diff --git a/.travis.yml b/.travis.yml
index 48649c8..5561513 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,12 +3,12 @@ cache: cargo
 rust:
   - stable
   - beta
-  - 1.34.0
+  - 1.40.0
 jobs:
   include:
-    - rust: 1.34.0
+    - rust: 1.40.0
       env: TEST_MINIMAL_VERSIONS=1
-    - rust: 1.34.0
+    - rust: 1.40.0
       env: LINT=1
 before_install:
   - |
@@ -28,8 +28,7 @@ before_script:
         cargo +nightly update -Z minimal-versions
     fi
 script:
-  # TODO: Use `--workspace` instead of `--all` for Rust 1.39 or later.
-  - if [ "${LINT:-0}" -eq 0 ] ; then cargo build --verbose --all --all-features && cargo test --verbose --all --all-features ; fi
+  - if [ "${LINT:-0}" -eq 0 ] ; then cargo build --verbose --workspace --all-features && cargo test --verbose --workspace --all-features ; fi
   # Fail if the code is correctly formatted.
   - if [ "${LINT:-0}" -ne 0 ] ; then cargo fmt --all -- --check ; fi
   # Fail if the code has warnings.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index a3e98c8..7fddc3c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,33 @@
 
 ## [Unreleased]
 
+## [0.6.0]
+
+* Minimum supported Rust version is bumped to 1.40.0.
+* Add an FBX version field to `any::AnyTree::V7400` variant
+  (372a2f6e0314eed86cc2c493d2e2fc86aa226781).
+* Add `any::AnyTree::fbx_version()` method (372a2f6e0314eed86cc2c493d2e2fc86aa226781).
+
+### Breaking changes
+* Add an FBX version field to `any::AnyTree::V7400` variant
+  (372a2f6e0314eed86cc2c493d2e2fc86aa226781).
+    + This is mainly used by newly added `any::AnyTree::fbx_version()`, but also useful for users to
+      know FBX version.
+        - For example, when users want to re-export the tree, they might want to know FBX version of
+          the source document.
+
+### Added
+* Add `any::AnyTree::fbx_version()` method (372a2f6e0314eed86cc2c493d2e2fc86aa226781).
+    + Using this, users can get FBX version of the tree even if the `AnyTree` variant is unknown for
+      users.
+    + By this method, users can emit meaningful error message if the tree is returned as unknown
+      variant.
+
+### Non-breaking changes
+* Use `#[non_exhaustive]` instead of hidden dummy variants for enums
+  (b4c0cf53fcefb2dc13850e09ac1ff15bc57a68e5).
+    + Users won't affected by this internal change.
+
 ## [0.5.0]
 
 * `pull_parser::error::{DataError, OperationError, Warning}` is now nonexhaustive.
@@ -242,8 +269,9 @@
 
 Totally rewritten.
 
-[Unreleased]: <https://github.com/lo48576/fbxcel/compare/v0.5.0...develop>
-[0.4.4]: <https://github.com/lo48576/fbxcel/releases/tag/v0.5.0>
+[Unreleased]: <https://github.com/lo48576/fbxcel/compare/v0.6.0...develop>
+[0.6.0]: <https://github.com/lo48576/fbxcel/releases/tag/v0.6.0>
+[0.5.0]: <https://github.com/lo48576/fbxcel/releases/tag/v0.5.0>
 [0.4.4]: <https://github.com/lo48576/fbxcel/releases/tag/v0.4.4>
 [0.4.3]: <https://github.com/lo48576/fbxcel/releases/tag/v0.4.3>
 [0.4.2]: <https://github.com/lo48576/fbxcel/releases/tag/v0.4.2>
diff --git a/Cargo.toml b/Cargo.toml
index 7ae8a63..b266481 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "fbxcel"
-version = "0.5.0"
+version = "0.6.0"
 authors = ["YOSHIOKA Takuma <lo48576@hard-wi.red>"]
 edition = "2018"
 license = "MIT OR Apache-2.0"
diff --git a/README.md b/README.md
index be06ba8..214f69f 100644
--- a/README.md
+++ b/README.md
@@ -3,7 +3,7 @@
 [![Build Status](https://travis-ci.com/lo48576/fbxcel.svg?branch=develop)](https://travis-ci.com/lo48576/fbxcel)
 [![Latest version](https://img.shields.io/crates/v/fbxcel.svg)](https://crates.io/crates/fbxcel)
 [![Documentation](https://docs.rs/fbxcel/badge.svg)](https://docs.rs/fbxcel)
-![Minimum rustc version: 1.34](https://img.shields.io/badge/rustc-1.34+-lightgray.svg)
+![Minimum rustc version: 1.40](https://img.shields.io/badge/rustc-1.40+-lightgray.svg)
 
 `fbxcel` is an FBX library for Rust programming language.
 
