diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 2de7e23..19cdd28 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [1.31.0, stable, beta, nightly]
+        rust: [1.56.0, stable, beta, nightly]
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@master
@@ -39,7 +39,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: dtolnay/rust-toolchain@1.62.0
+      - uses: dtolnay/rust-toolchain@1.70.0
         with:
           components: rustfmt
       - run: cargo fmt --all --check
diff --git a/.github/workflows/master.yaml b/.github/workflows/master.yaml
index 98e8f8e..f23d140 100644
--- a/.github/workflows/master.yaml
+++ b/.github/workflows/master.yaml
@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [1.31.0, stable]
+        rust: [1.56.0, stable]
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@master
diff --git a/.github/workflows/pr.yaml b/.github/workflows/pr.yaml
index d221a73..e195455 100644
--- a/.github/workflows/pr.yaml
+++ b/.github/workflows/pr.yaml
@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [1.31.0, stable]
+        rust: [1.56.0, stable]
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@master
@@ -23,7 +23,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: dtolnay/rust-toolchain@1.62.0
+      - uses: dtolnay/rust-toolchain@1.70.0
         with:
           components: rustfmt
       - run: cargo fmt --all --check
diff --git a/Cargo.toml b/Cargo.toml
index 5addf90..a2f19e6 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -8,35 +8,41 @@ categories = [ "science" ]
 license = "MIT OR Apache-2.0"
 name = "num-derive"
 repository = "https://github.com/rust-num/num-derive"
-version = "0.3.3"
+version = "0.4.0"
 readme = "README.md"
 exclude = ["/bors.toml", "/ci/*", "/.github/*"]
-edition = "2018"
+edition = "2021"
+rust-version = "1.56.0"
 
 [dependencies]
 proc-macro2 = "1"
 quote = "1"
-syn = "1"
+syn = "2.0.5"
 
 [dev-dependencies]
-num = "0.3"
+num = "0.4"
 num-traits = "0.2"
 
-[features]
-full-syntax = ["syn/full"]
-
 [lib]
 name = "num_derive"
 proc-macro = true
 test = false
 
-# Most of the tests are left implicily detected, compiled for Rust 2018,
-# but let's try a few of them with the old 2015 edition too.
+# Most of the tests are left implicily detected, compiled for Rust 2021,
+# but let's try a few of them with the older editions too.
 
 [[test]]
 name = "newtype-2015"
 edition = "2015"
 
+[[test]]
+name = "newtype-2018"
+edition = "2018"
+
 [[test]]
 name = "trivial-2015"
 edition = "2015"
+
+[[test]]
+name = "trivial-2018"
+edition = "2018"
diff --git a/README.md b/README.md
index 9826cdf..20d66a6 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 [![crate](https://img.shields.io/crates/v/num-derive.svg)](https://crates.io/crates/num-derive)
 [![documentation](https://docs.rs/num-derive/badge.svg)](https://docs.rs/num-derive)
-[![minimum rustc 1.31](https://img.shields.io/badge/rustc-1.31+-red.svg)](https://rust-lang.github.io/rfcs/2495-min-rust-version.html)
+[![minimum rustc 1.56](https://img.shields.io/badge/rustc-1.56+-red.svg)](https://rust-lang.github.io/rfcs/2495-min-rust-version.html)
 [![build status](https://github.com/rust-num/num-derive/workflows/master/badge.svg)](https://github.com/rust-num/num-derive/actions)
 
 Procedural macros to derive numeric traits in Rust.
@@ -14,7 +14,7 @@ Add this to your `Cargo.toml`:
 ```toml
 [dependencies]
 num-traits = "0.2"
-num-derive = "0.3"
+num-derive = "0.4"
 ```
 
 and this to your crate root:
@@ -51,7 +51,7 @@ Release notes are available in [RELEASES.md](RELEASES.md).
 
 ## Compatibility
 
-The `num-derive` crate is tested for rustc 1.31 and greater.
+The `num-derive` crate is tested for rustc 1.56 and greater.
 
 ## License
 
diff --git a/RELEASES.md b/RELEASES.md
index c3e18cc..fbb34da 100644
--- a/RELEASES.md
+++ b/RELEASES.md
@@ -1,3 +1,11 @@
+# Release 0.4.0 (2023-06-29)
+
+- [Update to syn-2][54] -- thanks @maurer!
+  - This raises the minimum supported rustc to 1.56.
+  - The "full-syntax" feature has also been removed.
+
+[54]: https://github.com/rust-num/num-derive/pull/54
+
 # Release 0.3.3 (2020-10-29)
 
 - [Make `NumOps` work with `no_std`][41] -- thanks @jedrzejboczar!
diff --git a/bors.toml b/bors.toml
index 4c20337..f44ffa3 100644
--- a/bors.toml
+++ b/bors.toml
@@ -1,5 +1,5 @@
 status = [
-  "Test (1.31.0)",
+  "Test (1.56.0)",
   "Test (stable)",
   "Test (beta)",
   "Test (nightly)",
diff --git a/ci/rustup.sh b/ci/rustup.sh
index fed3e45..5289138 100755
--- a/ci/rustup.sh
+++ b/ci/rustup.sh
@@ -5,6 +5,6 @@
 set -ex
 
 ci=$(dirname $0)
-for version in 1.31.0 stable beta nightly; do
+for version in 1.56.0 stable beta nightly; do
     rustup run "$version" "$ci/test_full.sh"
 done
diff --git a/ci/test_full.sh b/ci/test_full.sh
index 513fbc4..d3fe896 100755
--- a/ci/test_full.sh
+++ b/ci/test_full.sh
@@ -3,7 +3,7 @@
 set -e
 
