From 1fadaa60f22cbefd4feab72d887c01f95f211cd3 Mon Sep 17 00:00:00 2001
From: sofia <sofia@teascade.net>
Date: Mon, 21 Jul 2025 10:33:37 +0300
Subject: [PATCH] Add mutability compatibility check to borrows

---
 reid/src/mir/implement.rs | 16 ++++++++++++----
 reid/src/mir/mod.rs       |  2 +-
 reid/src/mir/typecheck.rs |  2 ++
 reid_src/borrow_hard.reid |  2 +-
 4 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/reid/src/mir/implement.rs b/reid/src/mir/implement.rs
index 889155d..ba46bdc 100644
--- a/reid/src/mir/implement.rs
+++ b/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(
-                Box::new(val1.collapse_into(val2)?),
-                *mut1 && *mut2,
-            )),
+            (TypeKind::Borrow(val1, mut1), TypeKind::Borrow(val2, mut2)) => {
+                // Extracted to give priority for other collapse-error
+                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())),
         }
     }
diff --git a/reid/src/mir/mod.rs b/reid/src/mir/mod.rs
index e0bdb5c..f432a8c 100644
--- a/reid/src/mir/mod.rs
+++ b/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>),
diff --git a/reid/src/mir/typecheck.rs b/reid/src/mir/typecheck.rs
index 2a1cb25..6247610 100644
--- a/reid/src/mir/typecheck.rs
+++ b/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
diff --git a/reid_src/borrow_hard.reid b/reid_src/borrow_hard.reid
index 38fe32e..a074d51 100644
--- a/reid_src/borrow_hard.reid
+++ b/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);