diff --git a/crates/cargo-util-schemas/src/core/source_kind.rs b/crates/cargo-util-schemas/src/core/source_kind.rs
index 3794791114d..1c16a251371 100644
--- a/crates/cargo-util-schemas/src/core/source_kind.rs
+++ b/crates/cargo-util-schemas/src/core/source_kind.rs
@@ -15,6 +15,8 @@ pub enum SourceKind {
     LocalRegistry,
     /// A directory-based registry.
     Directory,
+    /// Package sources distributed with the rust toolchain
+    Builtin,
 }
 
 // The hash here is important for what folder packages get downloaded into.
@@ -40,6 +42,7 @@ impl SourceKind {
             SourceKind::SparseRegistry => None,
             SourceKind::LocalRegistry => Some("local-registry"),
             SourceKind::Directory => Some("directory"),
+            SourceKind::Builtin => Some("builtin"),
         }
     }
 }
@@ -71,6 +74,10 @@ impl Ord for SourceKind {
             (_, SourceKind::Directory) => Ordering::Greater,
 
             (SourceKind::Git(a), SourceKind::Git(b)) => a.cmp(b),
+            (SourceKind::Git(_), _) => Ordering::Less,
+            (_, SourceKind::Git(_)) => Ordering::Greater,
+
+            (SourceKind::Builtin, SourceKind::Builtin) => Ordering::Equal,
         }
     }
 }
diff --git a/src/workspace/parser/mod.rs b/src/workspace/parser/mod.rs
index 0f862ac2ecc..52076921beb 100644
--- a/src/workspace/parser/mod.rs
+++ b/src/workspace/parser/mod.rs
@@ -2455,7 +2455,7 @@ fn to_dependency_source_id<P: ResolveToPath + Clone>(
         if kind == Some(DepKind::Build) {
             bail!("builtin dependency `{name_in_toml}` cannot be used as a build dependency")
         }
-        todo!("SourceKind::Builtin");
+        return SourceId::for_builtin();
     }
 
     match (
diff --git a/src/workspace/source_id.rs b/src/workspace/source_id.rs
index aaf736a69ae..e9f54ea1a7d 100644
--- a/src/workspace/source_id.rs
+++ b/src/workspace/source_id.rs
@@ -176,6 +176,7 @@ impl SourceId {
                 let url = url.into_url()?;
                 SourceId::new(SourceKind::Path, url, None)
             }
+            "builtin" => SourceId::for_builtin(),
             kind => Err(anyhow::format_err!("unsupported source protocol: {}", kind)),
         }
     }
@@ -204,6 +205,12 @@ impl SourceId {
         SourceId::new(SourceKind::Path, url, None)
     }
 
+    /// Creates a `SourceId` for the builtin packages in the configured toolchain.
+    pub fn for_builtin() -> CargoResult<SourceId> {
+        let url = "builtin://".into_url()?;
+        SourceId::new(SourceKind::Builtin, url, None)
+    }
+
     /// Creates a `SourceId` from a filesystem path.
     ///
     /// `path`: an absolute path.
@@ -404,6 +411,7 @@ impl SourceId {
                 }
                 Ok(Box::new(PathSource::new(&path, self, gctx)))
             }
+            SourceKind::Builtin => todo!("builtin source"),
             SourceKind::Registry | SourceKind::SparseRegistry => {
                 Ok(Box::new(RegistrySource::remote(self, gctx)?))
             }
@@ -664,6 +672,7 @@ impl fmt::Display for SourceId {
                 Ok(())
             }
             SourceKind::Path => write!(f, "{}", url_display(&self.inner.url)),
+            SourceKind::Builtin => write!(f, "builtin {}", url_display(&self.inner.url)),
             SourceKind::Registry | SourceKind::SparseRegistry => {
                 write!(f, "registry `{}`", self.display_registry_name())
             }
@@ -846,6 +855,10 @@ mod tests {
         assert_data_eq!(gen_hash(source_id), str!["473480029881867801"].raw());
         assert_data_eq!(short_hash(&source_id), str!["199e591d94239206"].raw());
 
+        let source_id = SourceId::for_builtin().unwrap();
+        assert_data_eq!(gen_hash(source_id), str!["8471877086995577839"].raw());
+        assert_data_eq!(short_hash(&source_id), str!["ef7b241f53279275"].raw());
+
         let path = &ws_root.join("crate");
         let source_id = SourceId::for_local_registry(path).unwrap();
         #[cfg(not(windows))]
diff --git a/tests/testsuite/builtin_dep.rs b/tests/testsuite/builtin_dep.rs
index dd16c219241..806511263f0 100644
--- a/tests/testsuite/builtin_dep.rs
+++ b/tests/testsuite/builtin_dep.rs
@@ -27,7 +27,7 @@ fn builtin_dep_accepted() {
         .with_stderr_data(str![[r#"
 
 thread [..] panicked at [..]
-not yet implemented: SourceKind::Builtin
+not yet implemented: builtin source
 [NOTE] run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 
 "#]])