diff --git a/crates/wiggle/borrow/src/lib.rs b/crates/wiggle/borrow/src/lib.rs
index 288c50f931..086f350d2e 100644
--- a/crates/wiggle/borrow/src/lib.rs
+++ b/crates/wiggle/borrow/src/lib.rs
@@ -31,11 +31,17 @@ impl BorrowChecker {
     pub fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         self.bc.borrow_mut().mut_borrow(r)
     }
-    pub fn unborrow(&self, h: BorrowHandle) {
-        self.bc.borrow_mut().unborrow(h)
+    pub fn shared_unborrow(&self, h: BorrowHandle) {
+        self.bc.borrow_mut().shared_unborrow(h)
     }
-    pub fn is_borrowed(&self, r: Region) -> bool {
-        self.bc.borrow().is_borrowed(r)
+    pub fn mut_unborrow(&self, h: BorrowHandle) {
+        self.bc.borrow_mut().mut_unborrow(h)
+    }
+    pub fn is_shared_borrowed(&self, r: Region) -> bool {
+        self.bc.borrow().is_shared_borrowed(r)
+    }
+    pub fn is_mut_borrowed(&self, r: Region) -> bool {
+        self.bc.borrow().is_mut_borrowed(r)
     }
 }
 
@@ -68,12 +74,11 @@ impl InnerBorrowChecker {
         !(self.shared_borrows.is_empty() && self.mut_borrows.is_empty())
     }
 
-    fn is_borrowed(&self, r: Region) -> bool {
-        !self
-            .shared_borrows
-            .values()
-            .chain(self.mut_borrows.values())
-            .all(|b| !b.overlaps(r))
+    fn is_shared_borrowed(&self, r: Region) -> bool {
+        self.shared_borrows.values().any(|b| b.overlaps(r))
+    }
+    fn is_mut_borrowed(&self, r: Region) -> bool {
+        self.mut_borrows.values().any(|b| b.overlaps(r))
     }
 
     fn new_handle(&mut self) -> Result<BorrowHandle, GuestError> {
@@ -95,7 +100,7 @@ impl InnerBorrowChecker {
     }
 
     fn shared_borrow(&mut self, r: Region) -> Result<BorrowHandle, GuestError> {
-        if !self.mut_borrows.values().all(|b| !b.overlaps(r)) {
+        if self.is_mut_borrowed(r) {
             return Err(GuestError::PtrBorrowed(r));
         }
         let h = self.new_handle()?;
@@ -104,7 +109,7 @@ impl InnerBorrowChecker {
     }
 
     fn mut_borrow(&mut self, r: Region) -> Result<BorrowHandle, GuestError> {
-        if self.is_borrowed(r) {
+        if self.is_shared_borrowed(r) || self.is_mut_borrowed(r) {
             return Err(GuestError::PtrBorrowed(r));
         }
         let h = self.new_handle()?;
@@ -112,11 +117,12 @@ impl InnerBorrowChecker {
         Ok(h)
     }
 
-    fn unborrow(&mut self, h: BorrowHandle) {
-        let removed = self.mut_borrows.remove(&h);
-        if removed.is_none() {
-            let _ = self.shared_borrows.remove(&h);
-        }
+    fn shared_unborrow(&mut self, h: BorrowHandle) {
+        let _ = self.shared_borrows.remove(&h);
+    }
+
+    fn mut_unborrow(&mut self, h: BorrowHandle) {
+        let _ = self.mut_borrows.remove(&h);
     }
 }
 
