From 8a5bb9d5acb664448e2f8410662b59e078c6f0f0 Mon Sep 17 00:00:00 2001
From: Lukas Wirth <lukastw97@gmail.com>
Date: Fri, 1 Mar 2024 15:09:12 +0100
Subject: [PATCH] Only the ROOT syntax context has `None` `outer_expn`

---
 crates/hir-expand/src/hygiene.rs  | 30 ++++++++++++++++++------------
 crates/hir-expand/src/mod_path.rs |  2 +-
 crates/span/src/hygiene.rs        |  1 +
 3 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/crates/hir-expand/src/hygiene.rs b/crates/hir-expand/src/hygiene.rs
index 7f0174fa5a9..ac2bab280d5 100644
--- a/crates/hir-expand/src/hygiene.rs
+++ b/crates/hir-expand/src/hygiene.rs
@@ -62,7 +62,7 @@ pub(super) fn apply_mark(
     transparency: Transparency,
 ) -> SyntaxContextId {
     if transparency == Transparency::Opaque {
-        return apply_mark_internal(db, ctxt, Some(call_id), transparency);
+        return apply_mark_internal(db, ctxt, call_id, transparency);
     }
 
     let call_site_ctxt = db.lookup_intern_macro_call(call_id).call_site.ctx;
@@ -73,7 +73,7 @@ pub(super) fn apply_mark(
     };
 
     if call_site_ctxt.is_root() {
-        return apply_mark_internal(db, ctxt, Some(call_id), transparency);
+        return apply_mark_internal(db, ctxt, call_id, transparency);
     }
 
     // Otherwise, `expn_id` is a macros 1.0 definition and the call site is in a
@@ -88,17 +88,19 @@ pub(super) fn apply_mark(
     for (call_id, transparency) in ctxt.marks(db) {
         call_site_ctxt = apply_mark_internal(db, call_site_ctxt, call_id, transparency);
     }
-    apply_mark_internal(db, call_site_ctxt, Some(call_id), transparency)
+    apply_mark_internal(db, call_site_ctxt, call_id, transparency)
 }
 
 fn apply_mark_internal(
     db: &dyn ExpandDatabase,
     ctxt: SyntaxContextId,
-    call_id: Option<MacroCallId>,
+    call_id: MacroCallId,
     transparency: Transparency,
 ) -> SyntaxContextId {
     use base_db::salsa;
 
+    let call_id = Some(call_id);
+
     let syntax_context_data = db.lookup_intern_syntax_context(ctxt);
     let mut opaque = syntax_context_data.opaque;
     let mut opaque_and_semitransparent = syntax_context_data.opaque_and_semitransparent;
@@ -148,7 +150,7 @@ pub trait SyntaxContextExt {
     fn parent_ctxt(self, db: &dyn ExpandDatabase) -> Self;
     fn remove_mark(&mut self, db: &dyn ExpandDatabase) -> (Option<MacroCallId>, Transparency);
     fn outer_mark(self, db: &dyn ExpandDatabase) -> (Option<MacroCallId>, Transparency);
-    fn marks(self, db: &dyn ExpandDatabase) -> Vec<(Option<MacroCallId>, Transparency)>;
+    fn marks(self, db: &dyn ExpandDatabase) -> Vec<(MacroCallId, Transparency)>;
 }
 
 impl SyntaxContextExt for SyntaxContextId {
@@ -170,7 +172,7 @@ fn remove_mark(&mut self, db: &dyn ExpandDatabase) -> (Option<MacroCallId>, Tran
         *self = data.parent;
         (data.outer_expn, data.outer_transparency)
     }
-    fn marks(self, db: &dyn ExpandDatabase) -> Vec<(Option<MacroCallId>, Transparency)> {
+    fn marks(self, db: &dyn ExpandDatabase) -> Vec<(MacroCallId, Transparency)> {
         let mut marks = marks_rev(self, db).collect::<Vec<_>>();
         marks.reverse();
         marks
@@ -181,11 +183,15 @@ fn marks(self, db: &dyn ExpandDatabase) -> Vec<(Option<MacroCallId>, Transparenc
 pub fn marks_rev(
     ctxt: SyntaxContextId,
     db: &dyn ExpandDatabase,
-) -> impl Iterator<Item = (Option<MacroCallId>, Transparency)> + '_ {
-    iter::successors(Some(ctxt), move |&mark| {
-        Some(mark.parent_ctxt(db)).filter(|&it| it != SyntaxContextId::ROOT)
-    })
-    .map(|ctx| ctx.outer_mark(db))
+) -> impl Iterator<Item = (MacroCallId, Transparency)> + '_ {
+    iter::successors(Some(ctxt), move |&mark| Some(mark.parent_ctxt(db)))
+        .take_while(|&it| !it.is_root())
+        .map(|ctx| {
+            let mark = ctx.outer_mark(db);
+            // We stop before taking the root expansion, as such we cannot encounter a `None` outer
+            // expansion, as only the ROOT has it.
+            (mark.0.unwrap(), mark.1)
+        })
 }
 
 pub(crate) fn dump_syntax_contexts(db: &dyn ExpandDatabase) -> String {
@@ -224,7 +230,7 @@ fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
             }
         }
 
-        pub fn fancy_debug(
+        fn fancy_debug(
             this: &SyntaxContextData,
             self_id: SyntaxContextId,
             db: &dyn ExpandDatabase,
diff --git a/crates/hir-expand/src/mod_path.rs b/crates/hir-expand/src/mod_path.rs
index 136b0935be2..0cf1fadec97 100644
--- a/crates/hir-expand/src/mod_path.rs
+++ b/crates/hir-expand/src/mod_path.rs
@@ -358,7 +358,7 @@ pub fn resolve_crate_root(db: &dyn ExpandDatabase, mut ctxt: SyntaxContextId) ->
         result_mark = Some(mark);
     }
 
-    result_mark.flatten().map(|call| db.lookup_intern_macro_call(call).def.krate)
+    result_mark.map(|call| db.lookup_intern_macro_call(call).def.krate)
 }
 
 pub use crate::name as __name;
diff --git a/crates/span/src/hygiene.rs b/crates/span/src/hygiene.rs
index 18da689922b..4f6d792201b 100644
--- a/crates/span/src/hygiene.rs
+++ b/crates/span/src/hygiene.rs
@@ -68,6 +68,7 @@ pub fn from_u32(u32: u32) -> Self {
 /// A syntax context describes a hierarchy tracking order of macro definitions.
 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
 pub struct SyntaxContextData {
+    /// Invariant: Only [`SyntaxContextId::ROOT`] has a [`None`] outer expansion.
     pub outer_expn: Option<MacroCallId>,
     pub outer_transparency: Transparency,
     pub parent: SyntaxContextId,