[ty] Eagerly simplify 'True' and 'False' constraints (#18998)

## Summary

Simplifies literal `True` and `False` conditions to `ALWAYS_TRUE` /
`ALWAYS_FALSE` during semantic index building. This allows us to eagerly
evaluate more constraints, which should help with performance (looks
like there is a tiny 1% improvement in instrumented benchmarks), but
also allows us to eliminate definitely-unreachable branches in
control-flow merging. This can lead to better type inference in some
cases because it allows us to retain narrowing constraints without
solving https://github.com/astral-sh/ty/issues/690 first:
```py
def _(c: int | None):
    if c is None:
        assert False
    
    reveal_type(c)  # int, previously: int | None
```

closes https://github.com/astral-sh/ty/issues/713

## Test Plan

* Regression test for https://github.com/astral-sh/ty/issues/713
* Made sure that all ecosystem diffs trace back to removed false
positives

crates/ty_python_semantic/src/semantic_index/builder.rs

@@ -35,8 +35,8 @@ use crate::semantic_index::place::{
     PlaceExprWithFlags, PlaceTableBuilder, Scope, ScopeId, ScopeKind, ScopedPlaceId,
 };
 use crate::semantic_index::predicate::{
-    PatternPredicate, PatternPredicateKind, Predicate, PredicateNode, ScopedPredicateId,
-    StarImportPlaceholderPredicate,
+    PatternPredicate, PatternPredicateKind, Predicate, PredicateNode, PredicateOrLiteral,
+    ScopedPredicateId, StarImportPlaceholderPredicate,
 };
 use crate::semantic_index::re_exports::exported_names;
 use crate::semantic_index::reachability_constraints::{
@@ -535,29 +535,34 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
     fn record_expression_narrowing_constraint(
         &mut self,
         precide_node: &ast::Expr,
-    ) -> Predicate<'db> {
+    ) -> PredicateOrLiteral<'db> {
         let predicate = self.build_predicate(precide_node);
         self.record_narrowing_constraint(predicate);
         predicate
     }
 
-    fn build_predicate(&mut self, predicate_node: &ast::Expr) -> Predicate<'db> {
+    fn build_predicate(&mut self, predicate_node: &ast::Expr) -> PredicateOrLiteral<'db> {
         let expression = self.add_standalone_expression(predicate_node);
-        Predicate {
-            node: PredicateNode::Expression(expression),
-            is_positive: true,
+
+        if let Some(boolean_literal) = predicate_node.as_boolean_literal_expr() {
+            PredicateOrLiteral::Literal(boolean_literal.value)
+        } else {
+            PredicateOrLiteral::Predicate(Predicate {
+                node: PredicateNode::Expression(expression),
+                is_positive: true,
+            })
         }
     }
 
     /// Adds a new predicate to the list of all predicates, but does not record it. Returns the
     /// predicate ID for later recording using
     /// [`SemanticIndexBuilder::record_narrowing_constraint_id`].
-    fn add_predicate(&mut self, predicate: Predicate<'db>) -> ScopedPredicateId {
+    fn add_predicate(&mut self, predicate: PredicateOrLiteral<'db>) -> ScopedPredicateId {
         self.current_use_def_map_mut().add_predicate(predicate)
     }
 
     /// Negates a predicate and adds it to the list of all predicates, does not record it.
-    fn add_negated_predicate(&mut self, predicate: Predicate<'db>) -> ScopedPredicateId {
+    fn add_negated_predicate(&mut self, predicate: PredicateOrLiteral<'db>) -> ScopedPredicateId {
         self.current_use_def_map_mut()
             .add_predicate(predicate.negated())
     }
@@ -569,7 +574,7 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
     }
 
