feat: drop support for termination_by' (leanprover#3033)

until around 7fe6881 the way to define well-founded recursions was to
specify a `WellFoundedRelation` on the argument explicitly. This was
rather low-level, for example one had to predict the packing of multiple
arguments into `PProd`s, the packing of mutual functions into `PSum`s,
and the cliques that were calculated.

Then the current `termination_by` syntax was introduced, where you
specify the termination argument at a higher level (one clause per
functions, unpacked arguments), and the `WellFoundedRelation` is found
using type class resolution.

The old syntax was kept around as `termination_by'`. This is not used
anywhere in the lean, std, mathlib or the theorem-proving-in-lean
repositories,
and three occurrences I found in the wild can do without

In particular, it should be possible to express anything that the old
syntax
supported also with the new one, possibly requiring a helper type with a
suitable instance, or the following generic wrapper that now lives in
std
```
def wrap {α : Sort u} {r : α → α → Prop} (h : WellFounded r) (x : α) : {x : α // Acc r x}
```

Since the old syntax is unused, has an unhelpful name and relies on
internals, this removes the support. Now is a good time before the
refactoring that's planned in leanprover#2921.

The test suite was updated without particular surprises.

The parametric `terminationHint` parser is gone, which means we can
match on syntax more easily now, in `expandDecreasingBy?`.