@@ -37,7 +37,7 @@ Currently there is no plan to support FBX ASCII format.
 
 ## Rust version
 
-Latest stable compiler (currently 1.34) is supported.
+Latest stable compiler (currently 1.40) is supported.
 
 ## License
 
diff --git a/examples/load-tree.rs b/examples/load-tree.rs
index 9593a2b..adee28f 100644
--- a/examples/load-tree.rs
+++ b/examples/load-tree.rs
@@ -16,7 +16,8 @@ pub fn main() {
     let reader = BufReader::new(file);
 
     match AnyTree::from_seekable_reader(reader).expect("Failed to load tree") {
-        AnyTree::V7400(tree, footer) => {
+        AnyTree::V7400(fbx_version, tree, footer) => {
+            println!("FBX version = {:#?}", fbx_version);
             println!("tree = {:#?}", tree);
             println!("footer = {:#?}", footer);
         }
diff --git a/src/pull_parser/any.rs b/src/pull_parser/any.rs
index 3c4d70a..0b9c5f4 100644
--- a/src/pull_parser/any.rs
+++ b/src/pull_parser/any.rs
@@ -16,11 +16,10 @@ pub use self::error::{Error, Result};
 mod error;
 
 /// FBX tree type with any supported version.
+#[non_exhaustive]
 pub enum AnyParser<R> {
     /// FBX 7.4 or later.
     V7400(pull_parser::v7400::Parser<R>),
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl<R: ParserSource> AnyParser<R> {
@@ -28,7 +27,6 @@ impl<R: ParserSource> AnyParser<R> {
     pub fn parser_version(&self) -> ParserVersion {
         match self {
             AnyParser::V7400(_) => pull_parser::v7400::Parser::<R>::PARSER_VERSION,
-            AnyParser::__Nonexhaustive => panic!("`__Nonexhaustive` should not be used"),
         }
     }
 
@@ -36,7 +34,6 @@ impl<R: ParserSource> AnyParser<R> {
     pub fn fbx_version(&self) -> FbxVersion {
         match self {
             AnyParser::V7400(parser) => parser.fbx_version(),
-            AnyParser::__Nonexhaustive => panic!("`__Nonexhaustive` should not be used"),
         }
     }
 }
@@ -69,7 +66,6 @@ pub fn from_reader<R: Read>(mut reader: R) -> Result<AnyParser<PlainSource<R>>>
             });
             Ok(AnyParser::V7400(parser))
         }
-        ParserVersion::__Nonexhaustive => unreachable!("`__Nonexhaustive` should never be used"),
     }
 }
 
@@ -88,6 +84,5 @@ pub fn from_seekable_reader<R: Read + Seek>(mut reader: R) -> Result<AnyParser<S
                 });
             Ok(AnyParser::V7400(parser))
         }
-        ParserVersion::__Nonexhaustive => unreachable!("`__Nonexhaustive` should never be used"),
     }
 }
diff --git a/src/pull_parser/any/error.rs b/src/pull_parser/any/error.rs
index a025025..f7ce9cc 100644
--- a/src/pull_parser/any/error.rs
+++ b/src/pull_parser/any/error.rs
@@ -9,20 +9,18 @@ pub type Result<T> = std::result::Result<T, Error>;
 
 /// Error.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
     /// Header error.
     Header(HeaderError),
     /// Unsupported version.
     UnsupportedVersion(FbxVersion),
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl error::Error for Error {
     fn source(&self) -> Option<&(dyn error::Error + 'static)> {
         match self {
             Error::Header(e) => Some(e),
-            Error::__Nonexhaustive => panic!("`__Nonexhaustive` should not be used"),
             _ => None,
         }
     }
@@ -33,7 +31,6 @@ impl fmt::Display for Error {
         match self {
             Error::Header(e) => write!(f, "FBX header error: {}", e),
             Error::UnsupportedVersion(ver) => write!(f, "Unsupported FBX version: {:?}", ver),
-            Error::__Nonexhaustive => panic!("`__Nonexhaustive` should not be used"),
         }
     }
 }
diff --git a/src/pull_parser/error/data.rs b/src/pull_parser/error/data.rs
index 3b92ec8..a9a6d20 100644
--- a/src/pull_parser/error/data.rs
+++ b/src/pull_parser/error/data.rs
@@ -6,6 +6,7 @@ use std::{error, fmt, string::FromUtf8Error};
 
