Add mutability compatibility check to borrows

reid/src/mir/implement.rs

@@ -385,10 +385,18 @@ impl Collapsable for TypeKind {
             (TypeKind::Vague(Vague::Unknown), other) | (other, TypeKind::Vague(Vague::Unknown)) => {
                 Ok(other.clone())
             }
-            (TypeKind::Borrow(val1, mut1), TypeKind::Borrow(val2, mut2)) => Ok(TypeKind::Borrow(
+            (TypeKind::Borrow(val1, mut1), TypeKind::Borrow(val2, mut2)) => {
-                Box::new(val1.collapse_into(val2)?),
+                // Extracted to give priority for other collapse-error
-                *mut1 && *mut2,
+                let collapsed = val1.collapse_into(val2)?;
-            )),
+                if mut1 == mut2 {
+                    Ok(TypeKind::Borrow(Box::new(collapsed), *mut1 && *mut2))
+                } else {
+                    Err(ErrorKind::TypesDifferMutability(
+                        self.clone(),
+                        other.clone(),
+                    ))
+                }
+            }
             _ => Err(ErrorKind::TypesIncompatible(self.clone(), other.clone())),
         }
     }

reid/src/mir/mod.rs

@@ -109,7 +109,7 @@ pub enum TypeKind {
     Array(Box<TypeKind>, u64),
     #[error("{0}")]
     CustomType(String),
-    #[error("Borrow({0})")]
+    #[error("Borrow({0}, {1})")]
     Borrow(Box<TypeKind>, bool),
     #[error("Ptr({0})")]
     Ptr(Box<TypeKind>),

reid/src/mir/typecheck.rs

@@ -59,6 +59,8 @@ pub enum ErrorKind {
     InvalidSetExpression,
     #[error("Can not deref {0}, as it is not a borrow")]
     AttemptedDerefNonBorrow(String),
+    #[error("Types {0} and {1} differ in mutability")]
+    TypesDifferMutability(TypeKind, TypeKind),
 }
 
 /// Struct used to implement a type-checking pass that can be performed on the

reid_src/borrow_hard.reid

@@ -5,7 +5,7 @@ fn changer(param: &mut u32) {
 }
 
 fn main() -> u32 {
-  let mut value = 6;
+  let value = 6;
 
   changer(&mut value);