Initialise Coverage object only when the option is used

compiler/src/dotty/tools/dotc/core/Contexts.scala

@@ -983,11 +983,10 @@ object Contexts {
     /** Was best effort file used during compilation? */
     private[core] var usedBestEffortTasty = false
 
-    /** Stores all instrumented statements (for InstrumentCoverage).
-     * We need this information to be persisted across different runs,
-     * so it's stored here.
+    /** If coverage option is used, it stores all instrumented statements (for InstrumentCoverage).
+     * We need this information to be persisted across different runs, so it's stored here.
      */
-    private[dotc] var coverage: Coverage = Coverage()
+    private[dotc] var coverage: Coverage | Null = null
 
     // Types state
     /** A table for hash consing unique types */

compiler/src/dotty/tools/dotc/transform/InstrumentCoverage.scala

@@ -55,13 +55,17 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
         .foreach(_.nn.delete())
     end if
 
+    // Initialise a coverage object if it does not exist yet
+    if ctx.base.coverage == null then
+      ctx.base.coverage = Coverage()
+
     coverageExcludeClasslikePatterns = ctx.settings.coverageExcludeClasslikes.value.map(_.r.pattern)
     coverageExcludeFilePatterns = ctx.settings.coverageExcludeFiles.value.map(_.r.pattern)
 
-    ctx.base.coverage.removeStatementsFromFile(ctx.compilationUnit.source.file.absolute.jpath)
+    ctx.base.coverage.nn.removeStatementsFromFile(ctx.compilationUnit.source.file.absolute.jpath)
     super.run
 
-    Serializer.serialize(ctx.base.coverage, outputPath, ctx.settings.sourceroot.value)
+    Serializer.serialize(ctx.base.coverage.nn, outputPath, ctx.settings.sourceroot.value)
 
   private def isClassIncluded(sym: Symbol)(using Context): Boolean =
     val fqn = sym.fullName.toText(ctx.printerFn(ctx)).show
@@ -105,7 +109,7 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
       * @return the statement's id
       */
     private def recordStatement(tree: Tree, pos: SourcePosition, branch: Boolean)(using ctx: Context): Int =
-      val id = ctx.base.coverage.nextStatementId()
+      val id = ctx.base.coverage.nn.nextStatementId()
 
       val sourceFile = pos.source
       val statement = Statement(
@@ -121,7 +125,7 @@ class InstrumentCoverage extends MacroTransform with IdentityDenotTransformer:
         treeName = tree.getClass.getSimpleName.nn,
         branch
       )
-      ctx.base.coverage.addStatement(statement)
+      ctx.base.coverage.nn.addStatement(statement)
       id
 
     /**