     /// Adds and records a narrowing constraint, i.e. adds it to all live bindings.
-    fn record_narrowing_constraint(&mut self, predicate: Predicate<'db>) {
+    fn record_narrowing_constraint(&mut self, predicate: PredicateOrLiteral<'db>) {
         let use_def = self.current_use_def_map_mut();
         let predicate_id = use_def.add_predicate(predicate);
         use_def.record_narrowing_constraint(predicate_id);
@@ -579,7 +584,7 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
     /// bindings.
     fn record_negated_narrowing_constraint(
         &mut self,
-        predicate: Predicate<'db>,
+        predicate: PredicateOrLiteral<'db>,
     ) -> ScopedPredicateId {
         let id = self.add_negated_predicate(predicate);
         self.record_narrowing_constraint_id(id);
@@ -603,7 +608,7 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
     /// we know that all statements that follow in this path of control flow will be unreachable.
     fn record_reachability_constraint(
         &mut self,
-        predicate: Predicate<'db>,
+        predicate: PredicateOrLiteral<'db>,
     ) -> ScopedReachabilityConstraintId {
         let predicate_id = self.add_predicate(predicate);
         self.record_reachability_constraint_id(predicate_id)
@@ -617,6 +622,7 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
         let reachability_constraint = self
             .current_reachability_constraints_mut()
             .add_atom(predicate_id);
+
         self.current_use_def_map_mut()
             .record_reachability_constraint(reachability_constraint);
         reachability_constraint
@@ -681,7 +687,7 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
         subject: Expression<'db>,
         pattern: &ast::Pattern,
         guard: Option<&ast::Expr>,
-    ) -> Predicate<'db> {
+    ) -> PredicateOrLiteral<'db> {
         // This is called for the top-level pattern of each match arm. We need to create a
         // standalone expression for each arm of a match statement, since they can introduce
         // constraints on the match subject. (Or more accurately, for the match arm's pattern,
@@ -705,10 +711,10 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
             guard,
             countme::Count::default(),
         );
-        let predicate = Predicate {
+        let predicate = PredicateOrLiteral::Predicate(Predicate {
             node: PredicateNode::Pattern(pattern_predicate),
             is_positive: true,
-        };
+        });
         self.record_narrowing_constraint(predicate);
         predicate
     }
@@ -1653,10 +1659,10 @@ impl<'ast> Visitor<'ast> for SemanticIndexBuilder<'_, 'ast> {
                         self.record_ambiguous_reachability();
                         self.visit_expr(guard);
                         let post_guard_eval = self.flow_snapshot();
-                        let predicate = Predicate {
+                        let predicate = PredicateOrLiteral::Predicate(Predicate {
                             node: PredicateNode::Expression(guard_expr),
                             is_positive: true,
-                        };
+                        });
                         self.record_negated_narrowing_constraint(predicate);
                         let match_success_guard_failure = self.flow_snapshot();
                         self.flow_restore(post_guard_eval);

crates/ty_python_semantic/src/semantic_index/predicate.rs

@@ -8,7 +8,7 @@
 //!   static reachability of a binding, and the reachability of a statement or expression.
 
 use ruff_db::files::File;
-use ruff_index::{IndexVec, newtype_index};
+use ruff_index::{Idx, IndexVec};
 use ruff_python_ast::Singleton;
 
 use crate::db::Db;
@@ -17,9 +17,42 @@ use crate::semantic_index::global_scope;
 use crate::semantic_index::place::{FileScopeId, ScopeId, ScopedPlaceId};
 
 // A scoped identifier for each `Predicate` in a scope.
-#[newtype_index]
-#[derive(Ord, PartialOrd, get_size2::GetSize)]
-pub(crate) struct ScopedPredicateId;
+#[derive(Clone, Debug, Copy, PartialOrd, Ord, PartialEq, Eq, Hash, get_size2::GetSize)]
+pub(crate) struct ScopedPredicateId(u32);
+
+impl ScopedPredicateId {
+    /// A special ID that is used for an "always true" predicate.
+    pub(crate) const ALWAYS_TRUE: ScopedPredicateId = ScopedPredicateId(0xffff_ffff);
+
+    /// A special ID that is used for an "always false" predicate.
+    pub(crate) const ALWAYS_FALSE: ScopedPredicateId = ScopedPredicateId(0xffff_fffe);
+
+    const SMALLEST_TERMINAL: ScopedPredicateId = Self::ALWAYS_FALSE;
+
+    fn is_terminal(self) -> bool {
+        self >= Self::SMALLEST_TERMINAL
+    }
+
+    #[cfg(test)]
+    pub(crate) fn as_u32(self) -> u32 {
+        self.0
+    }
+}
+
+impl Idx for ScopedPredicateId {
+    #[inline]
+    fn new(value: usize) -> Self {
+        assert!(value <= (Self::SMALLEST_TERMINAL.0 as usize));
+        #[expect(clippy::cast_possible_truncation)]
+        Self(value as u32)
+    }
+
+    #[inline]
+    fn index(self) -> usize {
+        debug_assert!(!self.is_terminal());
+        self.0 as usize
+    }
+}
 
