Fix #7683: Fix CompilationUnit handling in Context#withSource

In e07a728, I changed
Context#withSource to set a new CompilationUnit based on the source if
no compilation unit is currently set (always having a CompilationUnit
set is now needed since I moved the fresh name generator inside
CompilationUnit), however CompilationUnit#apply will error out if the
source file does not exist, which might happen for library code.

Fixed by adding a parameter to disable this check, but this is not very
pretty and we should really try to design a better way to distinguish
between user source files and source files we happen to know the name of
because it's stored in Tasty.

Author: smarter

compiler/src/dotty/tools/dotc/CompilationUnit.scala

@@ -83,9 +83,14 @@ object CompilationUnit {
     unit1
   }
 
-  def apply(source: SourceFile)(implicit ctx: Context): CompilationUnit = {
+  /** Create a compilation unit corresponding to `source`.
+   *  If `mustExist` is true, this will fail if `source` does not exist.
+   */
+  def apply(source: SourceFile, mustExist: Boolean = true)(implicit ctx: Context): CompilationUnit = {
     val src =
-      if (source.file.isDirectory) {
+      if (!mustExist)
+        source
+      else if (source.file.isDirectory) {
         ctx.error(s"expected file, received directory '${source.file.path}'")
         NoSource
       }

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

@@ -468,7 +468,10 @@ object Contexts {
         else {
           val newCtx = fresh.setSource(source)
           if (newCtx.compilationUnit == null)
-            newCtx.setCompilationUnit(CompilationUnit(source))
+            // `source` might correspond to a file not necessarily
+            // in the current project (e.g. when inlining library code),
+            // so set `mustExist` to false.
+            newCtx.setCompilationUnit(CompilationUnit(source, mustExist = false))
           sourceCtx = sourceCtx.updated(source, newCtx)
           newCtx
         }