 /// Data error.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum DataError {
     /// Data with broken compression.
     BrokenCompression(Compression, Box<dyn std::error::Error + Send + Sync>),
@@ -43,8 +44,6 @@ pub enum DataError {
     ///
     /// The former is the expected, the latter is a description of the actual value.
     UnexpectedAttribute(String, String),
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl error::Error for DataError {
@@ -88,7 +87,6 @@ impl fmt::Display for DataError {
                 "Unexpected attribute value or type: expected {}, got {}",
                 expected, got
             ),
-            DataError::__Nonexhaustive => unreachable!("`__Nonexhaustive` should never used"),
         }
     }
 }
diff --git a/src/pull_parser/error/operation.rs b/src/pull_parser/error/operation.rs
index b0e18ab..7c12291 100644
--- a/src/pull_parser/error/operation.rs
+++ b/src/pull_parser/error/operation.rs
@@ -6,6 +6,7 @@ use crate::{low::FbxVersion, pull_parser::ParserVersion};
 
 /// Invalid operation.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum OperationError {
     /// Attempt to parse more data while the parsing is aborted.
     AlreadyAborted,
@@ -13,8 +14,6 @@ pub enum OperationError {
     AlreadyFinished,
     /// Attempt to create a parser with unsupported FBX version.
     UnsupportedFbxVersion(ParserVersion, FbxVersion),
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl error::Error for OperationError {}
@@ -34,7 +33,6 @@ impl fmt::Display for OperationError {
                 "Unsupported FBX version: parser={:?}, fbx={:?}",
                 parser, fbx
             ),
-            OperationError::__Nonexhaustive => unreachable!("`__Nonexhaustive` should never used"),
         }
     }
 }
diff --git a/src/pull_parser/error/warning.rs b/src/pull_parser/error/warning.rs
index 93a750f..2f20e61 100644
--- a/src/pull_parser/error/warning.rs
+++ b/src/pull_parser/error/warning.rs
@@ -4,6 +4,7 @@ use std::{error, fmt};
 
 /// Warning.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Warning {
     /// Node name is empty.
     EmptyNodeName,
@@ -23,8 +24,6 @@ pub enum Warning {
     MissingNodeEndMarker,
     /// Unexpected value for footer fields (mainly for unknown fields).
     UnexpectedFooterFieldValue,
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl error::Error for Warning {}
@@ -44,7 +43,6 @@ impl fmt::Display for Warning {
             ),
             Warning::MissingNodeEndMarker => write!(f, "Missing node end marker"),
             Warning::UnexpectedFooterFieldValue => write!(f, "Unexpected footer field value"),
-            Warning::__Nonexhaustive => unreachable!("`__Nonexhaustive` should never used"),
         }
     }
 }
diff --git a/src/pull_parser/version.rs b/src/pull_parser/version.rs
index fbec983..4c13d97 100644
--- a/src/pull_parser/version.rs
+++ b/src/pull_parser/version.rs
@@ -9,11 +9,10 @@ use crate::low::FbxVersion;
 /// Some parser supports multiple versions of FBX binary.
 /// Each variants of this type corresponds to a parser implementation.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[non_exhaustive]
 pub enum ParserVersion {
     /// FBX 7.4 and 7.5.
     V7400,
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl ParserVersion {
diff --git a/src/tree.rs b/src/tree.rs
index b698f07..b06682d 100644
--- a/src/tree.rs
+++ b/src/tree.rs
@@ -25,7 +25,7 @@
 //! // To use readers without `std::io::Seek` implementation, use `from_reader`
 //! // instead.
 //! match AnyTree::from_seekable_reader(reader).expect("Failed to load tree") {
-//!     AnyTree::V7400(tree, footer) => {
+//!     AnyTree::V7400(fbx_version, tree, footer) => {
 //!         // You got a tree (and footer)! Do what you want!
 //!     }
 //!     // `AnyTree` is nonexhaustive.
diff --git a/src/tree/any.rs b/src/tree/any.rs
index 68a885b..818f41e 100644
--- a/src/tree/any.rs
+++ b/src/tree/any.rs
@@ -6,7 +6,7 @@ use log::warn;
 
 pub use self::error::{Error, Result};
 use crate::{
-    low,
+    low::{self, FbxVersion},
     pull_parser::{self, any::AnyParser},
     tree,
 };
@@ -14,14 +14,14 @@ use crate::{
 mod error;
 
