Auto merge of #2985 - RalfJung:retag-fields, r=saethlin

make full field retagging the default

The 'scalar' field retagging mode is clearly a hack -- it mirrors details of the codegen backend and how various structs are represented in LLVM. This means whether code has UB or not depends on surprising aspects, such as whether a struct has 2 or 3 (non-zero-sized) fields. Now that both hashbrown and scopeguard have released fixes to be compatible with field retagging, I think it is time to enable full field retagging by default.

`@saethlin` do you have an idea of how much fallout enabling full field retagging by default will cause? Do you have objections to enabling it by default?

Fixes https://github.com/rust-lang/miri/issues/2528

18 files changed, 140 insertions, 89 deletions

diff --git a/src/tools/miri/README.md b/src/tools/miri/README.md
index a965f44a4e6..eaf58340d7b 100644
--- a/src/tools/miri/README.md
+++ b/src/tools/miri/README.md
@@ -407,15 +407,11 @@ to Miri failing to detect cases of undefined behavior in a program.
   application instead of raising an error within the context of Miri (and halting
   execution). Note that code might not expect these operations to ever panic, so
   this flag can lead to strange (mis)behavior.
-* `-Zmiri-retag-fields` changes Stacked Borrows retagging to recurse into *all* fields.
-  This means that references in fields of structs/enums/tuples/arrays/... are retagged,
-  and in particular, they are protected when passed as function arguments.
-  (The default is to recurse only in cases where rustc would actually emit a `noalias` attribute.)
-* `-Zmiri-retag-fields=<all|none|scalar>` controls when Stacked Borrows retagging recurses into
-  fields. `all` means it always recurses (like `-Zmiri-retag-fields`), `none` means it never
-  recurses, `scalar` (the default) means it only recurses for types where we would also emit
-  `noalias` annotations in the generated LLVM IR (types passed as individual scalars or pairs of
-  scalars). Setting this to `none` is **unsound**.
+* `-Zmiri-retag-fields[=<all|none|scalar>]` controls when Stacked Borrows retagging recurses into
+  fields. `all` means it always recurses (the default, and equivalent to `-Zmiri-retag-fields`
+  without an explicit value), `none` means it never recurses, `scalar` means it only recurses for
+  types where we would also emit `noalias` annotations in the generated LLVM IR (types passed as
+  individual scalars or pairs of scalars). Setting this to `none` is **unsound**.
 * `-Zmiri-tag-gc=<blocks>` configures how often the pointer tag garbage collector runs. The default
   is to search for and remove unreachable tags once every `10000` basic blocks. Setting this to
   `0` disables the garbage collector, which causes some programs to have explosive memory usage
diff --git a/src/tools/miri/src/borrow_tracker/stacked_borrows/diagnostics.rs b/src/tools/miri/src/borrow_tracker/stacked_borrows/diagnostics.rs
index 5ec8d80fb32..9b0f13dd62c 100644
--- a/src/tools/miri/src/borrow_tracker/stacked_borrows/diagnostics.rs
+++ b/src/tools/miri/src/borrow_tracker/stacked_borrows/diagnostics.rs
@@ -6,11 +6,19 @@ use rustc_span::{Span, SpanData};
 use rustc_target::abi::Size;
 
 use crate::borrow_tracker::{
-    stacked_borrows::{err_sb_ub, Permission},
-    AccessKind, GlobalStateInner, ProtectorKind,
+    stacked_borrows::Permission, AccessKind, GlobalStateInner, ProtectorKind,
 };
 use crate::*;
 
+/// Error reporting
+fn err_sb_ub<'tcx>(
+    msg: String,
+    help: Vec<String>,
+    history: Option<TagHistory>,
+) -> InterpError<'tcx> {
+    err_machine_stop!(TerminationInfo::StackedBorrowsUb { msg, help, history })
+}
+
 #[derive(Clone, Debug)]
 pub struct AllocHistory {
     id: AllocId,
@@ -61,12 +69,15 @@ struct Invalidation {
 #[derive(Clone, Debug)]
 enum InvalidationCause {
     Access(AccessKind),
-    Retag(Permission, RetagCause),
+    Retag(Permission, RetagInfo),
 }
 
 impl Invalidation {
     fn generate_diagnostic(&self) -> (String, SpanData) {
-        let message = if let InvalidationCause::Retag(_, RetagCause::FnEntry) = self.cause {
+        let message = if matches!(
+            self.cause,
+            InvalidationCause::Retag(_, RetagInfo { cause: RetagCause::FnEntry, .. })
+        ) {
             // For a FnEntry retag, our Span points at the caller.
             // See `DiagnosticCx::log_invalidation`.
             format!(
@@ -87,8 +98,8 @@ impl fmt::Display for InvalidationCause {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             InvalidationCause::Access(kind) => write!(f, "{kind}"),
-            InvalidationCause::Retag(perm, kind) =>
-                write!(f, "{perm:?} {retag}", retag = kind.summary()),
+            InvalidationCause::Retag(perm, info) =>
+                write!(f, "{perm:?} {retag}", retag = info.summary()),
         }
     }
 }
@@ -129,13 +140,13 @@ impl<'ecx, 'mir, 'tcx> DiagnosticCxBuilder<'ecx, 'mir, 'tcx> {
 
