Auto merge of #1952 - RalfJung:self-referential, r=RalfJung

exclude mutable references to !Unpin types from uniqueness guarantees

This basically works around https://github.com/rust-lang/unsafe-code-guidelines/issues/148 by not requiring uniqueness any more for mutable references to self-referential generators. That corresponds to [the same work-around that was applied in rustc itself](b815532674/compiler/rustc_middle/src/ty/layout.rs (L2482)).

I am not entirely sure if this is a good idea since it might hide too many errors in case types are "accidentally" `!Unpin`. OTOH, our test suite still passes, and to my knowledge the vast majority of types is `Unpin`. (`place.layout.ty` is monomorphic, we should always exactly know which type this is.)

src/stacked_borrows.rs

@@ -9,7 +9,11 @@
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::Mutability;
 use rustc_middle::mir::RetagKind;
-use rustc_middle::ty::{self, layout::LayoutOf};
+use rustc_middle::ty::{
+    self,
+    layout::{HasParamEnv, LayoutOf},
+};
+use rustc_span::DUMMY_SP;
 use rustc_target::abi::Size;
 
 use crate::*;
@@ -657,8 +661,16 @@ fn reborrow(
         // Make sure that raw pointers and mutable shared references are reborrowed "weak":
         // There could be existing unique pointers reborrowed from them that should remain valid!
         let perm = match kind {
-            RefKind::Unique { two_phase: false } => Permission::Unique,
-            RefKind::Unique { two_phase: true } => Permission::SharedReadWrite,
+            RefKind::Unique { two_phase: false }
+                if place.layout.ty.is_unpin(this.tcx.at(DUMMY_SP), this.param_env()) =>
+            {
+                // Only if the type is unpin do we actually enforce uniqueness
+                Permission::Unique
+            }
+            RefKind::Unique { .. } => {
+                // Two-phase references and !Unpin references are treated as SharedReadWrite
+                Permission::SharedReadWrite
+            }
             RefKind::Raw { mutable: true } => Permission::SharedReadWrite,
             RefKind::Shared | RefKind::Raw { mutable: false } => {
                 // Shared references and *const are a whole different kind of game, the

tests/run-pass/stacked-borrows/generators-self-referential.rs

@@ -0,0 +1,34 @@
+// See https://github.com/rust-lang/unsafe-code-guidelines/issues/148:
+// this fails when Stacked Borrows is strictly applied even to `!Unpin` types.
+#![feature(generators, generator_trait)]
+
+use std::{
+    ops::{Generator, GeneratorState},
+    pin::Pin,
+};
+
+fn firstn() -> impl Generator<Yield = u64, Return = ()> {
+    static move || {
+        let mut num = 0;
+        let num = &mut num;
+
+        yield *num;
+        *num += 1; //~ ERROR: borrow stack
+
+        yield *num;
+        *num += 1;
+
+        yield *num;
+        *num += 1;
+    }
+}
+
+fn main() {
+    let mut generator_iterator = firstn();
+    let mut pin = unsafe { Pin::new_unchecked(&mut generator_iterator) };
+    let mut sum = 0;
+    while let GeneratorState::Yielded(x) = pin.as_mut().resume(()) {
+        sum += x;
+    }
+    assert_eq!(sum, 3);
+}