RELEASES.md

@@ -20,6 +20,22 @@ v4.5.0 (development in progress)
   ```
   [PR #2821](https://github.com/leanprover/lean4/pull/2821) and [RFC #2838](https://github.com/leanprover/lean4/issues/2838).
 
+* The low-level `termination_by'` clause is no longer supported.
+
+  Migration guide: Use `termination_by` instead, e.g.:
+  ```diff
+  -termination_by' measure (fun ⟨i, _⟩ => as.size - i)
+  +termination_by go i _ => as.size - i
+  ```
+
+  If the well-founded relation you want to use is not the one that the
+  `WellFoundedRelation` type class would infer for your termination argument,
+  you can use `WellFounded.wrap` from the std libarary to explicitly give one:
+  ```diff
+  -termination_by' ⟨r, hwf⟩
+  +termination_by _ x => hwf.wrap x
+  ```
+
 v4.4.0
 ---------
 

src/Lean/Elab/PreDefinition/Main.lean

@@ -100,8 +100,8 @@ def addPreDefinitions (preDefs : Array PreDefinition) (hints : TerminationHints)
   let preDefs ← preDefs.mapM ensureNoUnassignedMVarsAtPreDef
   let preDefs ← betaReduceLetRecApps preDefs
   let cliques := partitionPreDefs preDefs
-  let mut terminationBy ← liftMacroM <| WF.expandTerminationBy hints.terminationBy? (cliques.map fun ds => ds.map (·.declName))
-  let mut decreasingBy  ← liftMacroM <| WF.expandTerminationHint hints.decreasingBy? (cliques.map fun ds => ds.map (·.declName))
+  let mut terminationBy ← liftMacroM <| WF.expandTerminationBy? hints.terminationBy? (cliques.map fun ds => ds.map (·.declName))
+  let mut decreasingBy  ← liftMacroM <| WF.expandDecreasingBy? hints.decreasingBy? (cliques.map fun ds => ds.map (·.declName))
   let mut hasErrors := false
   for preDefs in cliques do
     trace[Elab.definition.scc] "{preDefs.map (·.declName)}"

src/Lean/Elab/PreDefinition/WF/GuessLex.lean

@@ -32,7 +32,6 @@ In addition to measures derived from `sizeOf xᵢ`, it also considers measures
 that assign an order to the functions themselves. This way we can support mutual
 function definitions where no arguments decrease from one function to another.
 
-
 The result of this module is a `TerminationWF`, which is then passed on to `wfRecursion`; this
 design is crucial so that whatever we infer in this module could also be written manually by the
 user. It would be bad if there are function definitions that can only be processed with the
@@ -590,7 +589,7 @@ def buildTermWF (declNames : Array Name) (varNamess : Array (Array Name))
         declName, vars, body,
         implicit := true
       }
-  return .ext termByElements
+  return termByElements
 
 
 /--

src/Lean/Elab/PreDefinition/WF/Rel.lean

@@ -34,10 +34,10 @@ private partial def unpackUnary (preDef : PreDefinition) (prefixSize : Nat) (mva
     let mut varNames ← xs.mapM fun x => x.fvarId!.getUserName
     if element.vars.size > varNames.size then
       throwErrorAt element.vars[varNames.size]! "too many variable names"
-    for i in [:element.vars.size] do
-      let varStx := element.vars[i]!
-      if varStx.isIdent then
-        varNames := varNames.set! (varNames.size - element.vars.size + i) varStx.getId
+    for h : i in [:element.vars.size] do
+      let varStx := element.vars[i]'h.2
+      if let `($ident:ident) := varStx then
+        varNames := varNames.set! (varNames.size - element.vars.size + i) ident.getId
     return varNames
   let mut mvarId := mvarId
   for localDecl in (← Term.getMVarDecl mvarId).lctx, varName in varNames[:prefixSize] do
@@ -60,25 +60,18 @@ def elabWFRel (preDefs : Array PreDefinition) (unaryPreDefName : Name) (fixedPre
   let expectedType := mkApp (mkConst ``WellFoundedRelation [u]) α
   trace[Elab.definition.wf] "elabWFRel start: {(← mkFreshTypeMVar).mvarId!}"
   withDeclName unaryPreDefName do
-  match wf with
-  | TerminationWF.core wfStx =>
-      let wfRel ← instantiateMVars (← withSynthesize <| elabTermEnsuringType wfStx expectedType)
-      let pendingMVarIds ← getMVars wfRel
-      discard <| logUnassignedUsingErrorInfos pendingMVarIds
-      k wfRel
-  | TerminationWF.ext elements =>
-    withRef (getRefFromElems elements) do
-      let mainMVarId := (← mkFreshExprSyntheticOpaqueMVar expectedType).mvarId!
-      let [fMVarId, wfRelMVarId, _] ← mainMVarId.apply (← mkConstWithFreshMVarLevels ``invImage) | throwError "failed to apply 'invImage'"
-      let (d, fMVarId) ← fMVarId.intro1
-      let subgoals ← unpackMutual preDefs fMVarId d
-      for (d, mvarId) in subgoals, element in elements, preDef in preDefs do
-        let mvarId ← unpackUnary preDef fixedPrefixSize mvarId d element
-        mvarId.withContext do
-          let value ← Term.withSynthesize <| elabTermEnsuringType element.body (← mvarId.getType)
-          mvarId.assign value
-      let wfRelVal ← synthInstance (← inferType (mkMVar wfRelMVarId))
-      wfRelMVarId.assign wfRelVal
-      k (← instantiateMVars (mkMVar mainMVarId))
