feat: add +all option to exact? and apply?

This PR adds a `+all` option to `exact?` and `apply?` that collects all
successful lemmas instead of stopping at the first complete solution.
This is useful when exploring what lemmas are available to prove a goal.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: kim-em

src/Init/Tactics.lean

@@ -1704,6 +1704,9 @@ structure LibrarySearchConfig where
   like `[*]`) are searched as a fallback when no concrete-keyed lemmas are found.
   Use `-star` to disable this fallback. -/
   star : Bool := true
+  /-- If true, collect all successful lemmas instead of stopping at the first complete solution.
+  Use `+all` to enable this behavior. -/
+  all : Bool := false
 
 /--
 Searches environment for definitions or theorems that can solve the goal using `exact`
@@ -1716,6 +1719,7 @@ ways to resolve the goal, and one wants to guide which lemma is used.
 Use `+grind` to enable `grind` as a fallback discharger for subgoals.
 Use `+try?` to enable `try?` as a fallback discharger for subgoals.
 Use `-star` to disable fallback to star-indexed lemmas (like `Empty.elim`, `And.left`).
+Use `+all` to collect all successful lemmas instead of stopping at the first.
 -/
 syntax (name := exact?) "exact?" optConfig (" using " (colGt ident),+)? : tactic
 
@@ -1729,6 +1733,7 @@ used when closing the goal.
 Use `+grind` to enable `grind` as a fallback discharger for subgoals.
 Use `+try?` to enable `try?` as a fallback discharger for subgoals.
 Use `-star` to disable fallback to star-indexed lemmas.
+Use `+all` to collect all successful lemmas instead of stopping at the first.
 -/
 syntax (name := apply?) "apply?" optConfig (" using " (colGt term),+)? : tactic
 

src/Lean/Elab/Tactic/LibrarySearch.lean

@@ -42,21 +42,37 @@ def exact? (ref : Syntax) (config : Parser.Tactic.LibrarySearchConfig)
     let allowFailure := fun g => do
       let g ← g.withContext (instantiateMVars (.mvar g))
       return required.all fun e => e.occurs g
-    match (← librarySearch goal tactic allowFailure (includeStar := config.star)) with
-    -- Found goal that closed problem
+    match (← librarySearch goal tactic allowFailure (includeStar := config.star)
+        (collectAll := config.all)) with
+    -- Found goal that closed problem (only when collectAll = false)
     | none =>
       addExactSuggestion ref (← instantiateMVars (mkMVar mvar)).headBeta (checkState? := initialState)
-    -- Found suggestions
+    -- Found suggestions (includes complete solutions when collectAll = true)
     | some suggestions =>
-      if requireClose then
+      -- Separate complete solutions (empty remaining goals) from incomplete ones
+      let (complete, incomplete) := suggestions.partition (·.1.isEmpty)
+      if requireClose && !config.all then
         let hint := if suggestions.isEmpty then "" else " Try `apply?` to see partial suggestions."
         throwError "`exact?` could not close the goal.{hint}"
+      if requireClose && config.all && complete.isEmpty then
+        let hint := if incomplete.isEmpty then "" else " Try `apply?` to see partial suggestions."
+        throwError "`exact?` could not close the goal.{hint}"
       reportOutOfHeartbeats `apply? ref
-      for (_, suggestionMCtx) in suggestions do
+      -- Collect complete solutions and show as a single "Try these:" message
+      let completeExprs ← complete.mapM fun (_, suggestionMCtx) =>
         withMCtx suggestionMCtx do
-          addExactSuggestion ref (← instantiateMVars (mkMVar mvar)).headBeta
-            (checkState? := initialState) (addSubgoalsMsg := true) (tacticErrorAsInfo := true)
-      if suggestions.isEmpty then logError "apply? didn't find any relevant lemmas"
+          return (← instantiateMVars (mkMVar mvar)).headBeta
+      if !completeExprs.isEmpty then
+        addExactSuggestions ref completeExprs (checkState? := initialState)
+      -- Show incomplete solutions only if not requireClose (i.e., for apply?)
+      if !requireClose then
+        let incompleteExprs ← incomplete.mapM fun (_, suggestionMCtx) =>
+          withMCtx suggestionMCtx do
+            return (← instantiateMVars (mkMVar mvar)).headBeta
+        if !incompleteExprs.isEmpty then
+          addExactSuggestions ref incompleteExprs
+            (checkState? := initialState) (addSubgoalsMsg := true)
+        if suggestions.isEmpty then logError "apply? didn't find any relevant lemmas"
       admitGoal goal (synthetic := false)
 
