introduce Polonius enum for -Zpolonius

this allows to opt into using the legacy version or the in-tree
prototype

compiler/rustc_session/src/config.rs

@@ -3166,6 +3166,7 @@ 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, LdImpl, LinkerPluginLto,
@@ -3276,6 +3277,7 @@ pub(crate) mod dep_tracking {
         OomStrategy,
         LanguageIdentifier,
         TraitSolver,
+        Polonius,
     );
 
     impl<T1, T2> DepTrackingHash for (T1, T2)
@@ -3414,3 +3416,30 @@ 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 experimentation
+    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)
+    }
+}

compiler/rustc_session/src/options.rs

@@ -415,6 +415,7 @@ mod desc {
     pub const parse_gcc_ld: &str = "one of: no value, `lld`";
     pub const parse_link_self_contained: &str = "one of: `y`, `yes`, `on`, `n`, `no`, `off`, or a list of enabled (`+` prefix) and disabled (`-` prefix) \
         components: `crto`, `libc`, `unwind`, `linker`, `sanitizers`, `mingw`";
+    pub const parse_polonius: &str = "either no value or `legacy` (the default), or `next`";
     pub const parse_stack_protector: &str =
         "one of (`none` (default), `basic`, `strong`, or `all`)";
     pub const parse_branch_protection: &str =
@@ -472,6 +473,21 @@ mod parse {
         }
     }
 
+    /// Parses whether polonius is enabled, and if so, which version.
+    pub(crate) fn parse_polonius(slot: &mut Polonius, v: Option<&str>) -> bool {
+        match v {
+            Some("legacy") | None => {
+                *slot = Polonius::Legacy;
+                true
+            }
+            Some("next") => {
+                *slot = Polonius::Next;
+                true
+            }
+            _ => false,
+        }
+    }
+
     /// Use this for any string option that has a static default.
     pub(crate) fn parse_string(slot: &mut String, v: Option<&str>) -> bool {
         match v {
@@ -1669,7 +1685,7 @@ options! {
         "whether to use the PLT when calling into shared libraries;
         only has effect for PIC code on systems with ELF binaries
         (default: PLT is disabled if full relro is enabled on x86_64)"),
-    polonius: bool = (false, parse_bool, [TRACKED],
+    polonius: Polonius = (Polonius::default(), parse_polonius, [TRACKED],
         "enable polonius-based borrow-checker (default: no)"),
     polymorphize: bool = (false, parse_bool, [TRACKED],
           "perform polymorphization analysis"),