chore: better error message when failing to find current package

src/Init/System/FilePath.lean

@@ -42,11 +42,9 @@ def normalizePath (fname : String) : String :=
     -- else if isCaseInsensitive then c.toLower
     else c
 
-def dirName (fname : String) : String :=
+def parent (fname : String) : Option String :=
   let fname := normalizePath fname
-  match fname.revPosOf pathSeparator with
-  | none => "."
-  | some pos => { str := fname, startPos := 0, stopPos := pos : Substring }.toString
+  fname.extract 0 <$> fname.revPosOf pathSeparator
 
 end FilePath
 

src/Init/System/IO.lean

@@ -328,7 +328,9 @@ def appPath : m String := liftM Prim.appPath
 
 def appDir : m String := do
   let p ← appPath
-  realPath (System.FilePath.dirName p)
+  let some p ← pure <| System.FilePath.parent p
+    | liftM (m := IO) <| throw <| IO.userError s!"System.IO.appDir: unexpected filename '{p}'"
+  realPath p
 
 def currentDir : m String := liftM Prim.currentDir
 

src/Lean/Util/Path.lean

@@ -65,11 +65,24 @@ def initSearchPath (path : Option String := none) : IO Unit :=
     let sp ← addSearchPathFromEnv sp
     searchPathRef.set sp
 
-def findOLean (mod : Name) : IO String := do
+partial def findOLean (mod : Name) : IO String := do
   let sp ← searchPathRef.get
-  let some fname ← sp.findWithExt ".olean" mod
-    | throw $ IO.userError $ "unknown package '" ++ mod.getRoot.toString ++ "'"
-  return fname
+  if let some fname ← sp.findWithExt ".olean" mod then
+    return fname
+  else
+    let pkg := mod.getRoot
+    let mut msg := s!"unknown package '{pkg}'"
+    let rec maybeThisOne dir := do
+      let dir := s!"{dir}{pathSep}{pkg}"
+      if ← IO.fileExists dir then
+        return some s!"\nYou might need to open '{dir}' as a workspace in your editor"
+      if let some dir ← System.FilePath.parent dir then
+        maybeThisOne dir
+      else
+       return none
+    if let some msg' ← maybeThisOne (← IO.currentDir) then
+      msg := msg ++ msg'
+    throw <| IO.userError msg
 
 /-- Infer module name of source file name. -/
 @[export lean_module_name_of_file]