 @[builtin_tactic Lean.Parser.Tactic.exact?]

src/Lean/Meta/Tactic/LibrarySearch.lean

@@ -334,15 +334,17 @@ Sequentially invokes a tactic `act` on each value in candidates on the current s
 
 The tactic `act` should return a list of meta-variables that still need to be resolved.
 If this list is empty, then no variables remain to be solved, and `tryOnEach` returns
-`none` with the environment set so each goal is resolved.
+`none` with the environment set so each goal is resolved (unless `collectAll` is true,
+in which case it continues searching and collects complete solutions in the array).
 
 If the action throws an internal exception with the `abortSpeculationId` id then
 further computation is stopped and intermediate results returned. If any other
 exception is thrown, then it is silently discarded.
 -/
 def tryOnEach
     (act : Candidate → MetaM (List MVarId))
-    (candidates : Array Candidate) :
+    (candidates : Array Candidate)
+    (collectAll : Bool := false) :
     MetaM (Option (Array (List MVarId × MetavarContext))) := do
   let mut a := #[]
   let s ← saveState
@@ -353,7 +355,7 @@ def tryOnEach
       if isAbortSpeculation e then
         break
     | .ok remaining =>
-      if remaining.isEmpty then
+      if remaining.isEmpty && !collectAll then
         return none
       let ctx ← getMCtx
       restoreState s
@@ -364,7 +366,8 @@ private def librarySearch' (goal : MVarId)
     (tactic : List MVarId → MetaM (List MVarId))
     (allowFailure : MVarId → MetaM Bool)
     (leavePercentHeartbeats : Nat)
-    (includeStar : Bool := true) :
+    (includeStar : Bool := true)
+    (collectAll : Bool := false) :
     MetaM (Option (Array (List MVarId × MetavarContext))) := do
   withTraceNode `Tactic.librarySearch (return m!"{librarySearchEmoji ·} {← goal.getType}") do
   profileitM Exception "librarySearch" (← getOptions) do
@@ -376,32 +379,35 @@ private def librarySearch' (goal : MVarId)
         librarySearchLemma cfg tactic allowFailure cand
     -- First pass: search with droppedKeys (excludes star-indexed lemmas)
     let candidates ← librarySearchSymm libSearchFindDecls goal
-    match ← tryOnEach act candidates with
-    | none => return none  -- Found a complete solution
+    match ← tryOnEach act candidates collectAll with
+    | none => return none  -- Found a complete solution (only when collectAll = false)
     | some results =>
       -- Only do star fallback if:
-      -- 1. No results from primary search
+      -- 1. No complete solutions from primary search (when collectAll, check for empty remaining)
       -- 2. includeStar is true
-      if !results.isEmpty || !includeStar then
+      let hasCompleteSolution := results.any (·.1.isEmpty)
+      if hasCompleteSolution || !results.isEmpty || !includeStar then
         return some results
       -- Second pass: try star-indexed lemmas (those with [*] or [Eq,*,*,*] keys)
       -- No need for librarySearchSymm since getStarLemmas ignores the goal type
       let starLemmas ← getStarLemmas
       if starLemmas.isEmpty then return some results
       let mctx ← getMCtx
       let starCandidates := starLemmas.map ((goal, mctx), ·)
-      tryOnEach act starCandidates
+      tryOnEach act starCandidates collectAll
 
 /--
 Tries to solve the goal by applying a library lemma
 then calling `tactic` on the resulting goals.
 
 Typically here `tactic` is `solveByElim`.
 
-If it successfully closes the goal, returns `none`.
+If it successfully closes the goal, returns `none` (unless `collectAll` is true).
 Otherwise, it returns `some a`, where `a : Array (List MVarId × MetavarContext)`,
 with an entry for each library lemma which was successfully applied,
 containing a list of the subsidiary goals, and the metavariable context after the application.
+When `collectAll` is true, complete solutions (with empty remaining goals) are also included
+in the array instead of returning early.
 
