fix: hovers on binders with metavariables (#4192)

this fixes #4078. It is an alternative fix to the one in #4137,
suggested
by @kmill.

Incidentially, it makes the unused variable linter better. My theory is
that
if we don’t reset the info when backtracking, the binder shows up more
than
once in the info tree, and then it is considered “used”, although there
are
just multiple binders.

src/Lean/Elab/Term.lean

@@ -1531,7 +1531,7 @@ partial def withAutoBoundImplicit (k : TermElabM α) : TermElabM α := do
           | ex => match isAutoBoundImplicitLocalException? ex with
             | some n =>
               -- Restore state, declare `n`, and try again
-              s.restore
+              s.restore (restoreInfo := true)
               withLocalDecl n .implicit (← mkFreshTypeMVar) fun x =>
                 withReader (fun ctx => { ctx with autoBoundImplicits := ctx.autoBoundImplicits.push x } ) do
                   loop (← saveState)

tests/lean/1018unknowMVarIssue.lean.expected.out

@@ -9,10 +9,6 @@ x : Fam2 α✝ β
 a : α
 ⊢ α
 [Elab.info] • command @ ⟨7, 0⟩-⟨10, 19⟩ @ Lean.Elab.Command.elabDeclaration
-      • α : Type @ ⟨7, 13⟩-⟨7, 14⟩ @ Lean.Elab.Term.elabIdent
-        • [.] α : some Sort.{?_uniq} @ ⟨7, 13⟩-⟨7, 14⟩
-        • α : Type @ ⟨7, 13⟩-⟨7, 14⟩
-      • a (isBinder := true) : α @ ⟨7, 9⟩-⟨7, 10⟩
       • α : Type @ ⟨7, 13⟩-⟨7, 14⟩ @ Lean.Elab.Term.elabIdent
         • [.] α : some Sort.{?_uniq} @ ⟨7, 13⟩-⟨7, 14⟩
         • α : Type @ ⟨7, 13⟩-⟨7, 14⟩

tests/lean/interactive/4078.lean

@@ -0,0 +1,17 @@
+def foo1 {β} (i : ∀ α, List α) : List β := i β
+                  --^ textDocument/hover
+
+def foo2 (i : ∀ α, List α) : List β := i β
+              --^ textDocument/hover
+
+def foo3 (i : (α : _) → List α) : List β := i β
+              --^ textDocument/hover
+
+def foo4 (i : (α : id _) → List α) : List β := i β
+              --^ textDocument/hover
+
+def foo5 (i : (α : Type _) → List α) : List β := i β
+              --^ textDocument/hover
+
+def foo6 (i : (α : Type 0) → List α) : List β := i β
+              --^ textDocument/hover

tests/lean/interactive/4078.lean.expected.out

@@ -0,0 +1,42 @@
+{"textDocument": {"uri": "file:///4078.lean"},
+ "position": {"line": 0, "character": 20}}
+{"range":
+ {"start": {"line": 0, "character": 20}, "end": {"line": 0, "character": 21}},
+ "contents": {"value": "```lean\nα : Type u_1\n```", "kind": "markdown"}}
+{"textDocument": {"uri": "file:///4078.lean"},
+ "position": {"line": 3, "character": 16}}
+{"range":
+ {"start": {"line": 3, "character": 16}, "end": {"line": 3, "character": 17}},
+ "contents": {"value": "```lean\nα : Type u_1\n```", "kind": "markdown"}}
+{"textDocument": {"uri": "file:///4078.lean"},
+ "position": {"line": 6, "character": 16}}
+{"range":
+ {"start": {"line": 6, "character": 14}, "end": {"line": 6, "character": 30}},
+ "contents":
+ {"value":
+  "```lean\nType (u_1 + 1)\n```\n***\nThe dependent arrow. `(x : α) → β` is equivalent to `∀ x : α, β`, but we usually\nreserve the latter for propositions. Also written as `Π x : α, β` (the \"Pi-type\")\nin the literature. ",
+  "kind": "markdown"}}
+{"textDocument": {"uri": "file:///4078.lean"},
+ "position": {"line": 9, "character": 16}}
+{"range":
+ {"start": {"line": 9, "character": 14}, "end": {"line": 9, "character": 33}},
+ "contents":
+ {"value":
+  "```lean\nType (u_1 + 1)\n```\n***\nThe dependent arrow. `(x : α) → β` is equivalent to `∀ x : α, β`, but we usually\nreserve the latter for propositions. Also written as `Π x : α, β` (the \"Pi-type\")\nin the literature. ",
+  "kind": "markdown"}}
+{"textDocument": {"uri": "file:///4078.lean"},
+ "position": {"line": 12, "character": 16}}
+{"range":
+ {"start": {"line": 12, "character": 14}, "end": {"line": 12, "character": 35}},
+ "contents":
+ {"value":
+  "```lean\nType (u_1 + 1)\n```\n***\nThe dependent arrow. `(x : α) → β` is equivalent to `∀ x : α, β`, but we usually\nreserve the latter for propositions. Also written as `Π x : α, β` (the \"Pi-type\")\nin the literature. ",
+  "kind": "markdown"}}
+{"textDocument": {"uri": "file:///4078.lean"},
+ "position": {"line": 15, "character": 16}}
+{"range":
+ {"start": {"line": 15, "character": 14}, "end": {"line": 15, "character": 35}},
+ "contents":
+ {"value":
+  "```lean\nType 1\n```\n***\nThe dependent arrow. `(x : α) → β` is equivalent to `∀ x : α, β`, but we usually\nreserve the latter for propositions. Also written as `Π x : α, β` (the \"Pi-type\")\nin the literature. ",
+  "kind": "markdown"}}