fix: set default value of pp.instantiateMVars to true and make the option be effective (#4558)

Before, `pp.instantiateMVars` generally had no effect because most call
sites for the pretty printer instantiated metavariables first, but now
this functionality is entrusted upon the `pp.instantiateMVars` option.

This also has an effect in hovers, where metavariables can be unfolded
one assignment at a time. However, the goal state still sees all
metavariables instantiated due to the fact that the algorithm relies on
expression equality post-instantiation (see
`Lean.Widget.goalToInteractive`).

Closes #4406

src/Lean/PrettyPrinter/Delaborator/Options.lean

@@ -80,7 +80,7 @@ register_builtin_option pp.numericTypes : Bool := {
   descr    := "(pretty printer) display types of numeric literals"
 }
 register_builtin_option pp.instantiateMVars : Bool := {
-  defValue := false -- TODO: default to true?
+  defValue := true
   group    := "pp"
   descr    := "(pretty printer) instantiate mvars before delaborating"
 }

src/Lean/Server/FileWorker/WidgetRequests.lean

@@ -50,7 +50,7 @@ def makePopup : WithRpcRef InfoWithCtx → RequestM (RequestTask InfoPopup)
   | ⟨i⟩ => RequestM.asTask do
     i.ctx.runMetaM i.info.lctx do
       let type? ← match (← i.info.type?) with
-        | some type => some <$> (ppExprTagged =<< instantiateMVars type)
+        | some type => some <$> ppExprTagged type
         | none => pure none
       let exprExplicit? ← match i.info with
         | Elab.Info.ofTermInfo ti =>

src/Lean/Util/PPExt.lean

@@ -69,8 +69,8 @@ builtin_initialize ppExt : EnvExtension PPFns ←
   registerEnvExtension ppFnsRef.get
 
 def ppExprWithInfos (ctx : PPContext) (e : Expr) : IO FormatWithInfos := do
-  let e := instantiateMVarsCore ctx.mctx e |>.1
   if pp.raw.get ctx.opts then
+    let e := instantiateMVarsCore ctx.mctx e |>.1
     return format (toString e)
   else
     try

src/Lean/Widget/InteractiveCode.lean

@@ -76,7 +76,7 @@ where
 
 def ppExprTagged (e : Expr) (explicit : Bool := false) : MetaM CodeWithInfos := do
   if pp.raw.get (← getOptions) then
-    return .text (toString e)
+    return .text (toString (← instantiateMVars e))
   let delab := open PrettyPrinter.Delaborator in
     if explicit then
       withOptionAtCurrPos pp.tagAppFns.name true do
@@ -86,6 +86,11 @@ def ppExprTagged (e : Expr) (explicit : Bool := false) : MetaM CodeWithInfos :=
     else
       withOptionAtCurrPos pp.proofs.name true do
         delab
+  let mut e := e
+  -- When hovering over a metavariable, we want to see its value, even if `pp.instantiateMVars` is false.
+  if explicit && e.isMVar then
+    if let some e' ← getExprMVarAssignment? e.mvarId! then
+      e := e'
   let ⟨fmt, infos⟩ ← PrettyPrinter.ppExprWithInfos e (delab := delab)
   let tt := TaggedText.prettyTagged fmt
   let ctx := {

tests/lean/run/4406.lean

@@ -0,0 +1,24 @@
+import Lean.Elab.Command
+
+/-!
+# Issue 4406: make sure `pp.instantiateMVars` has an effect
+-/
+
+open Lean Meta
+
+set_option pp.mvars false
+
+/-- info: ?_ -/
+#guard_msgs in
+set_option pp.instantiateMVars false in
+run_meta do
+  let mvarId ← mkFreshExprMVar (.some (.const ``Nat []))
+  mvarId.mvarId!.assign (mkNatLit 0)
+  logInfo m!"{mvarId}"
+
+/-- info: 0 -/
+#guard_msgs in
+run_meta do
+  let mvarId ← mkFreshExprMVar (.some (.const ``Nat []))
+  mvarId.mvarId!.assign (mkNatLit 0)
+  logInfo m!"{mvarId}"