fix(frontend): content assist error recovery

3 files changed, 33 insertions, 13 deletions

diff --git a/subprojects/frontend/src/language/problem.grammar b/subprojects/frontend/src/language/problem.grammar
index f4cf1712..704badab 100644
--- a/subprojects/frontend/src/language/problem.grammar
+++ b/subprojects/frontend/src/language/problem.grammar
@@ -16,6 +16,11 @@
 @top Problem { statement* }
 
 statement {
+  Assertion {
+    kw<"default">? (NotOp | UnknownOp)? RelationName
+    ParameterList<AssertionArgument>
+    (":" Expr)? "."
+  } |
   ProblemDeclaration {
     kw<"problem"> QualifiedName "."
   } |
@@ -45,11 +50,6 @@ statement {
   //  RuleName ParameterList<Parameter>?
   //  RuleBody { ":" sep<OrOp, Conjunction> "==>" sep<OrOp, Consequent> "." }
   //} |
-  Assertion {
-    kw<"default">? (NotOp | UnknownOp)? RelationName
-    ParameterList<AssertionArgument>
-    (":" Expr)? "."
-  } |
   IndividualDeclaration {
     kw<"indiv"> sep<",", IndividualNodeName> "."
   } |
diff --git a/subprojects/frontend/src/xtext/ContentAssistService.ts b/subprojects/frontend/src/xtext/ContentAssistService.ts
index fa894e4d..78f61c06 100644
--- a/subprojects/frontend/src/xtext/ContentAssistService.ts
+++ b/subprojects/frontend/src/xtext/ContentAssistService.ts
@@ -33,16 +33,36 @@ interface IFoundToken {
 
 function findToken({ pos, state }: CompletionContext): IFoundToken | undefined {
   const token = syntaxTree(state).resolveInner(pos, -1);
-  if (token.firstChild !== null) {
-    // We only autocomplete terminal nodes. If the current node is nonterminal,
-    // returning `undefined` makes us autocomplete with the empty prefix instead.
+  const { from } = token;
+  if (from > pos) {
+    // We haven't found the token we want to complete.
+    // Complete with an empty prefix from `pos` instead.
+    // The other `return undefined;` lines also handle this condition.
+    return undefined;
+  }
+  // We look at the text at the beginning of the token.
+  // For QualifiedName tokens right before a comment, this may be a comment token.
+  const endIndex = token.firstChild?.from ?? token.to;
+  if (pos > endIndex) {
+    return undefined;
+  }
+  const text = state.sliceDoc(from, endIndex).trimEnd();
+  // Due to parser error recovery, we may get spurious whitespace
+  // at the end of the token.
+  const to = from + text.length;
+  if (to > endIndex) {
+    return undefined;
+  }
+  if (from > pos || endIndex < pos) {
+    // We haven't found the token we want to complete.
+    // Complete with an empty prefix from `pos` instead.
     return undefined;
   }
   return {
-    from: token.from,
-    to: token.to,
+    from,
+    to,
     implicitCompletion: token.type.prop(implicitCompletion) || false,
-    text: state.sliceDoc(token.from, token.to),
+    text,
   };
 }
 
diff --git a/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext b/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext
index 95a64737..187ebf1f 100644
--- a/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext
+++ b/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext
@@ -8,9 +8,9 @@ Problem:
         statements+=Statement*;
 
 Statement:
-        ClassDeclaration | EnumDeclaration |
+        Assertion | ClassDeclaration | EnumDeclaration |
         PredicateDefinition | FunctionDefinition | /* RuleDefinition | */
-        Assertion | ScopeDeclaration | IndividualDeclaration;
+        ScopeDeclaration | IndividualDeclaration;
 
 ClassDeclaration:
         abstract?="abstract"? "class"