diff --git a/Cargo.lock b/Cargo.lock
index 496b1335..fb94cf3a 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -290,6 +290,15 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "88bffebc5d80432c9b140ee17875ff173a8ab62faad5b257da912bd2f6c1c0a1"
 
+[[package]]
+name = "erased-serde"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4adbf0983fe06bd3a5c19c8477a637c2389feb0994eca7a59e3b961054aa7c0a"
+dependencies = [
+ "serde",
+]
+
 [[package]]
 name = "errno"
 version = "0.3.1"
@@ -649,9 +658,9 @@ dependencies = [
 
 [[package]]
 name = "quote"
-version = "1.0.26"
+version = "1.0.33"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4424af4bf778aae2051a77b60283332f386554255d722233d09fbfc7e30da2fc"
+checksum = "5267fca4496028628a95160fc423a33e8b2e6af8a5302579e322e4b520293cae"
 dependencies = [
  "proc-macro2",
 ]
@@ -734,22 +743,32 @@ checksum = "d29ab0c6d3fc0ee92fe66e2d99f700eab17a8d57d1c1d3b748380fb20baa78cd"
 
 [[package]]
 name = "serde"
-version = "1.0.160"
+version = "1.0.193"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bb2f3770c8bce3bcda7e149193a069a0f4365bda1fa5cd88e03bca26afc1216c"
+checksum = "25dd9975e68d0cb5aa1120c288333fc98731bd1dd12f561e468ea4728c042b89"
 dependencies = [
  "serde_derive",
 ]
 
+[[package]]
+name = "serde-untagged"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c38885c2d9d8f038478583b7acf8f6029d020ba4b20e9dcaeb8799d67a04aae7"
+dependencies = [
+ "erased-serde",
+ "serde",
+]
+
 [[package]]
 name = "serde_derive"
-version = "1.0.160"
+version = "1.0.193"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "291a097c63d8497e00160b166a967a4a79c64f3facdd01cbd7502231688d77df"
+checksum = "43576ca501357b9b071ac53cdc7da8ef0cbd9493d8df094cd821777ea6e894d3"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.15",
+ "syn 2.0.41",
 ]
 
 [[package]]
@@ -768,6 +787,9 @@ name = "serde_spanned"
 version = "0.6.4"
 dependencies = [
  "serde",
+ "serde-untagged",
+ "serde_derive",
+ "toml 0.8.8",
 ]
 
 [[package]]
@@ -829,9 +851,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "2.0.15"
+version = "2.0.41"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a34fcf3e8b60f57e6a14301a2e916d323af98b0ea63c599441eec8558660c822"
+checksum = "44c8b28c477cc3bf0e7966561e3460130e1255f7a1cf71931075f1c5e7a7e269"
 dependencies = [
  "proc-macro2",
  "quote",
diff --git a/crates/serde_spanned/Cargo.toml b/crates/serde_spanned/Cargo.toml
index f1a578dc..421dbbf8 100644
--- a/crates/serde_spanned/Cargo.toml
+++ b/crates/serde_spanned/Cargo.toml
@@ -26,3 +26,9 @@ pre-release-replacements = [
 
 [dependencies]
 serde = { version = "1.0.145", optional = true }
+
+[dev-dependencies]
+serde = "1"
+serde_derive = "1"
+serde-untagged = "0.1"
+toml = { path = "../toml" }
diff --git a/crates/serde_spanned/src/spanned.rs b/crates/serde_spanned/src/spanned.rs
index ac351ca8..f4042a86 100644
--- a/crates/serde_spanned/src/spanned.rs
+++ b/crates/serde_spanned/src/spanned.rs
@@ -35,6 +35,63 @@ pub struct Spanned<T> {
 }
 
 impl<T> Spanned<T> {
+    /// Create a spanned value encompassing the given byte range.
+    ///
+    /// # Example
+    ///
+    /// Transposing a `Spanned<Enum<T>>` into `Enum<Spanned<T>>`:
+    ///
+    /// ```
+    /// use serde::de::{Deserialize, Deserializer};
+    /// use serde_untagged::UntaggedEnumVisitor;
+    /// use toml::Spanned;
+    ///
+    /// pub enum Dependency {
+    ///     Simple(Spanned<String>),
+    ///     Detailed(Spanned<DetailedDependency>),
+    /// }
+    ///
+    /// impl<'de> Deserialize<'de> for Dependency {
+    ///     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    ///     where
+    ///         D: Deserializer<'de>,
+    ///     {
+    ///         enum DependencyKind {
+    ///             Simple(String),
+    ///             Detailed(DetailedDependency),
+    ///         }
+    ///
+    ///         impl<'de> Deserialize<'de> for DependencyKind {
+    ///             fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    ///             where
+    ///                 D: Deserializer<'de>,
+    ///             {
+    ///                 UntaggedEnumVisitor::new()
+    ///                     .expecting(
+    ///                         "a version string like \"0.9.8\" or a \
+    ///                             detailed dependency like { version = \"0.9.8\" }",
+    ///                     )
+    ///                     .string(|value| Ok(DependencyKind::Simple(value.to_owned())))
+    ///                     .map(|value| value.deserialize().map(DependencyKind::Detailed))
+    ///                     .deserialize(deserializer)
+    ///             }
+    ///         }
+    ///
+    ///         let spanned: Spanned<DependencyKind> = Deserialize::deserialize(deserializer)?;
+    ///         let range = spanned.span();
+    ///         Ok(match spanned.into_inner() {
+    ///             DependencyKind::Simple(simple) => Dependency::Simple(Spanned::new(range, simple)),
+    ///             DependencyKind::Detailed(detailed) => Dependency::Detailed(Spanned::new(range, detailed)),
+    ///         })
+    ///     }
+    /// }
+    /// #
+    /// # type DetailedDependency = std::collections::BTreeMap<String, String>;
+    /// ```
+    pub fn new(range: std::ops::Range<usize>, value: T) -> Self {
+        Spanned { span: range, value }
+    }
+
     /// Byte range
     pub fn span(&self) -> std::ops::Range<usize> {
         self.span.clone()