use modify over getting a mutable reference

compiler/mono/src/copy.rs

@@ -391,14 +391,14 @@ fn deep_copy_type_vars<'a>(
     // in one go (without looking at the UnificationTable) and clear the copy field
     let mut result = Vec::with_capacity_in(copied.len(), arena);
     for var in copied {
-        let descriptor = subs.get_ref_mut(var);
-
+        subs.modify(var, |descriptor| {
             if let Some(copy) = descriptor.copy.into_variable() {
                 result.push((var, copy));
                 descriptor.copy = OptVariable::NONE;
             } else {
                 debug_assert!(false, "{:?} marked as copied but it wasn't", var);
             }
+        })
     }
 
     debug_assert!(result.contains(&(var, cloned_var)));

compiler/mono/src/layout_soa.rs

@@ -124,7 +124,7 @@ impl FunctionLayout {
         subs: &Subs,
         var: Variable,
     ) -> Result<Self, LayoutError> {
-        let content = &subs.get_ref(var).content;
+        let content = &subs.get_content_without_compacting(var);
         Self::from_content(layouts, subs, var, content)
     }
 
@@ -232,7 +232,7 @@ impl LambdaSet {
         subs: &Subs,
         var: Variable,
     ) -> Result<Self, LayoutError> {
-        let content = &subs.get_ref(var).content;
+        let content = &subs.get_content_without_compacting(var);
         Self::from_content(layouts, subs, var, content)
     }
 
@@ -598,7 +598,7 @@ impl Layout {
         subs: &Subs,
         var: Variable,
     ) -> Result<Layout, LayoutError> {
-        let content = &subs.get_ref(var).content;
+        let content = &subs.get_content_without_compacting(var);
         Self::from_content(layouts, subs, var, content)
     }
 
@@ -612,7 +612,7 @@ impl Layout {
         subs: &Subs,
         var: Variable,
     ) -> Result<Layout, LayoutError> {
-        let content = &subs.get_ref(var).content;
+        let content = &subs.get_content_without_compacting(var);
 
         match content {
             Content::FlexVar(_) | Content::RigidVar(_) => Ok(Layout::VOID),

compiler/solve/src/solve.rs

@@ -2999,13 +2999,13 @@ fn instantiate_rigids_help(subs: &mut Subs, max_rank: Rank, initial: Variable) {
     // we have tracked all visited variables, and can now traverse them
     // in one go (without looking at the UnificationTable) and clear the copy field
     for var in visited {
-        let descriptor = subs.get_ref_mut(var);
-
+        subs.modify(var, |descriptor| {
             if descriptor.copy.is_some() {
                 descriptor.rank = Rank::NONE;
                 descriptor.mark = Mark::NONE;
                 descriptor.copy = OptVariable::NONE;
             }
+        });
     }
 }
 
@@ -3023,13 +3023,13 @@ fn deep_copy_var_in(
     // we have tracked all visited variables, and can now traverse them
     // in one go (without looking at the UnificationTable) and clear the copy field
     for var in visited {
-        let descriptor = subs.get_ref_mut(var);
-
+        subs.modify(var, |descriptor| {
             if descriptor.copy.is_some() {
                 descriptor.rank = Rank::NONE;
                 descriptor.mark = Mark::NONE;
                 descriptor.copy = OptVariable::NONE;
             }
+        });
     }
 
     copy

compiler/types/src/subs.rs

@@ -1784,7 +1784,7 @@ impl Subs {
 
     pub fn modify<F>(&mut self, key: Variable, mapper: F)
     where
-        F: Fn(&mut Descriptor),
+        F: FnOnce(&mut Descriptor),
     {
         mapper(self.get_ref_mut(key));
     }