feat: completion fallback (#5299)

When the elaborator doesn't provide us with any `CompletionInfo`, we
currently provide no completions whatsoever. But in many cases, we can
still provide some helpful identifier completions without elaborator
information. This PR adds a fallback mode for this situation.

There is more potential here, but this should be a good start.

In principle, this issue alleviates #5172 (since we now provide
completions in these contexts). I'll leave it up to an elaboration
maintainer whether we also want to ensure that the completion infos are
provided correctly in these cases.

src/Lean/Server/Completion.lean

@@ -666,6 +666,50 @@ private def tacticCompletion (params : CompletionParams) (ctx : ContextInfo) : I
       }, 1)
     return some { items := sortCompletionItems items, isIncomplete := true }
 
+/--
+If there are `Info`s that contain `hoverPos` and have a nonempty `LocalContext`,
+yields the closest one of those `Info`s.
+Otherwise, yields the closest `Info` that contains `hoverPos` and has an empty `LocalContext`.
+-/
+private def findClosestInfoWithLocalContextAt?
+    (hoverPos : String.Pos)
+    (infoTree : InfoTree)
+    : Option (ContextInfo × Info) :=
+  infoTree.visitM (m := Id) (postNode := choose) |>.join
+where
+  choose
+      (ctx : ContextInfo)
+      (info : Info)
+      (_ : PersistentArray InfoTree)
+      (childValues : List (Option (Option (ContextInfo × Info))))
+      : Option (ContextInfo × Info) :=
+    let bestChildValue := childValues.map (·.join) |>.foldl (init := none) fun v best =>
+      if isBetter v best then
+        v
+      else
+        best
+    if info.occursInOrOnBoundary hoverPos && isBetter (ctx, info) bestChildValue then
+      (ctx, info)
+    else
+      bestChildValue
+
+  isBetter (a b : Option (ContextInfo × Info)) : Bool :=
+    match a, b with
+    | none,   none   => false
+    | some _, none   => true
+    | none,   some _ => false
+    | some (_, ia), some (_, ib) =>
+      if !ia.lctx.isEmpty && ib.lctx.isEmpty then
+        true
+      else if ia.lctx.isEmpty && !ib.lctx.isEmpty then
+        false
+      else if ia.isSmaller ib then
+        true
+      else if ib.isSmaller ia then
+        false
+      else
+        false
+
 private def findCompletionInfoAt?
     (fileMap  : FileMap)
     (hoverPos : String.Pos)
@@ -675,9 +719,32 @@ private def findCompletionInfoAt?
   match infoTree.foldInfo (init := none) (choose hoverLine) with
   | some (hoverInfo, ctx, Info.ofCompletionInfo info) =>
     some (hoverInfo, ctx, info)
-  | _ =>
-    -- TODO try to extract id from `fileMap` and some `ContextInfo` from `InfoTree`
-    none
+  | _ => do
+    -- No completion info => Attempt providing identifier completions
+    let some (ctx, info) := findClosestInfoWithLocalContextAt? hoverPos infoTree
+      | none
+    let some stack := info.stx.findStack? (·.getRange?.any (·.contains hoverPos (includeStop := true)))
+      | none
+    let stack := stack.dropWhile fun (stx, _) => !(stx matches `($_:ident) || stx matches `($_:ident.))
+    let some (stx, _) := stack.head?
+      | none
+    let isDotIdCompletion := stack.any fun (stx, _) => stx matches `(.$_:ident)
+    if isDotIdCompletion then
+      -- An identifier completion is never useful in a dotId completion context.
+      none
+    let some (id, danglingDot) :=
+        match stx with
+        | `($id:ident) => some (id.getId, false)
+        | `($id:ident.) => some (id.getId, true)
+        | _ => none
+      | none
+    let tailPos := stx.getTailPos?.get!
+    let hoverInfo :=
+      if hoverPos < tailPos then
+        HoverInfo.inside (tailPos - hoverPos).byteIdx
+      else
+        HoverInfo.after
+    some (hoverInfo, ctx, .id stx id danglingDot info.lctx none)
 where
   choose
       (hoverLine : Nat)

src/Lean/Server/InfoUtils.lean

@@ -141,10 +141,12 @@ def Info.stx : Info → Syntax
   | ofOmissionInfo i       => i.stx
 
 def Info.lctx : Info → LocalContext
-  | Info.ofTermInfo i     => i.lctx
-  | Info.ofFieldInfo i    => i.lctx
-  | Info.ofOmissionInfo i => i.lctx
-  | _                     => LocalContext.empty
+  | .ofTermInfo i           => i.lctx
+  | .ofFieldInfo i          => i.lctx
+  | .ofOmissionInfo i       => i.lctx
+  | .ofMacroExpansionInfo i => i.lctx
+  | .ofCompletionInfo i     => i.lctx
+  | _                       => LocalContext.empty
 
