Auto merge of #113218 - lqd:polonius-scopes, r=jackh726

Compute NLL loan scopes using the polonius model

For a *location-insensitive* analysis (that is, without expressiveness improvements for users yet), this PR implements loans going out of scope using reachability and liveness, rather than checking if the issuing region's values contain a given CFG point. This is equivalent to NLL scopes and computes the same data.

r? `@matthewjasper`

A couple of notes:
- there are some assumptions about SCC representatives, placeholders, free regions, and member constraints that I believe hold, and they're documented in the code
- this passes all the UI tests with `-Zpolonius=next` -- the perf is [not terrible](https://github.com/rust-lang/rust/pull/112432#issuecomment-1749685862) and there are a bunch of ways to improve it in the future.
- there's a fixme left, hopefully Matthew you know a clean way to get the information it mentions.

compiler/rustc_session/src/config.rs

@@ -3172,6 +3172,7 @@ impl PpMode {
 /// we have an opt-in scheme here, so one is hopefully forced to think about
 /// how the hash should be calculated when adding a new command-line argument.
 pub(crate) mod dep_tracking {
+    use super::Polonius;
     use super::{
         BranchProtection, CFGuard, CFProtection, CrateType, DebugInfo, DebugInfoCompression,
         ErrorOutputType, InstrumentCoverage, InstrumentXRay, LinkerPluginLto, LocationDetail,
@@ -3281,6 +3282,7 @@ pub(crate) mod dep_tracking {
         OomStrategy,
         LanguageIdentifier,
         TraitSolver,
+        Polonius,
     );
 
     impl<T1, T2> DepTrackingHash for (T1, T2)
@@ -3419,3 +3421,35 @@ impl DumpMonoStatsFormat {
         }
     }
 }
+
+/// `-Zpolonius` values, enabling the borrow checker polonius analysis, and which version: legacy,
+/// or future prototype.
+#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+pub enum Polonius {
+    /// The default value: disabled.
+    Off,
+
+    /// Legacy version, using datalog and the `polonius-engine` crate. Historical value for `-Zpolonius`.
+    Legacy,
+
+    /// In-tree prototype, extending the NLL infrastructure.
+    Next,
+}
+
+impl Default for Polonius {
+    fn default() -> Self {
+        Polonius::Off
+    }
+}
+
+impl Polonius {
+    /// Returns whether the legacy version of polonius is enabled
+    pub fn is_legacy_enabled(&self) -> bool {
+        matches!(self, Polonius::Legacy)
+    }
+
+    /// Returns whether the "next" version of polonius is enabled
+    pub fn is_next_enabled(&self) -> bool {
+        matches!(self, Polonius::Next)
+    }
+}