 /// FBX tree type with any supported version.
+#[non_exhaustive]
 pub enum AnyTree {
     /// FBX 7.4 or later.
     V7400(
+        FbxVersion,
         tree::v7400::Tree,
         std::result::Result<Box<low::v7400::FbxFooter>, pull_parser::Error>,
     ),
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl AnyTree {
@@ -33,15 +33,15 @@ impl AnyTree {
     pub fn from_reader(reader: impl Read) -> Result<Self> {
         match pull_parser::any::from_reader(reader)? {
             AnyParser::V7400(mut parser) => {
+                let fbx_version = parser.fbx_version();
                 parser.set_warning_handler(|w, pos| {
                     warn!("WARNING: {} (pos={:?})", w, pos);
                     Ok(())
                 });
                 let tree_loader = tree::v7400::Loader::new();
                 let (tree, footer) = tree_loader.load(&mut parser)?;
-                Ok(AnyTree::V7400(tree, footer))
+                Ok(AnyTree::V7400(fbx_version, tree, footer))
             }
-            AnyParser::__Nonexhaustive => unreachable!("`__Nonexhaustive` should never be used"),
         }
     }
 
@@ -49,15 +49,22 @@ impl AnyTree {
     pub fn from_seekable_reader(reader: impl Read + Seek) -> Result<Self> {
         match pull_parser::any::from_seekable_reader(reader)? {
             AnyParser::V7400(mut parser) => {
+                let fbx_version = parser.fbx_version();
                 parser.set_warning_handler(|w, pos| {
                     warn!("WARNING: {} (pos={:?})", w, pos);
                     Ok(())
                 });
                 let tree_loader = tree::v7400::Loader::new();
                 let (tree, footer) = tree_loader.load(&mut parser)?;
-                Ok(AnyTree::V7400(tree, footer))
+                Ok(AnyTree::V7400(fbx_version, tree, footer))
             }
-            AnyParser::__Nonexhaustive => unreachable!("`__Nonexhaustive` should never be used"),
+        }
+    }
+
+    /// Returns the FBX version of the document the tree came from.
+    pub fn fbx_version(&self) -> FbxVersion {
+        match self {
+            Self::V7400(ver, _, _) => *ver,
         }
     }
 }
diff --git a/src/tree/any/error.rs b/src/tree/any/error.rs
index 8b2c602..19530be 100644
--- a/src/tree/any/error.rs
+++ b/src/tree/any/error.rs
@@ -9,6 +9,7 @@ pub type Result<T> = std::result::Result<T, Error>;
 
 /// Error.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
     /// Parser creation error.
     ParserCreation(pull_parser::any::Error),
@@ -16,8 +17,6 @@ pub enum Error {
     Parser(pull_parser::Error),
     /// Tree load error.
     Tree(Box<dyn error::Error + Send + Sync + 'static>),
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl error::Error for Error {
@@ -26,7 +25,6 @@ impl error::Error for Error {
             Error::ParserCreation(e) => Some(e),
             Error::Parser(e) => Some(e),
             Error::Tree(e) => Some(&**e),
-            Error::__Nonexhaustive => panic!("`__Nonexhaustive` should not be used"),
         }
     }
 }
@@ -37,7 +35,6 @@ impl fmt::Display for Error {
             Error::ParserCreation(e) => write!(f, "Failed to create a parser: {}", e),
             Error::Parser(e) => write!(f, "Parser error: {}", e),
             Error::Tree(e) => write!(f, "Tree load error: {}", e),
-            Error::__Nonexhaustive => panic!("`__Nonexhaustive` should not be used"),
         }
     }
 }
diff --git a/src/tree/v7400/error.rs b/src/tree/v7400/error.rs
index 07cacb1..52ebc2b 100644
--- a/src/tree/v7400/error.rs
+++ b/src/tree/v7400/error.rs
@@ -6,6 +6,7 @@ use crate::pull_parser::Error as ParserError;
 
 /// FBX data tree load error.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum LoadError {
     /// Bad parser.
     ///
@@ -13,8 +14,6 @@ pub enum LoadError {
     BadParser,
     /// Parser error.
     Parser(ParserError),
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl fmt::Display for LoadError {
@@ -22,7 +21,6 @@ impl fmt::Display for LoadError {
         match self {
             LoadError::BadParser => f.write_str("Attempt to use a bad parser"),
             LoadError::Parser(e) => write!(f, "Parser error: {}", e),
-            LoadError::__Nonexhaustive => panic!("`__Nonexhaustive` should not be used"),
         }
     }
 }