diff --git a/crates/wiggle/src/guest_type.rs b/crates/wiggle/src/guest_type.rs
index 422c3f2394..0fae52491e 100644
--- a/crates/wiggle/src/guest_type.rs
+++ b/crates/wiggle/src/guest_type.rs
@@ -85,7 +85,7 @@ macro_rules! primitives {
                     start: offset,
                     len: size,
                 };
-                if ptr.mem().is_borrowed(region) {
+                if ptr.mem().is_mut_borrowed(region) {
                     return Err(GuestError::PtrBorrowed(region));
                 }
                 Ok(unsafe { <$i>::from_le_bytes(*host_ptr.cast::<[u8; mem::size_of::<Self>()]>()) })
@@ -104,7 +104,7 @@ macro_rules! primitives {
                     start: offset,
                     len: size,
                 };
-                if ptr.mem().is_borrowed(region) {
+                if ptr.mem().is_shared_borrowed(region) || ptr.mem().is_mut_borrowed(region) {
                     return Err(GuestError::PtrBorrowed(region));
                 }
                 unsafe {
diff --git a/crates/wiggle/src/lib.rs b/crates/wiggle/src/lib.rs
index 497fec4a7a..063ab5bd4b 100644
--- a/crates/wiggle/src/lib.rs
+++ b/crates/wiggle/src/lib.rs
@@ -153,28 +153,36 @@ pub unsafe trait GuestMemory {
     /// safe to recursively call into a WebAssembly module, or to manipulate
     /// the WebAssembly memory by any other means.
     fn has_outstanding_borrows(&self) -> bool;
-    /// Check if a region of linear memory is borrowed. This is called during
-    /// any `GuestPtr::read` or `GuestPtr::write` operation to ensure that
-    /// wiggle is not reading or writing a region of memory which Rust believes
-    /// it has exclusive access to.
-    fn is_borrowed(&self, r: Region) -> bool;
-    /// Borrow a region of linear memory. This is used when constructing a
-    /// `GuestSlice` or `GuestStr`. Those types will give Rust `&mut` access
+    /// Check if a region of linear memory is exclusively borrowed. This is called during any
+    /// `GuestPtr::read` or `GuestPtr::write` operation to ensure that wiggle is not reading or
+    /// writing a region of memory which Rust believes it has exclusive access to.
+    fn is_mut_borrowed(&self, r: Region) -> bool;
+    /// Check if a region of linear memory has any shared borrows.
+    fn is_shared_borrowed(&self, r: Region) -> bool;
+    /// Exclusively borrow a region of linear memory. This is used when constructing a
+    /// `GuestSliceMut` or `GuestStrMut`. Those types will give Rust `&mut` access
     /// to the region of linear memory, therefore, the `GuestMemory` impl must
     /// guarantee that at most one `BorrowHandle` is issued to a given region,
     /// `GuestMemory::has_outstanding_borrows` is true for the duration of the
-    /// borrow, and that `GuestMemory::is_borrowed` of any overlapping region
+    /// borrow, and that `GuestMemory::is_mut_borrowed` of any overlapping region
     /// is false for the duration of the borrow.
     fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>;
+    /// Shared borrow a region of linear memory. This is used when constructing a
+    /// `GuestSlice` or `GuestStr`. Those types will give Rust `&` (shared reference) access
+    /// to the region of linear memory.
     fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError>;
-    /// Unborrow a previously borrowed region. As long as `GuestSlice` and
-    /// `GuestStr` are implemented correctly, a `BorrowHandle` should only be
+    /// Unborrow a previously borrowed mutable region. As long as `GuestSliceMut` and
+    /// `GuestStrMut` are implemented correctly, a mut `BorrowHandle` should only be
     /// unborrowed once.
-    fn unborrow(&self, h: BorrowHandle);
+    fn mut_unborrow(&self, h: BorrowHandle);
+    /// Unborrow a previously borrowed shared region. As long as `GuestSlice` and
+    /// `GuestStr` are implemented correctly, a shared `BorrowHandle` should only be
+    /// unborrowed once.
+    fn shared_unborrow(&self, h: BorrowHandle);
 }
 
-/// A handle to a borrow on linear memory. It is produced by `borrow` and
-/// consumed by `unborrow`. Only the `GuestMemory` impl should ever construct
+/// A handle to a borrow on linear memory. It is produced by `{mut, shared}_borrow` and
+/// consumed by `{mut, shared}_unborrow`. Only the `GuestMemory` impl should ever construct
 /// a `BorrowHandle` or inspect its contents.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub struct BorrowHandle(pub usize);
@@ -187,8 +195,11 @@ unsafe impl<'a, T: ?Sized + GuestMemory> GuestMemory for &'a T {
     fn has_outstanding_borrows(&self) -> bool {
         T::has_outstanding_borrows(self)
     }
-    fn is_borrowed(&self, r: Region) -> bool {
-        T::is_borrowed(self, r)
+    fn is_mut_borrowed(&self, r: Region) -> bool {
+        T::is_mut_borrowed(self, r)
+    }
+    fn is_shared_borrowed(&self, r: Region) -> bool {
+        T::is_shared_borrowed(self, r)
     }
     fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::mut_borrow(self, r)
@@ -196,8 +207,11 @@ unsafe impl<'a, T: ?Sized + GuestMemory> GuestMemory for &'a T {
     fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::shared_borrow(self, r)
     }
-    fn unborrow(&self, h: BorrowHandle) {
-        T::unborrow(self, h)
+    fn mut_unborrow(&self, h: BorrowHandle) {
+        T::mut_unborrow(self, h)
+    }
+    fn shared_unborrow(&self, h: BorrowHandle) {
+        T::shared_unborrow(self, h)
     }
 }
 