 // A collection of predicates for a given scope.
 pub(crate) type Predicates<'db> = IndexVec<ScopedPredicateId, Predicate<'db>>;
@@ -49,11 +82,22 @@ pub(crate) struct Predicate<'db> {
     pub(crate) is_positive: bool,
 }
 
-impl Predicate<'_> {
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, salsa::Update, get_size2::GetSize)]
+pub(crate) enum PredicateOrLiteral<'db> {
+    Literal(bool),
+    Predicate(Predicate<'db>),
+}
+
+impl PredicateOrLiteral<'_> {
     pub(crate) fn negated(self) -> Self {
-        Self {
-            node: self.node,
-            is_positive: !self.is_positive,
+        match self {
+            PredicateOrLiteral::Literal(value) => PredicateOrLiteral::Literal(!value),
+            PredicateOrLiteral::Predicate(Predicate { node, is_positive }) => {
+                PredicateOrLiteral::Predicate(Predicate {
+                    node,
+                    is_positive: !is_positive,
+                })
+            }
         }
     }
 }
@@ -169,11 +213,11 @@ impl<'db> StarImportPlaceholderPredicate<'db> {
     }
 }
 
-impl<'db> From<StarImportPlaceholderPredicate<'db>> for Predicate<'db> {
+impl<'db> From<StarImportPlaceholderPredicate<'db>> for PredicateOrLiteral<'db> {
     fn from(predicate: StarImportPlaceholderPredicate<'db>) -> Self {
-        Predicate {
+        PredicateOrLiteral::Predicate(Predicate {
             node: PredicateNode::StarImportPlaceholder(predicate),
             is_positive: true,
-        }
+        })
     }
 }

crates/ty_python_semantic/src/semantic_index/reachability_constraints.rs

@@ -388,12 +388,18 @@ impl ReachabilityConstraintsBuilder {
         &mut self,
         predicate: ScopedPredicateId,
     ) -> ScopedReachabilityConstraintId {
-        self.add_interior(InteriorNode {
-            atom: predicate,
-            if_true: ALWAYS_TRUE,
-            if_ambiguous: AMBIGUOUS,
-            if_false: ALWAYS_FALSE,
-        })
+        if predicate == ScopedPredicateId::ALWAYS_FALSE {
+            ScopedReachabilityConstraintId::ALWAYS_FALSE
+        } else if predicate == ScopedPredicateId::ALWAYS_TRUE {
+            ScopedReachabilityConstraintId::ALWAYS_TRUE
+        } else {
+            self.add_interior(InteriorNode {
+                atom: predicate,
+                if_true: ALWAYS_TRUE,
+                if_ambiguous: AMBIGUOUS,
+                if_false: ALWAYS_FALSE,
+            })
+        }
     }
 
     /// Adds a new reachability constraint that is the ternary NOT of an existing one.

crates/ty_python_semantic/src/semantic_index/use_def.rs

@@ -247,7 +247,8 @@ use crate::semantic_index::place::{
     FileScopeId, PlaceExpr, PlaceExprWithFlags, ScopeKind, ScopedPlaceId,
 };
 use crate::semantic_index::predicate::{
-    Predicate, Predicates, PredicatesBuilder, ScopedPredicateId, StarImportPlaceholderPredicate,
+    Predicate, PredicateOrLiteral, Predicates, PredicatesBuilder, ScopedPredicateId,
+    StarImportPlaceholderPredicate,
 };
 use crate::semantic_index::reachability_constraints::{
     ReachabilityConstraints, ReachabilityConstraintsBuilder, ScopedReachabilityConstraintId,
@@ -805,11 +806,25 @@ impl<'db> UseDefMapBuilder<'db> {
         );
     }
 
