));
         validation::validate_executable_document(&mut errors, schema, self);
         errors.into_result()
     }
@@ -677,7 +680,9 @@ impl FieldSet {
     }
 
     pub fn validate(&self, schema: &Schema) -> Result<(), DiagnosticList> {
-        let mut errors = DiagnosticList::new(Some(schema.sources.clone()), self.sources.clone());
+        let mut sources = IndexMap::clone(&schema.sources);
+        sources.extend(self.sources.iter().map(|(k, v)| (*k, v.clone())));
+        let mut errors = DiagnosticList::new(Arc::new(sources));
         validation::validate_field_set(&mut errors, schema, self);
         errors.into_result()
     }
diff --git a/crates/apollo-compiler/src/parser.rs b/crates/apollo-compiler/src/parser.rs
index efc95ba3b..18d9d736c 100644
--- a/crates/apollo-compiler/src/parser.rs
+++ b/crates/apollo-compiler/src/parser.rs
@@ -251,7 +251,7 @@ impl Parser {
         let file_id = FileId::new();
 
         let sources: crate::SourceMap = Arc::new([(file_id, source_file)].into());
-        let mut errors = DiagnosticList::new(None, sources.clone());
+        let mut errors = DiagnosticList::new(sources.clone());
         for (file_id, source) in sources.iter() {
             source.validate_parse_errors(&mut errors, *file_id)
         }
diff --git a/crates/apollo-compiler/src/schema/mod.rs b/crates/apollo-compiler/src/schema/mod.rs
index 601adaaf9..0b0f95972 100644
--- a/crates/apollo-compiler/src/schema/mod.rs
+++ b/crates/apollo-compiler/src/schema/mod.rs
@@ -314,7 +314,7 @@ impl Schema {
 
     /// Returns `Err` if invalid
     pub fn validate(&self) -> Result<(), DiagnosticList> {
-        let mut errors = DiagnosticList::new(None, self.sources.clone());
+        let mut errors = DiagnosticList::new(self.sources.clone());
         validation::validate_schema(&mut errors, self);
         errors.into_result()
     }
diff --git a/crates/apollo-compiler/src/validation/mod.rs b/crates/apollo-compiler/src/validation/mod.rs
index 7f74f26c9..c7d79ef12 100644
--- a/crates/apollo-compiler/src/validation/mod.rs
+++ b/crates/apollo-compiler/src/validation/mod.rs
@@ -35,20 +35,11 @@ pub use crate::database::FileId;
 pub use crate::node::NodeLocation;
 
 /// A collection of diagnostics returned by some validation method
-pub struct DiagnosticList(Box<DiagnosticListBoxed>);
-
-/// Box indirection to avoid large `Err` values:
-/// <https://rust-lang.github.io/rust-clippy/master/index.html#result_large_err>
-struct DiagnosticListBoxed {
-    sources: Sources,
+pub struct DiagnosticList {
+    sources: SourceMap,
     diagnostics_data: Vec<DiagnosticData>,
 }
 
-struct Sources {
-    schema_sources: Option<SourceMap>,
-    self_sources: SourceMap,
-}
-
 struct DiagnosticData {
     location: Option<NodeLocation>,
     details: Details,
@@ -56,7 +47,7 @@ struct DiagnosticData {
 
 /// A single diagnostic in a [`DiagnosticList`]
 pub struct Diagnostic<'a> {
-    sources: &'a Sources,
+    sources: &'a SourceMap,
     data: &'a DiagnosticData,
 }
 
@@ -300,19 +291,26 @@ impl<'a> Diagnostic<'a> {
 }
 
 impl DiagnosticList {
+    pub(crate) fn new(sources: SourceMap) -> Self {
+        Self {
+            sources,
+            diagnostics_data: Vec::new(),
+        }
+    }
+
     pub fn is_empty(&self) -> bool {
-        self.0.diagnostics_data.is_empty()
+        self.diagnostics_data.is_empty()
     }
 
     pub fn len(&self) -> usize {
-        self.0.diagnostics_data.len()
+        self.diagnostics_data.len()
     }
 
     pub fn iter(
         &self,
     ) -> impl Iterator<Item = Diagnostic<'_>> + DoubleEndedIterator + ExactSizeIterator {
-        self.0.diagnostics_data.iter().map(|data| Diagnostic {
-            sources: &self.0.sources,
+        self.diagnostics_data.iter().map(|data| Diagnostic {
+            sources: &self.sources,
             data,
         })
     }
@@ -325,36 +323,20 @@ impl DiagnosticList {
         format!("{self:#}")
     }
 
-    pub(crate) fn new(schema_sources: Option<SourceMap>, self_sources: SourceMap) -> Self {
-        Self(Box::new(DiagnosticListBoxed {
-            sources: Sources {
-                schema_sources,
-                self_sources,
-            },
-            diagnostics_data: Vec::new(),
-        }))
-    }
-
     pub(crate) fn push(&mut self, location: Option<NodeLocation>, details: Details) {
-        self.0
-            .diagnostics_data
+        self.diagnostics_data
             .push(DiagnosticData { location, details })
     }
 
     pub(crate) fn into_result(mut self) -> Result<(), Self> {
-        if self.0.diagnostics_data.is_empty() {
+        if self.diagnostics_data.is_empty() {
             Ok(())
         } else {
-            self.sort();
+            self.diagnostics_data
+                .sort_by_key(|err| err.location.map(|loc| (loc.file_id(), loc.offset())));
             Err(self)
         }
     }
-
-    pub(crate) fn sort(&mut self) {
-        self.0
-            .diagnostics_data
-            .sort_by_key(|err| err.location.map(|loc| (loc.file_id(), loc.offset())))
-    }
 }
 
 /// Defaults to ANSI color codes if stderr is a terminal.
@@ -391,20 +373,14 @@ impl fmt::Display for Diagnostic<'_> {
         let color = !f.alternate();
         self.data
             .report(color)
-            .write(self.sources, Adaptor(f))
+            .write(Cache(self.sources), Adaptor(f))
             .map_err(|_| fmt::Error)
     }
 }
 
-impl Sources {
-    pub(crate) fn get(&self, file_id: &FileId) -> Option<&Arc<SourceFile>> {
-        self.self_sources
-            .get(file_id)
-            .or_else(|| self.schema_sources.as_ref()?.get(file_id))
-    }
-}
+struct Cache<'a>(&'a SourceMap);
 
-impl ariadne::Cache<FileId> for &'_ Sources {
+impl ariadne::Cache<FileId> for Cache<'_> {
     fn fetch(&mut self, file_id: &FileId) -> Result<&ariadne::Source, Box<dyn fmt::Debug + '_>> {
         struct NotFound(FileId);
         impl fmt::Debug for NotFound {
@@ -412,7 +388,7 @@ impl ariadne::Cache<FileId> for &'_ Sources {
                 write!(f, "source file not found: {:?}", self.0)
             }
         }
-        if let Some(source_file) = self.get(file_id) {
+        if let Some(source_file) = self.0.get(file_id) {
             Ok(source_file.ariadne())
         } else if *file_id == FileId::NONE || *file_id == FileId::HACK_TMP {
             static EMPTY: OnceLock<ariadne::Source> = OnceLock::new();
@@ -430,7 +406,7 @@ impl ariadne::Cache<FileId> for &'_ Sources {
                     self.0.path().display().fmt(f)
                 }
             }
-            let source_file = self.get(file_id)?;
+            let source_file = self.0.get(file_id)?;
             Some(Box::new(Path(source_file.clone())))
         } else {
             struct NoSourceFile;