fix: _root_ prefix in declarations

closes #490

src/Lean/Elab/DeclModifiers.lean

@@ -160,10 +160,20 @@ def checkIfShadowingStructureField (declName : Name) : m Unit := do
   | _ => pure ()
 
 def mkDeclName (currNamespace : Name) (modifiers : Modifiers) (shortName : Name) : m (Name × Name) := do
-  let name := (extractMacroScopes shortName).name
-  unless name.isAtomic || isFreshInstanceName name do
+  let mut shortName := shortName
+  let mut currNamespace := currNamespace
+  let view := extractMacroScopes shortName
+  let name := view.name
+  let isRootName := (`_root_).isPrefixOf name
+  if name == `_root_ then
+    throwError "invalid declaration name `_root_`, `_root_` is a prefix used to refer to the 'root' namespace"
+  unless name.isAtomic || isFreshInstanceName name || isRootName do
     throwError "atomic identifier expected '{shortName}'"
-  let declName := currNamespace ++ shortName
+  let declName := if isRootName then { view with name := name.replacePrefix `_root_ Name.anonymous }.review else currNamespace ++ shortName
+  if isRootName then
+    let .str p s _ := name | throwError "invalid declaration name '{name}'"
+    shortName := Name.mkSimple s
+    currNamespace := p.replacePrefix `_root_ Name.anonymous
   checkIfShadowingStructureField declName
   let declName ← applyVisibility modifiers.visibility declName
   match modifiers.visibility with

src/Lean/Elab/Declaration.lean

@@ -23,9 +23,12 @@ private def ensureValidNamespace (name : Name) : MacroM Unit := do
   | Name.num _ .. => Macro.throwError s!"invalid namespace '{name}', it must not contain numeric parts"
   | Name.anonymous => return ()
 
-/- Auxiliary function for `expandDeclNamespace?` -/
+/-- Auxiliary function for `expandDeclNamespace?` -/
 private def expandDeclIdNamespace? (declId : Syntax) : MacroM (Option (Name × Syntax)) := do
   let (id, _) := expandDeclIdCore declId
+  if (`_root_).isPrefixOf id then
+    ensureValidNamespace (id.replacePrefix `_root_ Name.anonymous)
+    return none
   let scpView := extractMacroScopes id
   match scpView.name with
   | Name.str Name.anonymous _ _ => return none
@@ -41,7 +44,10 @@ private def expandDeclIdNamespace? (declId : Syntax) : MacroM (Option (Name × S
       return some (pre, declId.setArg 0 id)
   | _ => return none
 
-/- given declarations such as `@[...] def Foo.Bla.f ...` return `some (Foo.Bla, @[...] def f ...)` -/
+/--
+  Given declarations such as `@[...] def Foo.Bla.f ...` return `some (Foo.Bla, @[...] def f ...)`
+  Remark: if the id starts with `_root_`, we return `none`.
+-/
 private def expandDeclNamespace? (stx : Syntax) : MacroM (Option (Name × Syntax)) := do
   if !stx.isOfKind `Lean.Parser.Command.declaration then
     return none

tests/lean/490.lean

@@ -1,3 +1,3 @@
 namespace Foo
-def _root_.bar := 1 -- Error
+def _root_.bar := 1 -- ok
 end Foo

tests/lean/490.lean.expected.out

@@ -1 +0,0 @@
-490.lean:2:0-2:19: error: invalid namespace '_root_', '_root_' is a reserved namespace

tests/lean/root.lean

@@ -0,0 +1,22 @@
+-*- mode: compilation; default-directory: "~/projects/lean4/build/release/" -*-
+Compilation started at Mon Jun 13 13:56:14
+
+/home/leodemoura/.elan/bin/lean  /home/leodemoura/projects/lean4/build/release/root.lean
+Bla.f : Nat → Nat
+/home/leodemoura/projects/lean4/build/release/root.lean:21:14: error: protected declarations must be in a namespace
+/home/leodemoura/projects/lean4/build/release/root.lean:29:4: error: invalid declaration name `_root_`, `_root_` is a prefix used to refer to the 'root' namespace
+/home/leodemoura/projects/lean4/build/release/root.lean:31:0: error: invalid namespace '_root_', '_root_' is a reserved namespace
+/home/leodemoura/projects/lean4/build/release/root.lean:33:0: error: invalid namespace 'f._root_', '_root_' is a reserved namespace
+/home/leodemoura/projects/lean4/build/release/root.lean:35:14: error: protected declarations must be in a namespace
+/home/leodemoura/projects/lean4/build/release/root.lean:41:7: error: unknown identifier 'h'
+/home/leodemoura/projects/lean4/build/release/root.lean:43:7: error: unknown identifier 'f'
+f : Nat → Nat
+_private.root.0.prv : Nat -> Nat
+/home/leodemoura/projects/lean4/build/release/root.lean:90:48: error: unsolved goals
+x : Nat
+⊢ isEven (x + 1 + 1) = isEven x
+/home/leodemoura/projects/lean4/build/release/root.lean:95:48: error: unsolved goals
+x : Nat
+⊢ isEven (x + 1 + 1) = isEven x
+
+Compilation exited abnormally with code 1 at Mon Jun 13 13:56:14

tests/lean/root.lean.expected.out

@@ -0,0 +1,11 @@
+root.lean:1:0: error: expected command
+root.lean:6:80: error: expected 'abbrev', 'axiom', 'builtin_initialize', 'class', 'constant', 'def', 'example', 'inductive', 'initialize', 'instance', 'structure' or 'theorem'
+root.lean:7:0: error: expected identifier
+root.lean:7:125: error: expected ':'
+root.lean:8:0: error: expected identifier
+root.lean:8:90: error: missing end of character literal
+root.lean:9:0: error: expected identifier
+root.lean:9:90: error: missing end of character literal
+root.lean:10:0: error: expected identifier
+root.lean:10:80: error: expected 'abbrev', 'axiom', 'builtin_initialize', 'class', 'constant', 'def', 'example', 'inductive', 'initialize', 'instance', 'structure' or 'theorem'
+root.lean:11:0: error: expected identifier