-    pub(super) fn add_predicate(&mut self, predicate: Predicate<'db>) -> ScopedPredicateId {
-        self.predicates.add_predicate(predicate)
+    pub(super) fn add_predicate(
+        &mut self,
+        predicate: PredicateOrLiteral<'db>,
+    ) -> ScopedPredicateId {
+        match predicate {
+            PredicateOrLiteral::Predicate(predicate) => self.predicates.add_predicate(predicate),
+            PredicateOrLiteral::Literal(true) => ScopedPredicateId::ALWAYS_TRUE,
+            PredicateOrLiteral::Literal(false) => ScopedPredicateId::ALWAYS_FALSE,
+        }
     }
 
     pub(super) fn record_narrowing_constraint(&mut self, predicate: ScopedPredicateId) {
+        if predicate == ScopedPredicateId::ALWAYS_TRUE
+            || predicate == ScopedPredicateId::ALWAYS_FALSE
+        {
+            // No need to record a narrowing constraint for `True` or `False`.
+            return;
+        }
+
         let narrowing_constraint = predicate.into();
         for state in &mut self.place_states {
             state

crates/ty_python_semantic/src/semantic_index/use_def/place_state.rs

@@ -431,6 +431,7 @@ impl PlaceState {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use ruff_index::Idx;
 
     use crate::semantic_index::predicate::ScopedPredicateId;
 
@@ -514,7 +515,7 @@ mod tests {
             false,
             true,
         );
-        let predicate = ScopedPredicateId::from_u32(0).into();
+        let predicate = ScopedPredicateId::new(0).into();
         sym.record_narrowing_constraint(&mut narrowing_constraints, predicate);
 
         assert_bindings(&narrowing_constraints, &sym, &["1<0>"]);
@@ -533,7 +534,7 @@ mod tests {
             false,
             true,
         );
-        let predicate = ScopedPredicateId::from_u32(0).into();
+        let predicate = ScopedPredicateId::new(0).into();
         sym1a.record_narrowing_constraint(&mut narrowing_constraints, predicate);
 
         let mut sym1b = PlaceState::undefined(ScopedReachabilityConstraintId::ALWAYS_TRUE);
@@ -543,7 +544,7 @@ mod tests {
             false,
             true,
         );
-        let predicate = ScopedPredicateId::from_u32(0).into();
+        let predicate = ScopedPredicateId::new(0).into();
         sym1b.record_narrowing_constraint(&mut narrowing_constraints, predicate);
 
         sym1a.merge(
@@ -562,7 +563,7 @@ mod tests {
             false,
             true,
         );
-        let predicate = ScopedPredicateId::from_u32(1).into();
+        let predicate = ScopedPredicateId::new(1).into();
         sym2a.record_narrowing_constraint(&mut narrowing_constraints, predicate);
 
         let mut sym1b = PlaceState::undefined(ScopedReachabilityConstraintId::ALWAYS_TRUE);
@@ -572,7 +573,7 @@ mod tests {
             false,
             true,
         );
-        let predicate = ScopedPredicateId::from_u32(2).into();
+        let predicate = ScopedPredicateId::new(2).into();
         sym1b.record_narrowing_constraint(&mut narrowing_constraints, predicate);
 
         sym2a.merge(
@@ -591,7 +592,7 @@ mod tests {
             false,
             true,
         );
-        let predicate = ScopedPredicateId::from_u32(3).into();
+        let predicate = ScopedPredicateId::new(3).into();
         sym3a.record_narrowing_constraint(&mut narrowing_constraints, predicate);
 
         let sym2b = PlaceState::undefined(ScopedReachabilityConstraintId::ALWAYS_TRUE);