 (Always succeeds, and the metavariable context stored in the monad is reverted,
 unless the goal was completely solved.)
@@ -414,9 +420,10 @@ def librarySearch (goal : MVarId)
       fun g => solveByElim [] (maxDepth := 6) (exfalso := false) g)
     (allowFailure : MVarId → MetaM Bool := fun _ => pure true)
     (leavePercentHeartbeats : Nat := 10)
-    (includeStar : Bool := true) :
+    (includeStar : Bool := true)
+    (collectAll : Bool := false) :
     MetaM (Option (Array (List MVarId × MetavarContext))) := do
-  librarySearch' goal tactic allowFailure leavePercentHeartbeats includeStar
+  librarySearch' goal tactic allowFailure leavePercentHeartbeats includeStar collectAll
 
 end LibrarySearch
 

tests/lean/librarySearch.lean

@@ -90,16 +90,31 @@ info: Try this:
 #guard_msgs in
 example (X : Type) (P : Prop) (x : X) (h : ∀ x : X, x = x → P) : P := by show_term solve_by_elim
 
--- Could be any number of results (`fun x => x`, `id`, etc)
-#guard_msgs (drop info) in
+/--
+info: Try this:
+  [apply] exact fun a => a
+-/
+#guard_msgs in
 example (α : Prop) : α → α := by show_term solve_by_elim
 
 -- These examples work via star-indexed fallback.
-#guard_msgs (drop info) in
+/--
+info: Try this:
+  [apply] exact fun a => Classical.byContradiction a
+-/
+#guard_msgs in
 example (p : Prop) : (¬¬p) → p := by apply?
-#guard_msgs (drop info) in
+/--
+info: Try this:
+  [apply] exact h.left
+-/
+#guard_msgs in
 example (a b : Prop) (h : a ∧ b) : a := by apply?
-#guard_msgs (drop info) in
+/--
+info: Try this:
+  [apply] exact fun a a_1 => Classical.byContradiction fun a_2 => a a_2 a_1
+-/
+#guard_msgs in
 example (P Q : Prop) : (¬ Q → ¬ P) → (P → Q) := by apply?
 
 /--
@@ -264,9 +279,13 @@ info: Try this:
 example {a b c : Nat} (h₁ : a ∣ c) (h₂ : a ∣ b + c) : a ∣ b := by apply?
 
 -- These examples work via star-indexed fallback.
-#guard_msgs (drop info) in
+/--
+info: Try this:
+  [apply] exact h.elim
+-/
+#guard_msgs in
 example {α : Sort u} (h : Empty) : α := by apply?
-#guard_msgs (drop info) in
+#guard_msgs in
 example (f : A → C) (g : B → C) : (A ⊕ B) → C := by apply?
 
 opaque f : Nat → Nat

tests/lean/run/library_search_all.lean

@@ -0,0 +1,59 @@
+/-!
+# Tests for `exact? +all`
+
+This tests the `+all` option which collects all successful lemmas
+instead of stopping at the first complete solution.
+-/
+
+set_option linter.unusedVariables false
+
+-- Create a custom inductive with limited lemmas for controlled testing
+inductive MyTrue : Prop where
+  | intro
+
+-- Define exactly three ways to prove MyTrue
+theorem myTrue1 : MyTrue := MyTrue.intro
+theorem myTrue2 : MyTrue := MyTrue.intro
+theorem myTrue3 : MyTrue := MyTrue.intro
+
+-- Test: without +all, we get a single suggestion and the goal is solved
+/--
+info: Try this:
+  [apply] exact myTrue3
+-/
+#guard_msgs in
+example : MyTrue := by exact?
+
+-- Test: with +all, exact? finds all lemmas (including the constructor)
+-- The goal is admitted (sorry), and all suggestions are shown in one message
+/--
+info: Try these:
+  [apply] exact myTrue3
+  [apply] exact MyTrue.intro
+  [apply] exact myTrue1
+  [apply] exact myTrue2
+---
+warning: declaration uses 'sorry'
+-/
+#guard_msgs in
+example : MyTrue := by exact? +all
+
+-- Test: apply? also supports +all
+/--
+info: Try these:
+  [apply] exact myTrue3
+  [apply] exact MyTrue.intro
+  [apply] exact myTrue1
+  [apply] exact myTrue2
+---
+warning: declaration uses 'sorry'
+-/
+#guard_msgs in
+example : MyTrue := by apply? +all
+
+-- Test: verify that +all without any matches still errors appropriately
+/--
+error: `exact?` could not close the goal. Try `apply?` to see partial suggestions.
+-/
+#guard_msgs in
+example (n : Nat) (h : n = n + 1) : False := by exact? +all