From 6072e6492b577f1c749d94346478bdd09bec7fb0 Mon Sep 17 00:00:00 2001
From: Xiangfei Ding <dingxiangfei2009@protonmail.ch>
Date: Fri, 9 Jan 2026 22:53:40 +0000
Subject: [PATCH] rustc_index: Convenient debugging view of the IndexMap

This small utility provides a familiar key-value view of an `IndexMap`
in the debugging output.

Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>

Signed-off-by: Xiangfei Ding <dingxiangfei2009@protonmail.ch>
---
 compiler/rustc_index/src/vec.rs | 40 ++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_index/src/vec.rs b/compiler/rustc_index/src/vec.rs
index 13f0dda180be9..97aad8e6e8c04 100644
--- a/compiler/rustc_index/src/vec.rs
+++ b/compiler/rustc_index/src/vec.rs
@@ -197,6 +197,11 @@ impl<I: Idx, T> IndexVec<I, T> {
     pub fn append(&mut self, other: &mut Self) {
         self.raw.append(&mut other.raw);
     }
+
+    #[inline]
+    pub fn debug_map_view(&self) -> IndexSliceMapView<'_, I, T> {
+        IndexSliceMapView(self.as_slice())
+    }
 }
 
 /// `IndexVec` is often used as a map, so it provides some map-like APIs.
@@ -220,11 +225,44 @@ impl<I: Idx, T> IndexVec<I, Option<T>> {
     pub fn contains(&self, index: I) -> bool {
         self.get(index).and_then(Option::as_ref).is_some()
     }
+
+    /// This debug view will skip printing `None` entries.
+    /// This is useful when the slice is actually like a map and `None` means
+    /// a value is absent under that key.
+    #[inline]
+    pub fn debug_map_view_compact(&self) -> IndexSliceMapViewCompact<'_, I, T> {
+        IndexSliceMapViewCompact(self.as_slice())
+    }
 }
 
+pub struct IndexSliceMapView<'a, I: Idx, T>(&'a IndexSlice<I, T>);
+pub struct IndexSliceMapViewCompact<'a, I: Idx, T>(&'a IndexSlice<I, Option<T>>);
+
 impl<I: Idx, T: fmt::Debug> fmt::Debug for IndexVec<I, T> {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&self.raw, fmt)
+        fmt::Debug::fmt(self.as_slice(), fmt)
+    }
+}
+
+impl<'a, I: Idx, T: fmt::Debug> fmt::Debug for IndexSliceMapView<'a, I, T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut entries = fmt.debug_map();
+        for (idx, val) in self.0.iter_enumerated() {
+            entries.entry(&idx, val);
+        }
+        entries.finish()
+    }
+}
+
+impl<'a, I: Idx, T: fmt::Debug> fmt::Debug for IndexSliceMapViewCompact<'a, I, T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut entries = fmt.debug_map();
+        for (idx, val) in self.0.iter_enumerated() {
+            if let Some(val) = val {
+                entries.entry(&idx, val);
+            }
+        }
+        entries.finish()
     }
 }