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rust-bors merged 1 commit into from
Sep 5, 2026
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A more readable debug map for IndexMaps #159792

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40 changes: 39 additions & 1 deletion compiler/rustc_index/src/vec.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand All @@ -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)
}
}

@cjgillot cjgillot Jul 26, 2026

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Should we just make this the impl for IndexVec and IndexSlice?

@dingxiangfei2009 dingxiangfei2009 Sep 3, 2026

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Sorry, life happened and it took me a while to come back to this.

Yes, we could potentially. I figured that I frequently need to search the value under one particular index in a slice and it has not been possible.

The printout would be a bit verbose, however. Let's see how the dev ergonomics would be.

@dingxiangfei2009 dingxiangfei2009 Sep 4, 2026

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Alright, since we do not have min_specialization in this crate yet, I will print the keys even when the index type is u32. The debug print seems to be more readable now and I like how layout printout looks like given that indices are clearly labelled.

@dingxiangfei2009 dingxiangfei2009 Sep 4, 2026 •
edited
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Ah, probably this is still not a good idea.

It turns out, out of convenience, MIR pretty-print uses debug print of IndexVec and IndexSlice. My smaller local tests did not catch this. With this MIR dump got noisier than I felt comfortable.

I would call this impl switch a deferred upgrade then. We probably would fix the MIR printing in another PR, I can imagine.


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()
}
}

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