Call the method fork instead of clone and add proper comments

compiler/rustc_data_structures/src/snapshot_map/mod.rs

@@ -13,6 +13,7 @@ mod tests;
 pub type SnapshotMapStorage<K, V> = SnapshotMap<K, V, FxHashMap<K, V>, ()>;
 pub type SnapshotMapRef<'a, K, V, L> = SnapshotMap<K, V, &'a mut FxHashMap<K, V>, &'a mut L>;
 
+#[derive(Clone)]
 pub struct SnapshotMap<K, V, M = FxHashMap<K, V>, L = VecLog<UndoLog<K, V>>> {
     map: M,
     undo_log: L,
@@ -30,6 +31,7 @@ where
     }
 }
 
+#[derive(Clone)]
 pub enum UndoLog<K, V> {
     Inserted(K),
     Overwrite(K, V),

compiler/rustc_infer/src/infer/at.rs

@@ -51,6 +51,29 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     ) -> At<'a, 'tcx> {
         At { infcx: self, cause, param_env }
     }
+
+    /// Forks the inference context, creating a new inference context with the same inference
+    /// variables in the same state. This can be used to "branch off" many tests from the same
+    /// common state. Used in coherence.
+    pub fn fork(&self) -> Self {
+        Self {
+            tcx: self.tcx.clone(),
+            defining_use_anchor: self.defining_use_anchor.clone(),
+            reveal_defining_opaque_types: self.reveal_defining_opaque_types,
+            in_progress_typeck_results: self.in_progress_typeck_results.clone(),
+            inner: self.inner.clone(),
+            skip_leak_check: self.skip_leak_check.clone(),
+            lexical_region_resolutions: self.lexical_region_resolutions.clone(),
+            selection_cache: self.selection_cache.clone(),
+            evaluation_cache: self.evaluation_cache.clone(),
+            reported_trait_errors: self.reported_trait_errors.clone(),
+            reported_closure_mismatch: self.reported_closure_mismatch.clone(),
+            tainted_by_errors_flag: self.tainted_by_errors_flag.clone(),
+            err_count_on_creation: self.err_count_on_creation,
+            in_snapshot: self.in_snapshot.clone(),
+            universe: self.universe.clone(),
+        }
+    }
 }
 
 pub trait ToTrace<'tcx>: Relate<'tcx> + Copy {

compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs

@@ -61,6 +61,7 @@ pub(crate) fn resolve<'tcx>(
 
 /// Contains the result of lexical region resolution. Offers methods
 /// to lookup up the final value of a region variable.
+#[derive(Clone)]
 pub struct LexicalRegionResolutions<'tcx> {
     values: IndexVec<RegionVid, VarValue<'tcx>>,
     error_region: ty::Region<'tcx>,

compiler/rustc_infer/src/infer/mod.rs

@@ -130,6 +130,7 @@ impl RegionckMode {
 /// `RefCell` and are involved with taking/rolling back snapshots. Snapshot
 /// operations are hot enough that we want only one call to `borrow_mut` per
 /// call to `start_snapshot` and `rollback_to`.
+#[derive(Clone)]
 pub struct InferCtxtInner<'tcx> {
     /// Cache for projections. This cache is snapshotted along with the infcx.
     ///

compiler/rustc_infer/src/infer/region_constraints/mod.rs

@@ -28,7 +28,7 @@ mod leak_check;
 
 pub use rustc_middle::infer::MemberConstraint;
 
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct RegionConstraintStorage<'tcx> {
     /// For each `RegionVid`, the corresponding `RegionVariableOrigin`.
     var_infos: IndexVec<RegionVid, RegionVariableInfo>,

compiler/rustc_infer/src/infer/type_variable.rs

@@ -14,6 +14,7 @@ use std::ops::Range;
 use rustc_data_structures::undo_log::{Rollback, UndoLogs};
 
 /// Represents a single undo-able action that affects a type inference variable.
+#[derive(Clone)]
 pub(crate) enum UndoLog<'tcx> {
     EqRelation(sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>),
     SubRelation(sv::UndoLog<ut::Delegate<ty::TyVid>>),
@@ -58,6 +59,7 @@ impl<'tcx> Rollback<UndoLog<'tcx>> for TypeVariableStorage<'tcx> {
     }
 }
 
+#[derive(Clone)]
 pub struct TypeVariableStorage<'tcx> {
     values: sv::SnapshotVecStorage<Delegate>,
 
@@ -137,6 +139,7 @@ pub enum TypeVariableOriginKind {
     LatticeVariable,
 }
 
+#[derive(Clone)]
 pub(crate) struct TypeVariableData {
     origin: TypeVariableOrigin,
 }
@@ -165,6 +168,7 @@ impl<'tcx> TypeVariableValue<'tcx> {
     }
 }
 
+#[derive(Clone)]
 pub(crate) struct Instantiate;
 
 pub(crate) struct Delegate;

compiler/rustc_infer/src/infer/undo_log.rs

@@ -17,6 +17,7 @@ pub struct Snapshot<'tcx> {
 }
 
 /// Records the "undo" data for a single operation that affects some form of inference variable.
+#[derive(Clone)]
 pub(crate) enum UndoLog<'tcx> {
     TypeVariables(type_variable::UndoLog<'tcx>),
     ConstUnificationTable(sv::UndoLog<ut::Delegate<ty::ConstVid<'tcx>>>),
@@ -84,6 +85,7 @@ impl<'tcx> Rollback<UndoLog<'tcx>> for InferCtxtInner<'tcx> {
 
 /// The combined undo log for all the various unification tables. For each change to the storage
 /// for any kind of inference variable, we record an UndoLog entry in the vector here.
+#[derive(Clone)]
 pub(crate) struct InferCtxtUndoLogs<'tcx> {
     logs: Vec<UndoLog<'tcx>>,
     num_open_snapshots: usize,

compiler/rustc_infer/src/traits/project.rs

@@ -70,7 +70,7 @@ pub struct ProjectionCache<'a, 'tcx> {
     undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
 }
 
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct ProjectionCacheStorage<'tcx> {
     map: SnapshotMapStorage<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
 }

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -379,7 +379,7 @@ fn negative_impl_exists<'cx, 'tcx>(
     region_context: DefId,
     o: &PredicateObligation<'tcx>,
 ) -> bool {
-    let infcx = selcx.infcx().clone();
+    let infcx = &selcx.infcx().fork();
     let tcx = infcx.tcx;
     o.flip_polarity(tcx)
         .map(|o| {