Type fallback refactorings

compiler/rustc_hir_typeck/src/fallback.rs

@@ -45,90 +45,102 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         }
     }
 
+    /// Tries to apply a fallback to all unresolved variables.
+    ///
+    /// - Unconstrained ints are replaced with `i32`.
+    ///
+    /// - Unconstrained floats are replaced with `f64`, except when there is a trait predicate
+    ///   `f32: From<{float}>`, in which case `f32` is used as the fallback instead.
+    ///
+    /// - Non-numerics may get replaced with `()` or `!`, depending on how they
+    ///   were categorized by [`Self::calculate_diverging_fallback`], crate's
+    ///   edition, and the setting of `#![rustc_never_type_options(fallback = ...)]`.
+    ///
+    /// Fallback becomes very dubious if we have encountered type-checking errors.
+    /// In that case, all variables fallback to Error.
+    ///
+    /// Sets [`FnCtxt::diverging_fallback_has_occurred`] if never type fallback
+    /// is performed during this call.
+    ///
+    /// Returns `true` if *any* kind of fallback has occurred during this call.
     fn fallback_types(&self) -> bool {
-        // Check if we have any unresolved variables. If not, no need for fallback.
-        let unresolved_variables = self.unresolved_variables();
+        let (unresolved_ty, unresolved_int, unresolved_float) = self.unresolved_root_variables();
 
-        if unresolved_variables.is_empty() {
+        // Check if we have any unresolved variables. If not, no need for fallback.
+        if unresolved_ty.is_empty() && unresolved_int.is_empty() && unresolved_float.is_empty() {
             return false;
         }
 
         let (diverging_fallback, diverging_fallback_ty) =
-            self.calculate_diverging_fallback(&unresolved_variables);
-        let fallback_to_f32 = self.calculate_fallback_to_f32(&unresolved_variables);
+            self.calculate_diverging_fallback(&unresolved_ty);
+        let fallback_to_f32 = self.calculate_fallback_to_f32(&unresolved_float);
 
         // We do fallback in two passes, to try to generate
         // better error messages.
         // The first time, we do *not* replace opaque types.
         let mut fallback_occurred = false;
-        for ty in unresolved_variables {
-            debug!("unsolved_variable = {:?}", ty);
+
+        for vid in unresolved_ty {
             fallback_occurred |= self.fallback_if_possible(
-                ty,
-                &diverging_fallback,
-                diverging_fallback_ty,
-                &fallback_to_f32,
+                vid,
+                || {
+                    diverging_fallback.contains(&vid).then(|| {
+                        self.diverging_fallback_has_occurred.set(true);
+                        diverging_fallback_ty
+                    })
+                },
+                |vid| (Ty::new_var(self.tcx, vid), self.type_var_origin(vid).span),
+            );
+        }
+
+        for vid in unresolved_int {
+            fallback_occurred |= self.fallback_if_possible(
+                vid,
+                || Some(self.tcx.types.i32),
+                // Int variables have no origin?..
+                |vid| (Ty::new_int_var(self.tcx, vid), DUMMY_SP),
+            );
+        }
+
+        for vid in unresolved_float {
+            fallback_occurred |= self.fallback_if_possible(
+                vid,
+                || {
+                    Some(if fallback_to_f32.contains(&vid) {
+                        self.tcx.types.f32
+                    } else {
+                        self.tcx.types.f64
+                    })
+                },
+                |vid| (Ty::new_float_var(self.tcx, vid), self.float_var_origin(vid).span),
             );
         }
 
