fix: improve error messages during parse errors.

core/src/main/kotlin/cash/grammar/kotlindsl/utils/Context.kt

@@ -10,15 +10,13 @@ import org.antlr.v4.runtime.misc.Interval
 public object Context {
 
   /**
-   * Returns the "full text", from [input], represented by [this][ParserRuleContext]. The full text
-   * includes tokens that are sent to hidden channels by the lexer. cf.
-   * [ParserRuleContext.text][ParserRuleContext.getText], which only considers tokens which have
-   * been added to the parse tree (i.e., not comments or whitespace).
+   * Returns the "full text", from [input], represented by [this][ParserRuleContext]. The full text includes tokens that
+   * are sent to hidden channels by the lexer. cf. [ParserRuleContext.text][ParserRuleContext.getText], which only
+   * considers tokens which have been added to the parse tree (i.e., not comments or whitespace).
    *
-   * Returns null if `this` has a null [ParserRuleContext.start] or [ParserRuleContext.stop], which
-   * can happen when, e.g., `this` is a
-   * [ScriptContext][com.squareup.cash.grammar.KotlinParser.ScriptContext]. (I don't
-   * fully understand why those tokens might be null.)
+   * Returns null if `this` has a null [ParserRuleContext.start] or [ParserRuleContext.stop], which can happen when,
+   * e.g., `this` is a [ScriptContext][com.squareup.cash.grammar.KotlinParser.ScriptContext]. (I don't fully understand
+   * why those tokens might be null.)
    */
   public fun ParserRuleContext.fullText(input: CharStream): String? {
     val a = start?.startIndex ?: return null

core/src/main/kotlin/cash/grammar/kotlindsl/utils/DependencyExtractor.kt

@@ -185,13 +185,16 @@ public class DependencyExtractor(
     }
 
     val precedingComment = comments.getCommentsToLeft(declaration)
+    val fullText = declaration.fullText(input)
+      ?: error("Could not determine 'full text' of dependency declaration. Failed to parse expression:\n  ${declaration.text}")
 
     return DependencyDeclaration(
       configuration = configuration,
-      identifier = identifier!!,
+      identifier = identifier
+        ?: error("Could not determine dependency identifier. Failed to parse expression:\n  `$fullText`"),
       capability = capability,
       type = type,
-      fullText = declaration.fullText(input)!!,
+      fullText = fullText,
       precedingComment = precedingComment,
     )
   }