hygiene: Make sure each Mark has an associated expansion info

The root expansion was missing one.
Expansions created for "derive containers" (see one of the next commits for the description) also didn't get expansion info.

src/librustc/ich/impls_syntax.rs

@@ -408,6 +408,7 @@ impl_stable_hash_for!(struct ::syntax_pos::hygiene::ExpnInfo {
 });
 
 impl_stable_hash_for!(enum ::syntax_pos::hygiene::ExpnKind {
+    Root,
     Macro(kind, descr),
     Desugaring(kind)
 });

src/librustc/lint/mod.rs

@@ -885,7 +885,7 @@ pub fn in_external_macro(sess: &Session, span: Span) -> bool {
     };
 
     match info.kind {
-        ExpnKind::Desugaring(DesugaringKind::ForLoop) => false,
+        ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop) => false,
         ExpnKind::Desugaring(_) => true, // well, it's "external"
         ExpnKind::Macro(MacroKind::Bang, _) => {
             if info.def_site.is_dummy() {

src/libsyntax/ext/expand.rs

@@ -5,7 +5,7 @@ use crate::source_map::{dummy_spanned, respan};
 use crate::config::StripUnconfigured;
 use crate::ext::base::*;
 use crate::ext::derive::{add_derived_markers, collect_derives};
-use crate::ext::hygiene::{Mark, SyntaxContext};
+use crate::ext::hygiene::{Mark, SyntaxContext, ExpnInfo, ExpnKind};
 use crate::ext::placeholders::{placeholder, PlaceholderExpander};
 use crate::feature_gate::{self, Features, GateIssue, is_builtin_attr, emit_feature_err};
 use crate::mut_visit::*;
@@ -847,7 +847,17 @@ struct InvocationCollector<'a, 'b> {
 
 impl<'a, 'b> InvocationCollector<'a, 'b> {
     fn collect(&mut self, fragment_kind: AstFragmentKind, kind: InvocationKind) -> AstFragment {
-        let mark = Mark::fresh(self.cx.current_expansion.mark, None);
+        // Expansion info for all the collected invocations is set upon their resolution,
+        // with exception of the "derive container" case which is not resolved and can get
+        // its expansion info immediately.
+        let expn_info = match &kind {
+            InvocationKind::Attr { attr: None, item, .. } => Some(ExpnInfo::default(
+                ExpnKind::Macro(MacroKind::Attr, sym::derive),
+                item.span(), self.cx.parse_sess.edition,
+            )),
+            _ => None,
+        };
+        let mark = Mark::fresh(self.cx.current_expansion.mark, expn_info);
         self.invocations.push(Invocation {
             kind,
             fragment_kind,

src/libsyntax_pos/hygiene.rs

@@ -59,6 +59,9 @@ pub struct Mark(u32);
 #[derive(Debug)]
 struct MarkData {
     parent: Mark,
+    /// Each mark should have an associated expansion info, but sometimes there's a delay between
+    /// creation of a mark and obtaining its info (e.g. macros are collected first and then
+    /// resolved later), so we use an `Option` here.
     expn_info: Option<ExpnInfo>,
 }
 
@@ -155,11 +158,11 @@ crate struct HygieneData {
 }
 
 impl HygieneData {
-    crate fn new() -> Self {
+    crate fn new(edition: Edition) -> Self {
         HygieneData {
             marks: vec![MarkData {
                 parent: Mark::root(),
-                expn_info: None,
+                expn_info: Some(ExpnInfo::default(ExpnKind::Root, DUMMY_SP, edition)),
             }],
             syntax_contexts: vec![SyntaxContextData {
                 outer_mark: Mark::root(),
@@ -183,7 +186,15 @@ impl HygieneData {
     }
 
     fn expn_info(&self, mark: Mark) -> Option<&ExpnInfo> {
-        self.marks[mark.0 as usize].expn_info.as_ref()
+        if mark != Mark::root() {
+            Some(self.marks[mark.0 as usize].expn_info.as_ref()
+                     .expect("no expansion info for a mark"))
+        } else {
+            // FIXME: Some code relies on `expn_info().is_none()` meaning "no expansion".
+            // Introduce a method for checking for "no expansion" instead and always return
+            // `ExpnInfo` from this function instead of the `Option`.
+            None
+        }
     }
 
     fn is_descendant_of(&self, mut mark: Mark, ancestor: Mark) -> bool {
@@ -670,6 +681,8 @@ impl ExpnInfo {
 /// Expansion kind.
 #[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
 pub enum ExpnKind {
+    /// No expansion, aka root expansion. Only `Mark::root()` has this kind.
+    Root,
     /// Expansion produced by a macro.
     /// FIXME: Some code injected by the compiler before HIR lowering also gets this kind.
     Macro(MacroKind, Symbol),
@@ -680,6 +693,7 @@ pub enum ExpnKind {
 impl ExpnKind {
     pub fn descr(&self) -> Symbol {
         match *self {
+            ExpnKind::Root => kw::PathRoot,
             ExpnKind::Macro(_, descr) => descr,
             ExpnKind::Desugaring(kind) => Symbol::intern(kind.descr()),
         }

src/libsyntax_pos/lib.rs

@@ -60,7 +60,7 @@ impl Globals {
         Globals {
             symbol_interner: Lock::new(symbol::Interner::fresh()),
             span_interner: Lock::new(span_encoding::SpanInterner::default()),
-            hygiene_data: Lock::new(hygiene::HygieneData::new()),
+            hygiene_data: Lock::new(hygiene::HygieneData::new(edition)),
             edition,
         }
     }
@@ -442,6 +442,7 @@ impl Span {
             // Don't print recursive invocations.
             if !info.call_site.source_equal(&prev_span) {
                 let (pre, post) = match info.kind {
+                    ExpnKind::Root => break,
                     ExpnKind::Desugaring(..) => ("desugaring of ", ""),
                     ExpnKind::Macro(macro_kind, _) => match macro_kind {
                         MacroKind::Bang => ("", "!"),