fix: correctly merge restrictions (#234)

Inner restrictions are not being merged correctly right now

crates/compilers/src/compilers/restrictions.rs

@@ -8,6 +8,7 @@ use semver::VersionReq;
 /// Abstraction over set of restrictions for given [`crate::Compiler::Settings`].
 pub trait CompilerSettingsRestrictions: Copy + Debug + Sync + Send + Clone + Default {
     /// Combines this restriction with another one. Returns `None` if restrictions are incompatible.
+    #[must_use]
     fn merge(self, other: Self) -> Option<Self>;
 }
 
@@ -20,15 +21,18 @@ pub struct RestrictionsWithVersion<T> {
 }
 
 impl<T: CompilerSettingsRestrictions> RestrictionsWithVersion<T> {
-    pub fn merge(&mut self, other: Self) {
+    /// Tries to merge the given restrictions with the other [`RestrictionsWithVersion`]. Returns
+    /// `None` if restrictions are incompatible.
+    pub fn merge(mut self, other: Self) -> Option<Self> {
         if let Some(version) = other.version {
             if let Some(self_version) = self.version.as_mut() {
                 self_version.comparators.extend(version.comparators);
             } else {
                 self.version = Some(version.clone());
             }
         }
-        self.restrictions.merge(other.restrictions);
+        self.restrictions = self.restrictions.merge(other.restrictions)?;
+        Some(self)
     }
 }