From 117418fd10b806c989a5fee3053fc8b097387e2c Mon Sep 17 00:00:00 2001
From: Neven Villani <vanille@crans.org>
Date: Mon, 1 May 2023 14:54:56 +0200
Subject: [PATCH] Payoff: much simpler event selection

Error messages unchanged
---
 .../tree_borrows/diagnostics.rs               | 57 +++++--------------
 1 file changed, 15 insertions(+), 42 deletions(-)

diff --git a/src/borrow_tracker/tree_borrows/diagnostics.rs b/src/borrow_tracker/tree_borrows/diagnostics.rs
index 72b23884bb..0aba9aeedf 100644
--- a/src/borrow_tracker/tree_borrows/diagnostics.rs
+++ b/src/borrow_tracker/tree_borrows/diagnostics.rs
@@ -196,44 +196,19 @@ impl History {
         History { events: Vec::new(), created: self.created, tag: self.tag }
     }
 
-    /// Reconstruct the history relevant to `error_offset` knowing that
-    /// its permission followed `complete_transition`.
-    ///
-    /// Here's how we do this:
-    /// - we know `full := complete_transition` the transition of the permission from
-    /// its initialization to the state just before the error was caused,
-    /// we want to find a chain of events that produces `full`
-    /// - we decompose `full` into `pre o post` where
-    /// `pre` is the best applicable transition from recorded events
-    /// - we select the event that caused `pre` and iterate
-    /// to find the chain of events that produces `full := post`
-    ///
-    /// To find the "best applicable transition" for full:
-    /// - eliminate events that cannot be applied because their offset is too big
-    /// - eliminate events that cannot be applied because their starting point is wrong
-    /// - select the one that happened closest to the range of interest
-    fn extract_relevant(&self, complete_transition: PermTransition, error_offset: Size) -> Self {
-        let mut selected_events: Vec<Event> = Vec::new();
-        let mut full = complete_transition;
-        while !full.is_noop() {
-            let (pre, post) = self
+    /// Reconstruct the history relevant to `error_offset` by filtering
+    /// only events whose range contains the offset we are interested in.
+    fn extract_relevant(&self, error_offset: u64) -> Self {
+        History {
+            events: self
                 .events
                 .iter()
-                .filter(|e| e.offset <= error_offset)
-                .filter_map(|pre_canditate| {
-                    full.apply_start(pre_canditate.transition)
-                        .map(|post_canditate| (pre_canditate, post_canditate))
-                })
-                .max_by_key(|(pre_canditate, _post_candidate)| pre_canditate.offset)
-                .unwrap();
-            // If this occurs we will loop infinitely !
-            // Make sure to only put non-noop transitions in `History`.
-            assert!(!pre.transition.is_noop());
-            full = post;
-            selected_events.push(pre.clone());
+                .filter(|e| e.offset.start <= error_offset && error_offset < e.offset.end)
+                .cloned()
+                .collect::<Vec<_>>(),
+            created: self.created,
+            tag: self.tag,
         }
-
-        History { events: selected_events, created: self.created, tag: self.tag }
     }
 }
 
@@ -260,12 +235,11 @@ impl TbError<'_> {
     /// Produce a UB error.
     pub fn build<'tcx>(self) -> InterpError<'tcx> {
         use TransitionError::*;
-        let started_as = self.conflicting_info.history.created.1;
         let kind = self.access_kind;
         let accessed = self.accessed_info;
         let conflicting = self.conflicting_info;
         let accessed_is_conflicting = accessed.tag == conflicting.tag;
-        let (pre_error, title, details, conflicting_tag_name) = match self.error_kind {
+        let (title, details, conflicting_tag_name) = match self.error_kind {
             ChildAccessForbidden(perm) => {
                 let conflicting_tag_name =
                     if accessed_is_conflicting { "accessed" } else { "conflicting" };
@@ -279,7 +253,7 @@ impl TbError<'_> {
                 details.push(format!(
                     "the {conflicting_tag_name} tag {conflicting} has state {perm} which forbids child {kind}es"
                 ));
-                (perm, title, details, conflicting_tag_name)
+                (title, details, conflicting_tag_name)
             }
             ProtectedTransition(transition) => {
                 let conflicting_tag_name = "protected";
@@ -296,7 +270,7 @@ impl TbError<'_> {
                         loss = transition.summary(),
                     ),
                 ];
-                (transition.started(), title, details, conflicting_tag_name)
+                (title, details, conflicting_tag_name)
             }
             ProtectedDealloc => {
                 let conflicting_tag_name = "strongly protected";
@@ -307,16 +281,15 @@ impl TbError<'_> {
                     ),
                     format!("the {conflicting_tag_name} tag {conflicting} disallows deallocations"),
                 ];
-                (started_as, title, details, conflicting_tag_name)
+                (title, details, conflicting_tag_name)
             }
         };
-        let pre_transition = PermTransition::from(started_as, pre_error).unwrap();
         let mut history = HistoryData::default();
         if !accessed_is_conflicting {
             history.extend(self.accessed_info.history.forget(), "accessed", false);
         }
         history.extend(
-            self.conflicting_info.history.extract_relevant(pre_transition, self.error_offset),
+            self.conflicting_info.history.extract_relevant(self.error_offset),
             conflicting_tag_name,
             true,
         );