fix: check is valid structure projection when pretty printing

For structure projections, the pretty printer assumed that the expression was type correct. Now it checks that the object being projected is of the correct type. Such terms appear in type mismatch errors.

Also, fixes and improves `#print` for structures. The types of projections now use MessageData (so are now hoverable), and the type of `self` is now the correct type.

Closes leanprover#4670

src/Lean/Elab/Print.lean

@@ -78,19 +78,19 @@ private def printStructure (id : Name) (levelParams : List Name) (numParams : Na
   logInfo m
 where
   doFields := liftTermElabM do
-    forallTelescope (← getConstInfo id).type fun params type =>
-      withLocalDeclD `self type fun self => do
+    forallTelescope (← getConstInfo id).type fun params _ =>
+      withLocalDeclD `self (mkAppN (Expr.const id (levelParams.map .param)) params) fun self => do
         let params := params.push self
-        let mut m : Format := ""
+        let mut m : MessageData := ""
         for field in fields do
           match getProjFnForField? (← getEnv) id field with
           | some proj =>
             let field : Format := if isPrivateName proj then "private " ++ toString field else toString field
             let cinfo ← getConstInfo proj
             let ftype ← instantiateForall cinfo.type params
-            m := m ++ Format.line ++ field ++ " : " ++ (← ppExpr ftype) -- Why ppExpr here?
+            m := m ++ Format.line ++ field ++ " : " ++ ftype
           | none => panic! "missing structure field info"
-        return m
+        addMessageContext m
 
 private def printIdCore (id : Name) : CommandElabM Unit := do
   let env ← getEnv

src/Lean/PrettyPrinter/Delaborator/FieldNotation.lean

@@ -89,6 +89,8 @@ def fieldNotationCandidate? (f : Expr) (args : Array Expr) (useGeneralizedFieldN
   -- Handle structure projections
   try
     let (field, numParams, _) ← projInfo c
+    unless numParams + 1 ≤ args.size do return none
+    unless (← whnf <| ← inferType args[numParams]!).isAppOf c.getPrefix do return none
     return (field, numParams)
   catch _ => pure ()
   -- Handle generalized field notation