Unrolled build for #163051 in rollup 163139

Rollup merge of #163051 - asquared31415:actually_covariant_unsafe_cell, r=WaffleLapkin

Make CovariantUnsafeCell actually covariant

The "test" in `core` was not actually being run, due to core being weird. (filed #163045 to track that)

The existing mechanism for inserting variance manually was only *priming* the solver, which happened to work for the other two types using this mechanism, but does not work for weakening variance explicitly. I changed this mechanism to additionally prevent the variance from being updated when solving.

This blocks #159838

r? @WaffleLapkin as the implementor of `CovariantUnsafeCell`

Author: bors

compiler/rustc_hir_analysis/src/variance/solve.rs

@@ -5,6 +5,7 @@
 //! optimal solution to the constraints. The final variance for each
 //! inferred is then written into the `variance_map` in the tcx.
 
+use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def_id::DefIdMap;
 use rustc_middle::ty;
 use tracing::debug;
@@ -43,16 +44,26 @@ struct SolveContext<'a, 'tcx> {
 pub(crate) fn solve_constraints<'tcx>(
     constraints_cx: ConstraintContext<'_, 'tcx>,
 ) -> ty::CrateVariancesMap<'tcx> {
-    let ConstraintContext { terms_cx, constraints, .. } = constraints_cx;
+    let ConstraintContext { terms_cx, mut constraints, .. } = constraints_cx;
 
+    let mut overridden_inferreds = FxHashSet::default();
     let mut solutions = vec![ty::Bivariant; terms_cx.inferred_terms.len()];
+    // prime the solutions for certain lang items which have hard-coded variance
     for (id, variances) in &terms_cx.lang_items {
         let InferredIndex(start) = terms_cx.inferred_starts[id];
         for (i, &variance) in variances.iter().enumerate() {
             solutions[start + i] = variance;
+            overridden_inferreds.insert(start + i);
         }
     }
 
+    // ensure the solutions for overridden inferreds are never constrained by anything else
+    if !overridden_inferreds.is_empty() {
+        constraints.retain(|Constraint { inferred: InferredIndex(inferred), .. }| {
+            !overridden_inferreds.contains(inferred)
+        });
+    }
+
     let mut solutions_cx = SolveContext { terms_cx, constraints, solutions };
     solutions_cx.solve();
     let variances = solutions_cx.create_map();

library/core/src/cell/covariant_unsafe_cell.rs

@@ -170,14 +170,3 @@ impl<T: ?Sized> fmt::Debug for CovariantUnsafeCell<T> {
         f.debug_struct("CovariantUnsafeCell").finish_non_exhaustive()
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    fn _covarience<'short, 'long: 'short>(
-        x: CovariantUnsafeCell<&'long ()>,
-    ) -> CovariantUnsafeCell<&'short ()> {
-        x
-    }
-}

tests/ui/variance/covariant_unsafe_cell_is_covariant.rs

@@ -0,0 +1,14 @@
+//@ build-pass
+
+#![feature(covariant_unsafe_cell)]
+
+use std::cell::CovariantUnsafeCell;
+
+/// this function compiling ensures that CovariantUnsafeCell is actually covariant
+fn _assert_covariance<'short, 'long: 'short>(
+    x: CovariantUnsafeCell<&'long ()>,
+) -> CovariantUnsafeCell<&'short ()> {
+    x
+}
+
+fn main () {}

tests/ui/variance/variance-types.rs

@@ -1,5 +1,5 @@
-#![allow(dead_code)]
 #![feature(rustc_attrs)]
+#![feature(covariant_unsafe_cell)]
 
 use std::cell::Cell;
 
@@ -38,4 +38,19 @@ enum Enum<A,B,C> { //~ ERROR [A: +, B: -, C: o]
     Zed(Covariant<C>,Contravariant<C>)
 }
 
+#[rustc_dump_variances]
+struct PhantomDataIsCovariant<A> { //~ ERROR [A: +]
+    t: std::marker::PhantomData<A>
+}
+
+#[rustc_dump_variances]
+struct UnsafeCellIsInvariant<A> { //~ ERROR [A: o]
+    t: std::cell::UnsafeCell<A>
+}
+
+#[rustc_dump_variances]
+struct CovariantUnsafeCellIsCovariant<A> { //~ ERROR [A: +]
+    t: std::cell::CovariantUnsafeCell<A>
+}
+
 pub fn main() { }

tests/ui/variance/variance-types.stderr

@@ -34,5 +34,23 @@ error: [A: +, B: -, C: o]
 LL | enum Enum<A,B,C> {
    | ^^^^^^^^^^^^^^^^
 
-error: aborting due to 6 previous errors
+error: [A: +]
+  --> $DIR/variance-types.rs:42:1
+   |
+LL | struct PhantomDataIsCovariant<A> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: [A: o]
+  --> $DIR/variance-types.rs:47:1
+   |
+LL | struct UnsafeCellIsInvariant<A> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: [A: +]
+  --> $DIR/variance-types.rs:52:1
+   |
+LL | struct CovariantUnsafeCellIsCovariant<A> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 9 previous errors