@@ -208,8 +222,11 @@ unsafe impl<'a, T: ?Sized + GuestMemory> GuestMemory for &'a mut T {
     fn has_outstanding_borrows(&self) -> bool {
         T::has_outstanding_borrows(self)
     }
-    fn is_borrowed(&self, r: Region) -> bool {
-        T::is_borrowed(self, r)
+    fn is_mut_borrowed(&self, r: Region) -> bool {
+        T::is_mut_borrowed(self, r)
+    }
+    fn is_shared_borrowed(&self, r: Region) -> bool {
+        T::is_shared_borrowed(self, r)
     }
     fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::mut_borrow(self, r)
@@ -217,8 +234,11 @@ unsafe impl<'a, T: ?Sized + GuestMemory> GuestMemory for &'a mut T {
     fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::shared_borrow(self, r)
     }
-    fn unborrow(&self, h: BorrowHandle) {
-        T::unborrow(self, h)
+    fn mut_unborrow(&self, h: BorrowHandle) {
+        T::mut_unborrow(self, h)
+    }
+    fn shared_unborrow(&self, h: BorrowHandle) {
+        T::shared_unborrow(self, h)
     }
 }
 
@@ -229,8 +249,11 @@ unsafe impl<T: ?Sized + GuestMemory> GuestMemory for Box<T> {
     fn has_outstanding_borrows(&self) -> bool {
         T::has_outstanding_borrows(self)
     }
-    fn is_borrowed(&self, r: Region) -> bool {
-        T::is_borrowed(self, r)
+    fn is_mut_borrowed(&self, r: Region) -> bool {
+        T::is_mut_borrowed(self, r)
+    }
+    fn is_shared_borrowed(&self, r: Region) -> bool {
+        T::is_shared_borrowed(self, r)
     }
     fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::mut_borrow(self, r)
@@ -238,8 +261,11 @@ unsafe impl<T: ?Sized + GuestMemory> GuestMemory for Box<T> {
     fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::shared_borrow(self, r)
     }
-    fn unborrow(&self, h: BorrowHandle) {
-        T::unborrow(self, h)
+    fn mut_unborrow(&self, h: BorrowHandle) {
+        T::mut_unborrow(self, h)
+    }
+    fn shared_unborrow(&self, h: BorrowHandle) {
+        T::shared_unborrow(self, h)
     }
 }
 
@@ -250,8 +276,11 @@ unsafe impl<T: ?Sized + GuestMemory> GuestMemory for Rc<T> {
     fn has_outstanding_borrows(&self) -> bool {
         T::has_outstanding_borrows(self)
     }
-    fn is_borrowed(&self, r: Region) -> bool {
-        T::is_borrowed(self, r)
+    fn is_mut_borrowed(&self, r: Region) -> bool {
+        T::is_mut_borrowed(self, r)
+    }
+    fn is_shared_borrowed(&self, r: Region) -> bool {
+        T::is_shared_borrowed(self, r)
     }
     fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::mut_borrow(self, r)
@@ -259,8 +288,11 @@ unsafe impl<T: ?Sized + GuestMemory> GuestMemory for Rc<T> {
     fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::shared_borrow(self, r)
     }
-    fn unborrow(&self, h: BorrowHandle) {
-        T::unborrow(self, h)
+    fn mut_unborrow(&self, h: BorrowHandle) {
+        T::mut_unborrow(self, h)
+    }
+    fn shared_unborrow(&self, h: BorrowHandle) {
+        T::shared_unborrow(self, h)
     }
 }
 
@@ -271,8 +303,11 @@ unsafe impl<T: ?Sized + GuestMemory> GuestMemory for Arc<T> {
     fn has_outstanding_borrows(&self) -> bool {
         T::has_outstanding_borrows(self)
     }
-    fn is_borrowed(&self, r: Region) -> bool {
-        T::is_borrowed(self, r)
+    fn is_mut_borrowed(&self, r: Region) -> bool {
+        T::is_mut_borrowed(self, r)
+    }
+    fn is_shared_borrowed(&self, r: Region) -> bool {
+        T::is_shared_borrowed(self, r)
     }
     fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::mut_borrow(self, r)
@@ -280,8 +315,11 @@ unsafe impl<T: ?Sized + GuestMemory> GuestMemory for Arc<T> {
     fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         T::shared_borrow(self, r)
     }
-    fn unborrow(&self, h: BorrowHandle) {
-        T::unborrow(self, h)
+    fn mut_unborrow(&self, h: BorrowHandle) {
+        T::mut_unborrow(self, h)
+    }
+    fn shared_unborrow(&self, h: BorrowHandle) {
+        T::shared_unborrow(self, h)
     }
 }
 
