feat: equations for simple functions

Functions without pattern matching or wf recursion don't have
any equational lemma autogenerated for themselves so we have to
generate it explicitly. The implementation is largely adapted from
structural equation code in the compiler.

DocGen4/Process.lean

@@ -177,17 +177,29 @@ def processEq (eq : Name) : MetaM CodeWithInfos := do
   let final := stripArgs type
   prettyPrintTerm final
 
+def valueToEq (v : DefinitionVal) : MetaM Expr := withLCtx {} {} do
+  let env ← getEnv
+  withOptions (tactic.hygienic.set . false) do
+    lambdaTelescope v.value fun xs body => do
+      let us := v.levelParams.map mkLevelParam
+      let type ← mkEq (mkAppN (Lean.mkConst v.name us) xs) body
+      let type ← mkForallFVars xs type
+      type
+
 def DefinitionInfo.ofDefinitionVal (v : DefinitionVal) : MetaM DefinitionInfo := do
   let info ← Info.ofConstantVal v.toConstantVal
-  let mut eqs? := none
   try
-    eqs? ← getEqnsFor? v.name
+    let eqs? ← getEqnsFor? v.name
+    match eqs? with
+    | some eqs =>
+      let prettyEqs ← eqs.mapM processEq
+      DefinitionInfo.mk info v.safety v.hints prettyEqs
+    | none =>
+      let eq ← prettyPrintTerm $ stripArgs (←valueToEq v)
+      DefinitionInfo.mk info v.safety v.hints (some #[eq])
   catch err =>
     IO.println s!"WARNING: Failed to calculate equational lemmata for {v.name}: {←err.toMessageData.toString}"
-
-  let prettyEqs ← eqs?.mapM (λ eqs => eqs.mapM processEq)
-  return DefinitionInfo.mk info v.safety v.hints prettyEqs
-
+    return DefinitionInfo.mk info v.safety v.hints none
 
 def getConstructorType (ctor : Name) : MetaM CodeWithInfos := do
   let env ← getEnv