+  withRef (getRefFromElems wf) do
+    let mainMVarId := (← mkFreshExprSyntheticOpaqueMVar expectedType).mvarId!
+    let [fMVarId, wfRelMVarId, _] ← mainMVarId.apply (← mkConstWithFreshMVarLevels ``invImage) | throwError "failed to apply 'invImage'"
+    let (d, fMVarId) ← fMVarId.intro1
+    let subgoals ← unpackMutual preDefs fMVarId d
+    for (d, mvarId) in subgoals, element in wf, preDef in preDefs do
+      let mvarId ← unpackUnary preDef fixedPrefixSize mvarId d element
+      mvarId.withContext do
+        let value ← Term.withSynthesize <| elabTermEnsuringType element.body (← mvarId.getType)
+        mvarId.assign value
+    let wfRelVal ← synthInstance (← inferType (mkMVar wfRelMVarId))
+    wfRelMVarId.assign wfRelVal
+    k (← instantiateMVars (mkMVar mainMVarId))
 
 end Lean.Elab.WF

src/Lean/Elab/PreDefinition/WF/TerminationHint.lean

@@ -5,114 +5,112 @@ Authors: Leonardo de Moura
 -/
 import Lean.Parser.Command
 
+set_option autoImplicit false
+
 namespace Lean.Elab.WF
 
-/-! # Support for `decreasing_by` and `termination_by'` notations -/
+/-! # Support for `decreasing_by` -/
 
-structure TerminationHintValue where
+structure DecreasingByTactic where
   ref   : Syntax
-  value : Syntax
+  value : Lean.TSyntax `Lean.Parser.Tactic.tacticSeq
   deriving Inhabited
 
-inductive TerminationHint where
+inductive DecreasingBy where
   | none
-  | one (val : TerminationHintValue)
-  | many (map : NameMap TerminationHintValue)
+  | one (val : DecreasingByTactic)
+  | many (map : NameMap DecreasingByTactic)
   deriving Inhabited
 
 open Parser.Command in
 /--
-This function takes a user-specified `decreasing_by` or `termination_by'` clause (as `Sytnax`).
-If it is a `terminathionHintMany` (a set of clauses guarded by the function name) then
+This function takes a user-specified `decreasing_by` clause (as `Sytnax`).
+If it is a `decreasingByMany` (a set of clauses guarded by the function name) then
 * checks that all mentioned names exist in the current declaration
 * check that at most one function from each clique is mentioned
 and sort the entries by function name.
 -/
-def expandTerminationHint (terminationHint? : Option Syntax) (cliques : Array (Array Name)) : MacroM TerminationHint := do
-  let some terminationHint := terminationHint? | return TerminationHint.none
-  let ref := terminationHint
-  match terminationHint with
-  | `(terminationByCore|termination_by' $hint1:terminationHint1)
-  | `(decreasingBy|decreasing_by $hint1:terminationHint1) =>
-    return TerminationHint.one { ref, value := hint1.raw[0] }
-  | `(terminationByCore|termination_by' $hints:terminationHintMany)
-  | `(decreasingBy|decreasing_by $hints:terminationHintMany) => do
-    let m ← hints.raw[0].getArgs.foldlM (init := {}) fun m arg =>
+def expandDecreasingBy? (decreasingBy? : Option Syntax) (cliques : Array (Array Name)) : MacroM DecreasingBy := do
+  let some decreasingBy := decreasingBy? | return DecreasingBy.none
+  let ref := decreasingBy
+  match decreasingBy with
+  | `(decreasingBy|decreasing_by $hint1:tacticSeq) =>
+    return DecreasingBy.one { ref, value := hint1 }
+  | `(decreasingBy|decreasing_by $hints:decreasingByMany) => do
+    let m ← hints.raw[0].getArgs.foldlM (init := {}) fun m arg => do
+      let arg : TSyntax `decreasingByElement := ⟨arg⟩ -- cannot use syntax pattern match with lookahead
+      let `(decreasingByElement| $declId:ident => $tac:tacticSeq) := arg | Macro.throwUnsupported
       let declName? := cliques.findSome? fun clique => clique.findSome? fun declName =>
-        if arg[0].getId.isSuffixOf declName then some declName else none
+        if declId.getId.isSuffixOf declName then some declName else none
       match declName? with
-      | none => Macro.throwErrorAt arg[0] s!"function '{arg[0].getId}' not found in current declaration"
-      | some declName => return m.insert declName { ref := arg, value := arg[2] }
+      | none => Macro.throwErrorAt declId s!"function '{declId.getId}' not found in current declaration"
+      | some declName => return m.insert declName { ref := arg, value := tac }
     for clique in cliques do
       let mut found? := Option.none
       for declName in clique do
         if let some { ref, .. } := m.find? declName then
           if let some found := found? then
             Macro.throwErrorAt ref s!"invalid termination hint element, '{declName}' and '{found}' are in the same clique"
           found? := some declName
-    return TerminationHint.many m
+    return DecreasingBy.many m
   | _ => Macro.throwUnsupported
 
-def TerminationHint.markAsUsed (t : TerminationHint) (clique : Array Name) : TerminationHint :=
+def DecreasingBy.markAsUsed (t : DecreasingBy) (clique : Array Name) : DecreasingBy :=
   match t with
