fix: skip SCIP generation of occurrences with empty locations

Sorbet can assign empty locations to synthesized AST nodes, including
Minitest `before` hooks inside `test_each`. Passing these locations to
`trimColonColonPrefix` triggers an assertion in debug builds and can
segfault in release builds as we've seen in production.

Skip definitions and references with empty locations before converting
them to SCIP ranges, and add a regression test for the Minitest case.

Signed-off-by: Emi <emi@sourcegraph.com>

Author: emidoots

scip_indexer/SCIPIndexer.cc

@@ -119,7 +119,12 @@ InlinedVector<int32_t, 4> fromSorbetLoc(const core::GlobalState &gs, core::Loc l
 }
 
 core::Loc trimColonColonPrefix(const core::GlobalState &gs, core::Loc baseLoc) {
-    ENFORCE(!baseLoc.empty());
+    // Sorbet can assign empty locations to synthesized AST nodes. There is no
+    // source prefix to trim in that case, and callers will skip emitting an
+    // occurrence because SCIP ranges must be non-empty.
+    if (!baseLoc.exists() || baseLoc.empty()) {
+        return baseLoc;
+    }
     auto source = baseLoc.source(gs);
     if (!source.has_value()) {
         return baseLoc;
@@ -291,6 +296,9 @@ class SCIPState {
                                     const SmallVec<scip::Relationship> &rels,
                                     optional<core::Loc> enclosingLoc = nullopt) {
         ENFORCE(!symbolString.empty());
+        if (!occLoc.exists() || occLoc.empty()) {
+            return absl::OkStatus();
+        }
         occLoc = trimColonColonPrefix(gs, occLoc);
         auto range = sorbet::scip_indexer::fromSorbetLoc(gs, occLoc);
         if (range.size() == 4) {
@@ -328,7 +336,11 @@ class SCIPState {
     void saveReferenceImpl(const core::GlobalState &gs, core::FileRef file, const string &symbolString,
                            const SmallVec<string> &overrideDocs, core::LocOffsets occLocOffsets, int32_t symbol_roles) {
         ENFORCE(!symbolString.empty());
-        auto occLoc = trimColonColonPrefix(gs, core::Loc(file, occLocOffsets));
+        auto occLoc = core::Loc(file, occLocOffsets);
+        if (!occLoc.exists() || occLoc.empty()) {
+            return;
+        }
+        occLoc = trimColonColonPrefix(gs, occLoc);
         scip::Occurrence occurrence;
         occurrence.set_symbol(symbolString);
         occurrence.set_symbol_roles(symbol_roles);