 def Info.pos? (i : Info) : Option String.Pos :=
   i.stx.getPos? (canonicalOnly := true)

tests/lean/interactive/completionFallback.lean

@@ -0,0 +1,31 @@
+-- When the elaborator doesn't provide `CompletionInfo`, try to provide identifier completions.
+-- As of when this test case was written, the elaborator did not provide `CompletionInfo` in these cases.
+
+-- https://github.com/leanprover/lean4/issues/5172
+
+inductive Direction where
+  | up
+  | right
+  | down
+  | left
+deriving Repr
+
+def angle (d: Direction) :=
+  match d with
+  | Direction. => 90
+            --^ textDocument/completion
+  | Direction.right => 0
+  | Direction.down => 270
+  | Direction.left => 180
+
+-- Ensure that test is stable when changes to the `And` namespace are made.
+structure CustomAnd (a b : Prop) : Prop where
+  ha : a
+  hb : b
+
+example : p ∨ (q ∧ r) → CustomAnd (p ∨ q) (p ∨ r) := by
+  intro h
+  cases h with
+  | inl hp => apply CustomAnd. (Or.intro_left q hp) (Or.intro_left r hp)
+                            --^ textDocument/completion
+  | inr hqr => apply CustomAnd.mk (Or.intro_right p hqr.left) (Or.intro_right p hqr.right)

tests/lean/interactive/completionFallback.lean.expected.out

@@ -0,0 +1,80 @@
+{"textDocument": {"uri": "file:///completionFallback.lean"},
+ "position": {"line": 14, "character": 14}}
+{"items":
+ [{"sortText": "0",
+   "label": "down",
+   "kind": 4,
+   "data":
+   {"params":
+    {"textDocument": {"uri": "file:///completionFallback.lean"},
+     "position": {"line": 14, "character": 14}},
+    "id": {"const": {"declName": "Direction.down"}}}},
+  {"sortText": "1",
+   "label": "left",
+   "kind": 4,
+   "data":
+   {"params":
+    {"textDocument": {"uri": "file:///completionFallback.lean"},
+     "position": {"line": 14, "character": 14}},
+    "id": {"const": {"declName": "Direction.left"}}}},
+  {"sortText": "2",
+   "label": "noConfusionType",
+   "kind": 3,
+   "data":
+   {"params":
+    {"textDocument": {"uri": "file:///completionFallback.lean"},
+     "position": {"line": 14, "character": 14}},
+    "id": {"const": {"declName": "Direction.noConfusionType"}}}},
+  {"sortText": "3",
+   "label": "right",
+   "kind": 4,
+   "data":
+   {"params":
+    {"textDocument": {"uri": "file:///completionFallback.lean"},
+     "position": {"line": 14, "character": 14}},
+    "id": {"const": {"declName": "Direction.right"}}}},
+  {"sortText": "4",
+   "label": "toCtorIdx",
+   "kind": 3,
+   "data":
+   {"params":
+    {"textDocument": {"uri": "file:///completionFallback.lean"},
+     "position": {"line": 14, "character": 14}},
+    "id": {"const": {"declName": "Direction.toCtorIdx"}}}},
+  {"sortText": "5",
+   "label": "up",
+   "kind": 4,
+   "data":
+   {"params":
+    {"textDocument": {"uri": "file:///completionFallback.lean"},
+     "position": {"line": 14, "character": 14}},
+    "id": {"const": {"declName": "Direction.up"}}}}],
+ "isIncomplete": true}
+{"textDocument": {"uri": "file:///completionFallback.lean"},
+ "position": {"line": 28, "character": 30}}
+{"items":
+ [{"sortText": "0",
+   "label": "ha",
+   "kind": 5,
+   "data":
+   {"params":
+    {"textDocument": {"uri": "file:///completionFallback.lean"},
+     "position": {"line": 28, "character": 30}},
+    "id": {"const": {"declName": "CustomAnd.ha"}}}},
+  {"sortText": "1",
+   "label": "hb",
+   "kind": 5,
+   "data":
+   {"params":
+    {"textDocument": {"uri": "file:///completionFallback.lean"},
+     "position": {"line": 28, "character": 30}},
+    "id": {"const": {"declName": "CustomAnd.hb"}}}},
+  {"sortText": "2",
+   "label": "mk",
+   "kind": 4,
+   "data":
+   {"params":
+    {"textDocument": {"uri": "file:///completionFallback.lean"},
+     "position": {"line": 28, "character": 30}},
+    "id": {"const": {"declName": "CustomAnd.mk"}}}}],
+ "isIncomplete": true}