Add readme for SemanticDB directory

semanticdb/readme.md

@@ -0,0 +1,50 @@
+# SemanticDB
+
+To run the tests use two terminals. In the first terminal compile the
+`semanticdb/input` project whenever sources change in
+`semanticdb/input/src/main/scala/**.scala`.
+
+```
+cd semanticdb/input
+sbt
+> compile
+```
+
+In the second terminal, run `sbt dotty-semanticdb/test` from the main dotty
+build
+
+```
+sbt
+> dotty-semanticdb/test
+```
+
+The tests assert that the TASTy to SemanticDB converter produces the same output
+as the semanticdb-scalac compiler plugin. Test failures result in diffs like
+this
+
+```diff
+Test dotty.semanticdb.Tests.testExample failed: java.lang.AssertionError:
+--- tasty
++++ scala2
+-class Example {
+-  val a: String = "1"
++class Example /*example/Example#*/  {
++  val a /*example/Example#a.*/ : String /*scala/Predef.String#*/  = "1"
+```
+
+The lines starting with `-` are the output from the TASTy converter and the
+lines starting with `+` are the output from the semanticdb-scalac compiler
+plugin.
+
+Once the TASTy converter is updated to emit correct `SymbolOccurrence`
+(https://scalameta.org/docs/semanticdb/specification.html#symboloccurrence) then
+the "tasty" output should become identical to "scala2" and include inline
+comments `class Example /* pkg/Example# */`. The inline comments are read as:
+"the symbol `pkg/Example#` was resolved next to the class name `Example`".
+
+The spec for SemanticDB symbols can be found here:
+
+- Scala symbols:
+  https://scalameta.org/docs/semanticdb/specification.html#scala-symbol
+- Java symbols:
+  https://scalameta.org/docs/semanticdb/specification.html#java-symbol