 CRATE=num-derive
-MSRV=1.31
+MSRV=1.56
 
 get_rust_version() {
   local array=($(rustc --version));
@@ -27,7 +27,7 @@ if ! check_version $MSRV ; then
   exit 1
 fi
 
-FEATURES=(full-syntax)
+FEATURES=()
 echo "Testing supported features: ${FEATURES[*]}"
 
 set -x
diff --git a/src/lib.rs b/src/lib.rs
index ef55e4b..26b9c77 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -71,19 +71,11 @@ use quote::quote;
 use syn::{Data, Fields, Ident};
 
 /// Try to parse the tokens, or else return a compilation error
-/// suggesting "full-syntax" if that's not already enabled.
 macro_rules! parse {
     ($tokens:ident as $type:ty) => {
         match syn::parse::<$type>($tokens) {
             Ok(parsed) => parsed,
-            Err(mut error) => {
-                if cfg!(not(feature = "full-syntax")) {
-                    let hint = syn::Error::new(
-                        Span::call_site(),
-                        r#"this might need the "full-syntax" feature of `num-derive`"#,
-                    );
-                    error.combine(hint);
-                }
+            Err(error) => {
                 return TokenStream::from(error.to_compile_error());
             }
         }
@@ -170,16 +162,22 @@ impl NumTraits {
         // retrieve its value, and use it to create an `Ident` to be used
         // to import the `num_traits` crate.
         for attr in &ast.attrs {
-            if let Ok(syn::Meta::NameValue(mnv)) = attr.parse_meta() {
-                if mnv.path.is_ident("num_traits") {
-                    if let syn::Lit::Str(lit_str) = mnv.lit {
-                        return NumTraits {
-                            import: syn::Ident::new(&lit_str.value(), lit_str.span()),
-                            explicit: true,
-                        };
-                    } else {
-                        panic!("#[num_traits] attribute value must be a str");
-                    }
+            if attr.path().is_ident("num_traits") {
+                if let Ok(syn::MetaNameValue {
+                    value:
+                        syn::Expr::Lit(syn::ExprLit {
+                            lit: syn::Lit::Str(ref lit_str),
+                            ..
+                        }),
+                    ..
+                }) = attr.meta.require_name_value()
+                {
+                    return NumTraits {
+                        import: syn::Ident::new(&lit_str.value(), lit_str.span()),
+                        explicit: true,
+                    };
+                } else {
+                    panic!("#[num_traits] attribute value must be a str");
                 }
             }
         }
@@ -954,5 +952,3 @@ pub fn float(input: TokenStream) -> TokenStream {
 
     import.wrap("Float", &name, impl_).into()
 }
-
-mod test;
diff --git a/src/test.rs b/src/test.rs
deleted file mode 100644
index c4cd7fe..0000000
--- a/src/test.rs
+++ /dev/null
@@ -1,31 +0,0 @@
-//! This module uses doc-tests on modules for `compile_fail`
-
-// We need "syn/full" to parse macros.
-// Use `--nocapture` to check the quality of the error message.
-#[cfg(not(feature = "full-syntax"))]
-/// ```compile_fail
-/// macro_rules! get_an_isize {
-///     () => (0_isize)
-/// }
-///
-/// #[derive(num_derive::FromPrimitive)]
-/// pub enum CLikeEnum {
-///     VarA = get_an_isize!(), // error without "syn/full"
-///     VarB = 2,
-/// }
-/// ```
-mod issue16 {}
-
-#[cfg(feature = "full-syntax")]
-/// ```
-/// macro_rules! get_an_isize {
-///     () => (0_isize)
-/// }
-///
-/// #[derive(num_derive::FromPrimitive)]
-/// pub enum CLikeEnum {
-///     VarA = get_an_isize!(), // ok with "syn/full"
-///     VarB = 2,
-/// }
-/// ```
-mod issue16 {}
diff --git a/tests/issue-16.rs b/tests/issue-16.rs
new file mode 100644
index 0000000..48cc2fb
--- /dev/null
+++ b/tests/issue-16.rs
@@ -0,0 +1,11 @@
+macro_rules! get_an_isize {
+    () => {
+        0_isize
+    };
+}
+
+#[derive(num_derive::FromPrimitive)]
+pub enum CLikeEnum {
+    VarA = get_an_isize!(),
+    VarB = 2,
+}
diff --git a/tests/newtype-2018.rs b/tests/newtype-2018.rs
new file mode 100644
index 0000000..814529e
--- /dev/null
+++ b/tests/newtype-2018.rs
@@ -0,0 +1,2 @@
+// Same source, just compiled for 2018 edition
+include!("newtype.rs");
diff --git a/tests/trivial-2018.rs b/tests/trivial-2018.rs
new file mode 100644
index 0000000..92b971b
--- /dev/null
+++ b/tests/trivial-2018.rs
@@ -0,0 +1,2 @@
+// Same source, just compiled for 2018 edition
+include!("trivial.rs");
diff --git a/tests/with_custom_values.rs b/tests/with_custom_values.rs
index 94c8445..9b202f4 100644
--- a/tests/with_custom_values.rs
+++ b/tests/with_custom_values.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![cfg(feature = "full-syntax")]
-
 extern crate num as num_renamed;
 #[macro_use]
 extern crate num_derive;