fexists can be called in the preprocessor

crates/dreamchecker/src/lib.rs

@@ -143,6 +143,7 @@ impl<'o> AssumptionSet<'o> {
                 ConstFn::Newlist => AssumptionSet::from_valid_instance(objtree.expect("/list")),
                 ConstFn::Sound => AssumptionSet::from_valid_instance(objtree.expect("/sound")),
                 ConstFn::Generator => AssumptionSet::from_valid_instance(objtree.expect("/generator")),
+                ConstFn::FileExists => AssumptionSet::default(),
                 ConstFn::Filter => AssumptionSet::default(),
                 ConstFn::File => AssumptionSet::default(),
             },

crates/dreammaker/src/constants.rs

@@ -154,6 +154,8 @@ pub enum ConstFn {
     File,
     /// The `generator()` type constructor.
     Generator,
+    /// `fexists()`, which can be used in preprocessor expressions.
+    FileExists,
 }
 
 /// A constant-evaluation error (usually type mismatch).
@@ -411,6 +413,7 @@ impl fmt::Display for ConstFn {
             ConstFn::Filter => "filter",
             ConstFn::File => "file",
             ConstFn::Generator => "generator",
+            ConstFn::FileExists => "fexists"
         })
     }
 }
@@ -812,6 +815,12 @@ impl<'a> ConstantFolder<'a> {
                         None => return Err(self.error("malformed nameof() call, expression appears to have no name")),
                     }
                 }
+                "fexists" => {
+                    if args.len() != 1 {
+                        return Err(self.error(format!("malformed fexists() call, must have 1 argument and instead has {}", args.len())));
+                    }
+                    Constant::Call(ConstFn::FileExists, self.arguments(args)?)
+                }
                 // other functions are no-goes
                 _ => return Err(self.error(format!("non-constant function call: {}", ident))),
             },

crates/dreammaker/tests/constants_tests.rs

@@ -110,3 +110,11 @@ fn rgb_hcy() {
         Constant::string("#000000"),
     );
 }
+
+#[test]
+fn fexists_noarg() {
+    assert_eq!(
+        eval("fexists()").unwrap_err().description(),
+        "malformed fexists() call, must have 1 argument and instead has 0",
+    );
+}