         fallback_occurred
     }
 
-    /// Tries to apply a fallback to `ty` if it is an unsolved variable.
-    ///
-    /// - Unconstrained ints are replaced with `i32`.
+    /// Applies fallback to `vid`, if possible.
     ///
-    /// - Unconstrained floats are replaced with `f64`, except when there is a trait predicate
-    ///   `f32: From<{float}>`, in which case `f32` is used as the fallback instead.
-    ///
-    /// - Non-numerics may get replaced with `()` or `!`, depending on how they
-    ///   were categorized by [`Self::calculate_diverging_fallback`], crate's
-    ///   edition, and the setting of `#![rustc_never_type_options(fallback = ...)]`.
+    /// - If `self.tainted_by_errors()` unifies the type represented by `vid` with error
+    /// - Otherwise, if `fallback` returns `Some`, unifies it with the output of `fallback`
+    /// - Otherwise, does nothing
     ///
-    /// Fallback becomes very dubious if we have encountered
-    /// type-checking errors. In that case, fallback to Error.
-    ///
-    /// Sets [`FnCtxt::diverging_fallback_has_occurred`] if never type fallback
-    /// is performed during this call.
-    fn fallback_if_possible(
+    /// Returns whatever fallback has been applied.
+    fn fallback_if_possible<V>(
         &self,
-        ty: Ty<'tcx>,
-        diverging_fallback: &UnordSet<Ty<'tcx>>,
-        diverging_fallback_ty: Ty<'tcx>,
-        fallback_to_f32: &UnordSet<FloatVid>,
+        vid: V,
+        fallback: impl FnOnce() -> Option<Ty<'tcx>>,
+        vid_to_ty_and_span: impl FnOnce(V) -> (Ty<'tcx>, Span),
     ) -> bool {
-        // Careful: we do NOT shallow-resolve `ty`. We know that `ty`
-        // is an unsolved variable, and we determine its fallback
-        // based solely on how it was created, not what other type
-        // variables it may have been unified with since then.
-        //
-        // The reason this matters is that other attempts at fallback
-        // may (in principle) conflict with this fallback, and we wish
-        // to generate a type error in that case. (However, this
-        // actually isn't true right now, because we're only using the
-        // builtin fallback rules. This would be true if we were using
-        // user-supplied fallbacks. But it's still useful to write the
-        // code to detect bugs.)
-        //
-        // (Note though that if we have a general type variable `?T`
-        // that is then unified with an integer type variable `?I`
-        // that ultimately never gets resolved to a special integral
-        // type, `?T` is not considered unsolved, but `?I` is. The
-        // same is true for float variables.)
-        let fallback = match ty.kind() {
-            _ if let Some(e) = self.tainted_by_errors() => Ty::new_error(self.tcx, e),
-            ty::Infer(ty::IntVar(_)) => self.tcx.types.i32,
-            ty::Infer(ty::FloatVar(vid)) if fallback_to_f32.contains(vid) => self.tcx.types.f32,
-            ty::Infer(ty::FloatVar(_)) => self.tcx.types.f64,
-            _ if diverging_fallback.contains(&ty) => {
-                self.diverging_fallback_has_occurred.set(true);
-                diverging_fallback_ty
-            }
-            _ => return false,
+        let fallback = if let Some(e) = self.tainted_by_errors() {
+            Ty::new_error(self.tcx, e)
+        } else if let Some(fallback) = fallback() {
+            fallback
+        } else {
+            return false;
         };
-        debug!("fallback_if_possible(ty={:?}): defaulting to `{:?}`", ty, fallback);
 
-        let span = ty.ty_vid().map_or(DUMMY_SP, |vid| self.infcx.type_var_origin(vid).span);
+        let (ty, span) = vid_to_ty_and_span(vid);
         self.demand_eqtype(span, ty, fallback);
         true
     }
@@ -146,15 +158,19 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
     ///
     /// foo(1.0);
     /// ```
-    fn calculate_fallback_to_f32(&self, unresolved_variables: &[Ty<'tcx>]) -> UnordSet<FloatVid> {
+    fn calculate_fallback_to_f32(
+        &self,
+        unresolved_variables: &[ty::FloatVid],
+    ) -> UnordSet<FloatVid> {
         // Short-circuit: if no unresolved variable is a float, no f32 fallback can apply,
         // so we can skip the (potentially very expensive) work in `from_float_for_f32_root_vids`.
         // Under the new solver, that function walks `visit_proof_tree` for every pending
         // obligation, which is O(N × proof_tree_size) and can dominate type-checking on crates
         // with many large pending obligations and no f32 involvement.
-        if unresolved_variables.iter().all(|ty| ty.float_vid().is_none()) {
+        if unresolved_variables.is_empty() {
             return UnordSet::new();
         }
+
         let roots: UnordSet<ty::FloatVid> = self.from_float_for_f32_root_vids();
         if roots.is_empty() {
             // Most functions have no `f32: From<{float}>` predicates, so short-circuit and return
@@ -166,10 +182,10 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         // top of that function for details.
         let fallback_to_f32 = unresolved_variables
             .iter()
-            .flat_map(|ty| ty.float_vid())
-            .filter(|vid| roots.contains(&self.root_float_var(*vid)))
-            .inspect(|vid| {
-                let origin = self.float_var_origin(*vid);
+            .copied()
+            .filter(|&vid| roots.contains(&self.root_float_var(vid)))
+            .inspect(|&vid| {
+                let origin = self.float_var_origin(vid);
                 // Show the entire literal in the suggestion to make it clearer.
                 let mut literal = self.tcx.sess.source_map().span_to_snippet(origin.span).ok();
                 // A `.` at the end of the literal is no longer necessary if `f32` is explicitly specified
@@ -195,8 +211,8 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
 
     fn calculate_diverging_fallback(
         &self,
-        unresolved_variables: &[Ty<'tcx>],
-    ) -> (UnordSet<Ty<'tcx>>, Ty<'tcx>) {
+        unresolved_variables: &[ty::TyVid],
+    ) -> (UnordSet<ty::TyVid>, Ty<'tcx>) {
         debug!("calculate_diverging_fallback({:?})", unresolved_variables);
 
         let diverging_fallback_ty = match self.diverging_fallback_behavior {
@@ -214,7 +230,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
 
