Auto merge of #111775 - compiler-errors:triple-check, r=Nilstrieb

Add extra debug assertions for equality for Adt/Variant/FieldDef

Would've made it easier to both catch and test https://github.com/rust-lang/rust/pull/111494. Maybe not worth it, since it does mean that the compiler is doing extra work when debug-assertions are enabled, but also that's what debug assertions are for :^)

This is a revival of #111523 because I think I pushed an empty branch and bors got a bit too excited it closed the PR.

2 files changed, 44 insertions, 5 deletions

diff --git a/compiler/rustc_middle/src/ty/adt.rs b/compiler/rustc_middle/src/ty/adt.rs
index ad9891a5dca..7c5c030c276 100644
--- a/compiler/rustc_middle/src/ty/adt.rs
+++ b/compiler/rustc_middle/src/ty/adt.rs
@@ -111,12 +111,30 @@ impl Ord for AdtDefData {
     }
 }
 
-/// There should be only one AdtDef for each `did`, therefore
-/// it is fine to implement `PartialEq` only based on `did`.
 impl PartialEq for AdtDefData {
     #[inline]
     fn eq(&self, other: &Self) -> bool {
-        self.did == other.did
+        // There should be only one `AdtDefData` for each `def_id`, therefore
+        // it is fine to implement `PartialEq` only based on `def_id`.
+        //
+        // Below, we exhaustively destructure `self` and `other` so that if the
+        // definition of `AdtDefData` changes, a compile-error will be produced,
+        // reminding us to revisit this assumption.
+
+        let Self { did: self_def_id, variants: _, flags: _, repr: _ } = self;
+        let Self { did: other_def_id, variants: _, flags: _, repr: _ } = other;
+
+        let res = self_def_id == other_def_id;
+
+        // Double check that implicit assumption detailed above.
+        if cfg!(debug_assertions) && res {
+            let deep = self.flags == other.flags
+                && self.repr == other.repr
+                && self.variants == other.variants;
+            assert!(deep, "AdtDefData for the same def-id has differing data");
+        }
+
+        res
     }
 }
 
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index be0d1e61a46..ecb191676c2 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -1882,7 +1882,20 @@ impl PartialEq for VariantDef {
 
         let Self { def_id: lhs_def_id, ctor: _, name: _, discr: _, fields: _, flags: _ } = &self;
         let Self { def_id: rhs_def_id, ctor: _, name: _, discr: _, fields: _, flags: _ } = other;
-        lhs_def_id == rhs_def_id
+
+        let res = lhs_def_id == rhs_def_id;
+
+        // Double check that implicit assumption detailed above.
+        if cfg!(debug_assertions) && res {
+            let deep = self.ctor == other.ctor
+                && self.name == other.name
+                && self.discr == other.discr
+                && self.fields == other.fields
+                && self.flags == other.flags;
+            assert!(deep, "VariantDef for the same def-id has differing data");
+        }
+
+        res
     }
 }
 
@@ -1937,7 +1950,15 @@ impl PartialEq for FieldDef {
 
         let Self { did: rhs_did, name: _, vis: _ } = other;
 
-        lhs_did == rhs_did
+        let res = lhs_did == rhs_did;
+
+        // Double check that implicit assumption detailed above.
+        if cfg!(debug_assertions) && res {
+            let deep = self.name == other.name && self.vis == other.vis;
+            assert!(deep, "FieldDef for the same def-id has differing data");
+        }
+
+        res
     }
 }