-  | TerminationHint.none   => TerminationHint.none
-  | TerminationHint.one .. => TerminationHint.none
-  | TerminationHint.many m => Id.run do
+  | DecreasingBy.none   => DecreasingBy.none
+  | DecreasingBy.one .. => DecreasingBy.none
+  | DecreasingBy.many m => Id.run do
     for declName in clique do
       if m.contains declName then
         let m := m.erase declName
         if m.isEmpty then
-          return TerminationHint.none
+          return DecreasingBy.none
         else
-          return TerminationHint.many m
+          return DecreasingBy.many m
     return t
 
-def TerminationHint.find? (t : TerminationHint) (clique : Array Name) : Option TerminationHintValue :=
+def DecreasingBy.find? (t : DecreasingBy) (clique : Array Name) : Option DecreasingByTactic :=
   match t with
-  | TerminationHint.none   => Option.none
-  | TerminationHint.one v  => some v
-  | TerminationHint.many m => clique.findSome? m.find?
+  | DecreasingBy.none   => Option.none
+  | DecreasingBy.one v  => some v
+  | DecreasingBy.many m => clique.findSome? m.find?
 
-def TerminationHint.ensureAllUsed (t : TerminationHint) : MacroM Unit := do
+def DecreasingBy.ensureAllUsed (t : DecreasingBy) : MacroM Unit := do
   match t with
-  | TerminationHint.one v   => Macro.throwErrorAt v.ref "unused termination hint element"
-  | TerminationHint.many m  => m.forM fun _ v => Macro.throwErrorAt v.ref "unused termination hint element"
+  | DecreasingBy.one v   => Macro.throwErrorAt v.ref "unused termination hint element"
+  | DecreasingBy.many m  => m.forM fun _ v => Macro.throwErrorAt v.ref "unused termination hint element"
   | _ => pure ()
 
-/-! # Support for `termination_by` and `termination_by'` notation -/
+/-! # Support for `termination_by` notation -/
 
 /-- A single `termination_by` clause -/
 structure TerminationByElement where
   ref       : Syntax
   declName  : Name
-  vars      : Array Syntax
-  body      : Syntax
+  vars      : TSyntaxArray [`ident, ``Lean.Parser.Term.hole]
+  body      : Term
   implicit  : Bool
   deriving Inhabited
 
-/-- `terminatoin_by` clauses, grouped by clique -/
+/-- `termination_by` clauses, grouped by clique -/
 structure TerminationByClique where
   elements : Array TerminationByElement
   used     : Bool := false
 
 /--
-A `termination_by'` or `termination_by` hint, as specified by the user
+A `termination_by` hint, as specified by the user
 -/
-inductive TerminationBy where
-  /-- `termination_by'` -/
-  | core (hint : TerminationHint)
-  /-- `termination_by` -/
-  | ext  (cliques : Array TerminationByClique)
+structure TerminationBy where
+  cliques : Array TerminationByClique
   deriving Inhabited
 
 /--
-A `termination_by'` or `termination_by` hint, as applicable to a single clique
+A `termination_by` hint, as applicable to a single clique
 -/
-inductive TerminationWF where
-  | core (stx : Syntax)
-  | ext  (clique : Array TerminationByElement)
+abbrev TerminationWF := Array TerminationByElement
 
-/-
+open Parser.Command in
+/--
 Expands the syntax for a `termination_by` clause, checking that
 * each function is mentioned once
 * each function mentioned actually occurs in the current declaration
@@ -125,8 +123,9 @@ def terminationByElement   := leading_parser ppLine >> (ident <|> hole) >> many
 def terminationBy          := leading_parser ppLine >> "termination_by " >> many1chIndent terminationByElement
 ```
 -/
-open Parser.Command in
-private def expandTerminationByNonCore (hint : Syntax) (cliques : Array (Array Name)) : MacroM TerminationBy := do
+def expandTerminationBy? (hint? : Option Syntax) (cliques : Array (Array Name)) :
+    MacroM TerminationBy := do
+  let some hint := hint?  | return { cliques := #[] }
   let `(terminationBy|termination_by $elementStxs*) := hint | Macro.throwUnsupported
   let mut alreadyFound : NameSet := {}
   let mut elseElemStx? := none
@@ -170,71 +169,45 @@ private def expandTerminationByNonCore (hint : Syntax) (cliques : Array (Array N
       result := result.push { elements }
   if !usedElse && elseElemStx?.isSome then
     withRef elseElemStx?.get! <| Macro.throwError s!"invalid `termination_by` syntax, unnecessary else-case"
-  return TerminationBy.ext result
-
-/--
-Expands the syntax for a `termination_by` or `termination_by'` clause, using the appropriate function
--/
-def expandTerminationBy (hint? : Option Syntax) (cliques : Array (Array Name)) : MacroM TerminationBy :=
-  if let some hint := hint? then
-    if hint.isOfKind ``Parser.Command.terminationByCore then
-      return TerminationBy.core (← expandTerminationHint hint? cliques)
-    else if hint.isOfKind ``Parser.Command.terminationBy then
-      expandTerminationByNonCore hint cliques
-    else
-      Macro.throwUnsupported
-  else
-      return TerminationBy.core TerminationHint.none
+  return ⟨result⟩
 
