refactor: infer.rs 'fn results' (#631)

* No need for `&self` to be mutable
* Make control-flow more obvious
* Also, in merge_equal_metas(), do if contains ... remove.unwrap in one
step

No change to semantics.

src/extension/infer.rs

@@ -440,7 +440,7 @@ impl UnificationContext {
  continue;
  }
 
- if let Some(cs) = self.constraints.get(m) {
+ if let Some(cs) = self.constraints.remove(m) {
  for c in cs
  .iter()
  .filter(|c| !matches!(c, Constraint::Equal(_)))
@@ -450,7 +450,6 @@ impl UnificationContext {
  {
  self.add_constraint(combined_meta, c.clone());
  }
- self.constraints.remove(m).unwrap();
  merged.insert(*m);
  // Record a new meta the first time that we use it; don't
  // bother recording a new meta if we don't add any
@@ -538,27 +537,21 @@ impl UnificationContext {
  /// available. When there are variables, we should leave the graph as it is,
  /// but make sure that no matter what they're instantiated to, the graph
  /// still makes sense (should pass the extension validation check)
- pub fn results(&mut self) -> Result<ExtensionSolution, InferExtensionError> {
+ pub fn results(&self) -> Result<ExtensionSolution, InferExtensionError> {
  // Check that all of the metavariables associated with nodes of the
  // graph are solved
  let mut results: ExtensionSolution = HashMap::new();
  for (loc, meta) in self.extensions.iter() {
- let rs = match self.get_solution(meta) {
- Some(rs) => Ok(rs.clone()),
- None => {
- // If it depends on some other live meta, that's bad news.
- // If it only depends on graph variables, then we don't have
- // a *solution*, but it's fine
- if self.live_var(meta).is_some() {
- Err(InferExtensionError::Unsolved { location: *loc })
- } else {
- continue;
- }
+ if let Some(rs) = self.get_solution(meta) {
+ if loc.1 == Direction::Incoming {
+ results.insert(loc.0, rs.clone());
  }
- }?;
- if loc.1 == Direction::Incoming {
- results.insert(loc.0, rs);
+ } else if self.live_var(meta).is_some() {
+  // If it depends on some other live meta, that's bad news.
+ return Err(InferExtensionError::Unsolved { location: *loc });
  }
+ // If it only depends on graph variables, then we don't have
+ // a *solution*, but it's fine
  }
  debug_assert!(self.live_metas().is_empty());
  Ok(results)