Make comparison of non-serialisable types illegal.

Fixes #940.

crates/aiken-lang/src/tests/check.rs

@@ -185,6 +185,24 @@ fn illegal_inhabitants_nested() {
     ))
 }
 
+#[test]
+fn illegal_function_comparison() {
+    let source_code = r#"
+    fn not(x: Bool) -> Bool {
+        todo
+    }
+
+    fn foo() -> Bool {
+        not == not
+    }
+    "#;
+
+    assert!(matches!(
+        dbg!(check_validator(parse(source_code))),
+        Err((_, Error::IllegalComparison { .. }))
+    ))
+}
+
 #[test]
 fn illegal_inhabitants_returned() {
     let source_code = r#"

crates/aiken-lang/src/tipo/error.rs

@@ -292,11 +292,24 @@ You can use '{discard}' and numbers to distinguish between similar names.
         Fuzzer = "Fuzzer".if_supports_color(Stdout, |s| s.cyan()),
     ))]
     IllegalTypeInData {
-        #[label]
+        #[label("non-serialisable inhabitants")]
         location: Span,
         tipo: Rc<Type>,
     },
 
+    #[error("I noticed an inadequate use of '=='.\n")]
+    #[diagnostic(code("illegal::comparison"))]
+    #[diagnostic(help(
+        r#"I can compare any value that is serializable to {Data}. This excludes values that are functions, {Fuzzer} or {MillerLoopResult} for example."#,
+        Data = "Data".if_supports_color(Stdout, |s| s.cyan()),
+        Fuzzer = "Fuzzer".if_supports_color(Stdout, |s| s.cyan()),
+        MillerLoopResult = "MillerLoopResult".if_supports_color(Stdout, |s| s.cyan()),
+    ))]
+    IllegalComparison {
+        #[label("non-serialisable operands")]
+        location: Span,
+    },
+
     #[error("I found a discarded expression not bound to a variable.\n")]
     #[diagnostic(code("implicit_discard"))]
     #[diagnostic(help(
@@ -1033,6 +1046,7 @@ impl ExtraData for Error {
             | Error::ExtraVarInAlternativePattern { .. }
             | Error::FunctionTypeInData { .. }
             | Error::IllegalTypeInData { .. }
+            | Error::IllegalComparison { .. }
             | Error::ImplicitlyDiscardedExpression { .. }
             | Error::IncorrectFieldsArity { .. }
             | Error::IncorrectFunctionCallArity { .. }

crates/aiken-lang/src/tipo/expr.rs

@@ -624,6 +624,11 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
 
                 self.unify(left.tipo(), right.tipo(), right.location(), false)?;
 
+                for tipo in &[left.tipo(), right.tipo()] {
+                    ensure_serialisable(false, tipo.clone(), location)
+                        .map_err(|_| Error::IllegalComparison { location })?;
+                }
+
                 return Ok(TypedExpr::BinOp {
                     location,
                     name,