 open Parser.Command in
-def TerminationWF.unexpand : TerminationWF → MetaM Syntax
-  | .ext elements => do
-    let elementStxs ← elements.mapM fun element => do
-      let fn : Ident := mkIdent (← unresolveNameGlobal element.declName)
-      let body : Term := ⟨element.body⟩
-      let vars : Array Ident := element.vars.map TSyntax.mk
-      `(terminationByElement|$fn $vars* => $body)
-    `(terminationBy|termination_by $elementStxs*)
-  | .core _ => unreachable! -- we don't synthetize termination_by' syntax
+def TerminationWF.unexpand (elements : TerminationWF) : MetaM Syntax := do
+  let elementStxs ← elements.mapM fun element => do
+    let fn : Ident := mkIdent (← unresolveNameGlobal element.declName)
+    `(terminationByElement|$fn $element.vars* => $element.body)
+  `(terminationBy|termination_by $elementStxs*)
 
 def TerminationBy.markAsUsed (t : TerminationBy) (cliqueNames : Array Name) : TerminationBy :=
-  match t with
-  | core hint   => core (hint.markAsUsed cliqueNames)
-  | ext cliques => ext <| cliques.map fun clique =>
+  .mk <| t.cliques.map fun clique =>
     if cliqueNames.any fun name => clique.elements.any fun elem => elem.declName == name then
       { clique with used := true }
     else
       clique
 
 def TerminationBy.find? (t : TerminationBy) (cliqueNames : Array Name) : Option TerminationWF :=
-  match t with
-  | core hint => hint.find? cliqueNames |>.map fun v => TerminationWF.core v.value
-  | ext cliques =>
-    cliques.findSome? fun clique =>
-      if cliqueNames.any fun name => clique.elements.any fun elem => elem.declName == name then
-        some <| TerminationWF.ext clique.elements
-      else
-        none
+  t.cliques.findSome? fun clique =>
+    if cliqueNames.any fun name => clique.elements.any fun elem => elem.declName == name then
+      some <| clique.elements
+    else
+      none
 
 def TerminationByClique.allImplicit (c : TerminationByClique) : Bool :=
   c.elements.all fun elem => elem.implicit
 
 def TerminationByClique.getExplicitElement? (c : TerminationByClique) : Option TerminationByElement :=
   c.elements.find? (!·.implicit)
 
-def TerminationBy.ensureAllUsed (t : TerminationBy) : MacroM Unit :=
-  match t with
-  | core hint => hint.ensureAllUsed
-  | ext cliques => do
-    let hasUsedAllImplicit := cliques.any fun c => c.allImplicit && c.used
-    let mut reportedAllImplicit := true
-    for clique in cliques do
-      unless clique.used do
-        if let some explicitElem := clique.getExplicitElement? then
-          Macro.throwErrorAt explicitElem.ref "unused termination hint element"
-        else if !hasUsedAllImplicit then
-          unless reportedAllImplicit do
-            reportedAllImplicit := true
-            Macro.throwErrorAt clique.elements[0]!.ref "unused termination hint element"
+def TerminationBy.ensureAllUsed (t : TerminationBy) : MacroM Unit := do
+  let hasUsedAllImplicit := t.cliques.any fun c => c.allImplicit && c.used
+  let mut reportedAllImplicit := true
+  for clique in t.cliques do
+    unless clique.used do
+      if let some explicitElem := clique.getExplicitElement? then
+        Macro.throwErrorAt explicitElem.ref "unused termination hint element"
+      else if !hasUsedAllImplicit then
+        unless reportedAllImplicit do
+          reportedAllImplicit := true
+          Macro.throwErrorAt clique.elements[0]!.ref "unused termination hint element"
 
 end Lean.Elab.WF