diff --git a/.travis.yml b/.travis.yml
index 6a08f1279..fe2effe23 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,7 +10,5 @@ addons:
 
 script:
   - rustup target add thumbv6m-none-eabi
+  - cargo test --verbose --features="compiletest"
   - ./scripts/build-examples.sh
-  - cargo test --verbose
-
-cache: cargo
diff --git a/Cargo.toml b/Cargo.toml
index 12915cb8e..73090a15e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -23,6 +23,9 @@ embedded-hal = { version = "0.2", features = ["unproven"] }
 lpc82x       = "0.4"
 nb           = "0.1.1"
 void         = { version = "1", default-features = false }
+# This should be in [dev-dependencies], but those can't be optional. Issue:
+# https://github.com/rust-lang/cargo/issues/1596
+compiletest_rs = { version = "0.3", optional = true }
 
 [dev-dependencies]
 cortex-m-rt = "0.5"
@@ -30,7 +33,10 @@ panic-abort = "0.2"
 
 
 [features]
-rt = ["lpc82x/rt"]
+# This is needed to make the compiletest stuff optional. It requires std, which
+# means we can't build it together with the examples.
+compiletest = ["compiletest_rs"]
+rt          = ["lpc82x/rt"]
 
 
 [[example]]
diff --git a/src/swm.rs b/src/swm.rs
index 86b99df95..56f1b5f71 100644
--- a/src/swm.rs
+++ b/src/swm.rs
@@ -690,7 +690,7 @@ impl<T, F, O, Is> UnassignFunction<F, Input>
      for Pin<T, pin_state::Swm<O, (Is,)>>
      where
         T: PinTrait,
-        F: FunctionTrait<T, Kind=Output>,
+        F: FunctionTrait<T, Kind=Input>,
 {
     type Unassigned = Pin<T, pin_state::Swm<O, Is>>;
 