@@ -771,7 +809,7 @@ impl<'a, T> std::ops::Deref for GuestSlice<'a, T> {
 
 impl<'a, T> Drop for GuestSlice<'a, T> {
     fn drop(&mut self) {
-        self.mem.unborrow(self.borrow)
+        self.mem.shared_unborrow(self.borrow)
     }
 }
 
@@ -799,7 +837,7 @@ impl<'a, T> std::ops::DerefMut for GuestSliceMut<'a, T> {
 
 impl<'a, T> Drop for GuestSliceMut<'a, T> {
     fn drop(&mut self) {
-        self.mem.unborrow(self.borrow)
+        self.mem.mut_unborrow(self.borrow)
     }
 }
 
@@ -820,7 +858,7 @@ impl<'a> std::ops::Deref for GuestStr<'a> {
 
 impl<'a> Drop for GuestStr<'a> {
     fn drop(&mut self) {
-        self.mem.unborrow(self.borrow)
+        self.mem.shared_unborrow(self.borrow)
     }
 }
 
@@ -848,7 +886,7 @@ impl<'a> std::ops::DerefMut for GuestStrMut<'a> {
 
 impl<'a> Drop for GuestStrMut<'a> {
     fn drop(&mut self) {
-        self.mem.unborrow(self.borrow)
+        self.mem.mut_unborrow(self.borrow)
     }
 }
 
diff --git a/crates/wiggle/test-helpers/src/lib.rs b/crates/wiggle/test-helpers/src/lib.rs
index 7ba2d5a6e4..99571367a5 100644
--- a/crates/wiggle/test-helpers/src/lib.rs
+++ b/crates/wiggle/test-helpers/src/lib.rs
@@ -125,8 +125,11 @@ unsafe impl GuestMemory for HostMemory {
     fn has_outstanding_borrows(&self) -> bool {
         self.bc.has_outstanding_borrows()
     }
-    fn is_borrowed(&self, r: Region) -> bool {
-        self.bc.is_borrowed(r)
+    fn is_shared_borrowed(&self, r: Region) -> bool {
+        self.bc.is_shared_borrowed(r)
+    }
+    fn is_mut_borrowed(&self, r: Region) -> bool {
+        self.bc.is_mut_borrowed(r)
     }
     fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         self.bc.mut_borrow(r)
@@ -134,8 +137,11 @@ unsafe impl GuestMemory for HostMemory {
     fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         self.bc.shared_borrow(r)
     }
-    fn unborrow(&self, h: BorrowHandle) {
-        self.bc.unborrow(h)
+    fn shared_unborrow(&self, h: BorrowHandle) {
+        self.bc.shared_unborrow(h)
+    }
+    fn mut_unborrow(&self, h: BorrowHandle) {
+        self.bc.mut_unborrow(h)
     }
 }
 
diff --git a/crates/wiggle/wasmtime/src/lib.rs b/crates/wiggle/wasmtime/src/lib.rs
index b102d874ba..dc43683c14 100644
--- a/crates/wiggle/wasmtime/src/lib.rs
+++ b/crates/wiggle/wasmtime/src/lib.rs
@@ -35,8 +35,11 @@ unsafe impl GuestMemory for WasmtimeGuestMemory {
     fn has_outstanding_borrows(&self) -> bool {
         self.bc.has_outstanding_borrows()
     }
-    fn is_borrowed(&self, r: Region) -> bool {
-        self.bc.is_borrowed(r)
+    fn is_shared_borrowed(&self, r: Region) -> bool {
+        self.bc.is_shared_borrowed(r)
+    }
+    fn is_mut_borrowed(&self, r: Region) -> bool {
+        self.bc.is_mut_borrowed(r)
     }
     fn shared_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         self.bc.shared_borrow(r)
@@ -44,7 +47,10 @@ unsafe impl GuestMemory for WasmtimeGuestMemory {
     fn mut_borrow(&self, r: Region) -> Result<BorrowHandle, GuestError> {
         self.bc.mut_borrow(r)
     }
-    fn unborrow(&self, h: BorrowHandle) {
-        self.bc.unborrow(h)
+    fn shared_unborrow(&self, h: BorrowHandle) {
+        self.bc.shared_unborrow(h)
+    }
+    fn mut_unborrow(&self, h: BorrowHandle) {
+        self.bc.mut_unborrow(h)
     }
 }