         // Extract the unsolved type inference variable vids; note that some
         // unsolved variables are integer/float variables and are excluded.
-        let unsolved_vids = unresolved_variables.iter().filter_map(|ty| ty.ty_vid());
+        let unsolved_vids = unresolved_variables.iter();
 
         // Compute the diverging root vids D -- that is, the root vid of
         // those type variables that (a) are the target of a coercion from
@@ -241,7 +257,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         // we find later that they are *also* reachable from some
         // other type variable outside this set.
         let mut diverging_vids = vec![];
-        for unsolved_vid in unsolved_vids {
+        for &unsolved_vid in unsolved_vids {
             let root_vid = self.root_var(unsolved_vid);
             debug!(
                 "calculate_diverging_fallback: unsolved_vid={:?} root_vid={:?} diverges={:?}",
@@ -271,7 +287,6 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         );
 
         for &diverging_vid in &diverging_vids {
-            let diverging_ty = Ty::new_var(self.tcx, diverging_vid);
             let root_vid = self.root_var(diverging_vid);
 
             self.lint_never_type_fallback_flowing_into_unsafe_code(
@@ -280,7 +295,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
                 root_vid,
             );
 
-            diverging_fallback.insert(diverging_ty);
+            diverging_fallback.insert(diverging_vid);
         }
 
         (diverging_fallback, diverging_fallback_ty)

compiler/rustc_infer/src/infer/mod.rs

@@ -752,27 +752,25 @@ impl<'tcx> InferCtxt<'tcx> {
         }
     }
 
-    pub fn unresolved_variables(&self) -> Vec<Ty<'tcx>> {
+    pub fn unresolved_root_variables(&self) -> (Vec<TyVid>, Vec<ty::IntVid>, Vec<ty::FloatVid>) {
         let mut inner = self.inner.borrow_mut();
-        let mut vars: Vec<Ty<'_>> = inner
-            .type_variables()
-            .unresolved_variables()
-            .into_iter()
-            .map(|t| Ty::new_var(self.tcx, t))
+        let ty = inner.type_variables().unresolved_root_variables().into_iter().collect();
+        let int = (0..inner.int_unification_table().len())
+            .map(|i| ty::IntVid::from_usize(i))
+            .filter(|&vid| {
+                inner.int_unification_table().probe_value(vid).is_unknown()
+                    && inner.int_unification_table().find(vid) == vid
+            })
             .collect();
-        vars.extend(
-            (0..inner.int_unification_table().len())
-                .map(|i| ty::IntVid::from_usize(i))
-                .filter(|&vid| inner.int_unification_table().probe_value(vid).is_unknown())
-                .map(|v| Ty::new_int_var(self.tcx, v)),
-        );
-        vars.extend(
-            (0..inner.float_unification_table().len())
-                .map(|i| ty::FloatVid::from_usize(i))
-                .filter(|&vid| inner.float_unification_table().probe_value(vid).is_unknown())
-                .map(|v| Ty::new_float_var(self.tcx, v)),
-        );
-        vars
+        let float = (0..inner.float_unification_table().len())
+            .map(|i| ty::FloatVid::from_usize(i))
+            .filter(|&vid| {
+                inner.float_unification_table().probe_value(vid).is_unknown()
+                    && inner.float_unification_table().find(vid) == vid
+            })
+            .collect();
+
+        (ty, int, float)
     }
 
     #[instrument(skip(self), level = "debug")]

compiler/rustc_infer/src/infer/type_variable.rs

@@ -286,12 +286,14 @@ impl<'tcx> TypeVariableTable<'_, 'tcx> {
         (range.clone(), range.map(|index| self.var_origin(index)).collect())
     }
 
-    /// Returns indices of all variables that are not yet
-    /// instantiated.
-    pub(crate) fn unresolved_variables(&mut self) -> Vec<ty::TyVid> {
+    /// Returns indices of all root variables that are not yet instantiated.
+    pub(crate) fn unresolved_root_variables(&mut self) -> Vec<ty::TyVid> {
         (0..self.num_vars())
             .filter_map(|i| {
                 let vid = ty::TyVid::from_usize(i);
+                if self.root_var(vid) != vid {
+                    return None;
+                }
                 match self.probe(vid) {
                     TypeVariableValue::Unknown { .. } => Some(vid),
                     TypeVariableValue::Known { .. } => None,