diff --git a/tests/compile-fail/swm/assign-function-to-multiple-pins.rs b/tests/compile-fail/swm/assign-function-to-multiple-pins.rs
new file mode 100644
index 000000000..4a10651aa
--- /dev/null
+++ b/tests/compile-fail/swm/assign-function-to-multiple-pins.rs
@@ -0,0 +1,27 @@
+extern crate lpc82x_hal;
+
+
+use lpc82x_hal::Peripherals;
+use lpc82x_hal::swm::{
+    self,
+    pin_state,
+    Pin,
+};
+
+
+fn main() {
+    let mut p = Peripherals::take().unwrap();
+
+    let     swm    = p.swm.split();
+    let mut syscon = p.syscon.split();
+
+    let pio0_0: Pin<_, pin_state::Unused> = swm.pins.pio0_0;
+    let pio0_1: Pin<_, pin_state::Unused> = swm.pins.pio0_1;
+
+    let u0_rxd: swm::Function<_, swm::state::Unassigned> =
+        swm.movable_functions.u0_rxd;
+
+    let (u0_rxd, _) = u0_rxd.assign(pio0_0.into_swm_pin(), &mut swm.handle);
+    let (u0_rxd, _) = u0_rxd.assign(pio0_1.into_swm_pin(), &mut swm.handle);
+    //~^ ERROR no method named `assign` found for type
+}
diff --git a/tests/compile-fail/swm/assign-multiple-output-functions.rs b/tests/compile-fail/swm/assign-multiple-output-functions.rs
new file mode 100644
index 000000000..4591cfb6e
--- /dev/null
+++ b/tests/compile-fail/swm/assign-multiple-output-functions.rs
@@ -0,0 +1,38 @@
+extern crate lpc82x_hal;
+
+
+use lpc82x_hal::Peripherals;
+use lpc82x_hal::swm::{
+    self,
+    pin_state,
+    Pin,
+};
+
+
+fn main() {
+    let mut p = Peripherals::take().unwrap();
+
+    let     swm    = p.swm.split();
+    let mut syscon = p.syscon.split();
+
+    let pio0_0: Pin<_, pin_state::Unused> = swm.pins.pio0_0;
+
+    let u0_rxd: swm::Function<_, swm::state::Unassigned> =
+        swm.movable_functions.u0_rxd;
+    let u0_txd: swm::Function<_, swm::state::Unassigned> =
+        swm.movable_functions.u0_txd;
+    let u1_rxd: swm::Function<_, swm::state::Unassigned> =
+        swm.movable_functions.u1_rxd;
+    let u1_txd: swm::Function<_, swm::state::Unassigned> =
+        swm.movable_functions.u1_txd;
+
+    let (u0_rxd, pio0_0) =
+        u0_rxd.assign(pio0_0.into_swm_pin(), &mut swm.handle);
+    let (u1_rxd, pio0_0) =
+        u1_rxd.assign(pio0_0, &mut swm.handle);
+    let (u0_txd, pio0_0) =
+        u0_txd.assign(pio0_0, &mut swm.handle);
+    let (u1_txd, pio0_0) =
+        u1_txd.assign(pio0_0, &mut swm.handle);
+    //~^ ERROR the trait bound
+}
diff --git a/tests/compile-fail/swm/unassign-function-from-wrong-pin.rs b/tests/compile-fail/swm/unassign-function-from-wrong-pin.rs
new file mode 100644
index 000000000..d8095c986
--- /dev/null
+++ b/tests/compile-fail/swm/unassign-function-from-wrong-pin.rs
@@ -0,0 +1,27 @@
+extern crate lpc82x_hal;
+
+
+use lpc82x_hal::Peripherals;
+use lpc82x_hal::swm::{
+    self,
+    pin_state,
+    Pin,
+};
+
+
+fn main() {
+    let mut p = Peripherals::take().unwrap();
+
+    let     swm    = p.swm.split();
+    let mut syscon = p.syscon.split();
+
+    let pio0_0: Pin<_, pin_state::Unused> = swm.pins.pio0_0;
+    let pio0_1: Pin<_, pin_state::Unused> = swm.pins.pio0_1;
+
+    let u0_rxd: swm::Function<_, swm::state::Unassigned> =
+        swm.movable_functions.u0_rxd;
+
+    let (u0_rxd, _) = u0_rxd.assign(pio0_0.into_swm_pin(), &mut swm.handle);
+    let (u0_rxd, _) = u0_rxd.unassign(pio0_1.into_swm_pin(), &mut swm.handle);
+    //~^ ERROR mismatched types [E0308]
+}
diff --git a/tests/compile-fail/swm/unassign-unassigned-input-function.rs b/tests/compile-fail/swm/unassign-unassigned-input-function.rs
new file mode 100644
index 000000000..e2d80291a
--- /dev/null
+++ b/tests/compile-fail/swm/unassign-unassigned-input-function.rs
@@ -0,0 +1,30 @@
+extern crate lpc82x_hal;
+
+
+use lpc82x_hal::Peripherals;
+use lpc82x_hal::swm::{
+    self,
+    pin_state,
+    Pin,
+};
+
+
+fn main() {
+    let mut p = Peripherals::take().unwrap();
+
+    let     swm    = p.swm.split();
+    let mut syscon = p.syscon.split();
+
+    let pio0_0: Pin<_, pin_state::Unused> = swm.pins.pio0_0;
+
+    let u0_rxd: swm::Function<_, swm::state::Unassigned> =
+        swm.movable_functions.u0_rxd;
+
+    let (u0_rxd, pio0_0) =
+        u0_rxd.assign(pio0_0.into_swm_pin(), &mut swm.handle);
+    let (u0_rxd, pio0_0) =
+        u0_rxd.unassign(pio0_0, &mut swm.handle);
+    let (u0_rxd, pio0_0) =
+        u0_rxd.unassign(pio0_0, &mut swm.handle);
+    //~^ ERROR no method named `unassign` found for type
+}
diff --git a/tests/compile-fail/swm/unassign-unassigned-output-function.rs b/tests/compile-fail/swm/unassign-unassigned-output-function.rs
new file mode 100644
index 000000000..48f25bf44
--- /dev/null
+++ b/tests/compile-fail/swm/unassign-unassigned-output-function.rs
@@ -0,0 +1,30 @@
+extern crate lpc82x_hal;
+
+
+use lpc82x_hal::Peripherals;
+use lpc82x_hal::swm::{
+    self,
+    pin_state,
+    Pin,
+};
+
+
+fn main() {
+    let mut p = Peripherals::take().unwrap();
+
+    let     swm    = p.swm.split();
+    let mut syscon = p.syscon.split();
+
+    let pio0_0: Pin<_, pin_state::Unused> = swm.pins.pio0_0;
+
+    let u0_txd: swm::Function<_, swm::state::Unassigned> =
+        swm.movable_functions.u0_txd;
+
+    let (u0_txd, pio0_0) =
+        u0_txd.assign(pio0_0.into_swm_pin(), &mut swm.handle);
+    let (u0_txd, pio0_0) =
+        u0_txd.unassign(pio0_0, &mut swm.handle);
+    let (u0_txd, pio0_0) =
+        u0_txd.unassign(pio0_0, &mut swm.handle);
+    //~^ ERROR no method named `unassign` found for type
+}
diff --git a/tests/compiletest.rs b/tests/compiletest.rs
new file mode 100644
index 000000000..7dac77a21
--- /dev/null
+++ b/tests/compiletest.rs
@@ -0,0 +1,28 @@
+#![cfg(feature = "compiletest")]
+
+
+extern crate compiletest_rs as compiletest;
+
+
+use std::path::PathBuf;
+
+
+#[test]
+fn compile_test() {
+    run_mode("compile-fail");
+}
+
+fn run_mode(mode: &'static str) {
+    let mut config = compiletest::Config::default();
+
+    config.mode     = mode.parse().expect("Failed to parse mode");
+    config.src_base = PathBuf::from(format!("tests/{}", mode));
+
+    // Needed by the compiler to find other crates
+    config.link_deps();
+
+    // Fixes E0464, "multiple matching crates"
+    config.clean_rmeta();
+
+    compiletest::run_tests(&config);
+}