     pub fn retag(
         machine: &'ecx MiriMachine<'mir, 'tcx>,
-        cause: RetagCause,
+        info: RetagInfo,
         new_tag: BorTag,
         orig_tag: ProvenanceExtra,
         range: AllocRange,
     ) -> Self {
         let operation =
-            Operation::Retag(RetagOp { cause, new_tag, orig_tag, range, permission: None });
+            Operation::Retag(RetagOp { info, new_tag, orig_tag, range, permission: None });
 
         DiagnosticCxBuilder { machine, operation }
     }
@@ -179,7 +190,7 @@ enum Operation {
 
 #[derive(Debug, Clone)]
 struct RetagOp {
-    cause: RetagCause,
+    info: RetagInfo,
     new_tag: BorTag,
     orig_tag: ProvenanceExtra,
     range: AllocRange,
@@ -187,6 +198,12 @@ struct RetagOp {
 }
 
 #[derive(Debug, Clone, Copy, PartialEq)]
+pub struct RetagInfo {
+    pub cause: RetagCause,
+    pub in_field: bool,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum RetagCause {
     Normal,
     InPlaceFnPassing,
@@ -258,11 +275,11 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
     pub fn log_invalidation(&mut self, tag: BorTag) {
         let mut span = self.machine.current_span();
         let (range, cause) = match &self.operation {
-            Operation::Retag(RetagOp { cause, range, permission, .. }) => {
-                if *cause == RetagCause::FnEntry {
+            Operation::Retag(RetagOp { info, range, permission, .. }) => {
+                if info.cause == RetagCause::FnEntry {
                     span = self.machine.caller_span();
                 }
-                (*range, InvalidationCause::Retag(permission.unwrap(), *cause))
+                (*range, InvalidationCause::Retag(permission.unwrap(), *info))
             }
             Operation::Access(AccessOp { kind, range, .. }) =>
                 (*range, InvalidationCause::Access(*kind)),
@@ -372,9 +389,13 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
             self.history.id,
             self.offset.bytes(),
         );
+        let mut helps = vec![operation_summary(&op.info.summary(), self.history.id, op.range)];
+        if op.info.in_field {
+            helps.push(format!("errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling"));
+        }
         err_sb_ub(
             format!("{action}{}", error_cause(stack, op.orig_tag)),
-            Some(operation_summary(&op.cause.summary(), self.history.id, op.range)),
+            helps,
             op.orig_tag.and_then(|orig_tag| self.get_logs_relevant_to(orig_tag, None)),
         )
     }
@@ -397,7 +418,7 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
         );
         err_sb_ub(
             format!("{action}{}", error_cause(stack, op.tag)),
-            Some(operation_summary("an access", self.history.id, op.range)),
+            vec![operation_summary("an access", self.history.id, op.range)],
             op.tag.and_then(|tag| self.get_logs_relevant_to(tag, None)),
         )
     }
@@ -423,7 +444,7 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
             Operation::Dealloc(_) =>
                 err_sb_ub(
                     format!("deallocating while item {item:?} is {protected} by call {call_id:?}",),
-                    None,
+                    vec![],
                     None,
                 ),
             Operation::Retag(RetagOp { orig_tag: tag, .. })
@@ -432,7 +453,7 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
                     format!(
                         "not granting access to tag {tag:?} because that would remove {item:?} which is {protected} because it is an argument of call {call_id:?}",
                     ),
-                    None,
+                    vec![],
                     tag.and_then(|tag| self.get_logs_relevant_to(tag, Some(item.tag()))),
                 ),
         }
@@ -450,7 +471,7 @@ impl<'history, 'ecx, 'mir, 'tcx> DiagnosticCx<'history, 'ecx, 'mir, 'tcx> {
                 alloc_id = self.history.id,
                 cause = error_cause(stack, op.tag),
             ),
-            None,
+            vec![],
             op.tag.and_then(|tag| self.get_logs_relevant_to(tag, None)),
         )
     }
@@ -496,14 +517,18 @@ fn error_cause(stack: &Stack, prov_extra: ProvenanceExtra) -> &'static str {
     }
 }
 
-impl RetagCause {
+impl RetagInfo {
     fn summary(&self) -> String {
-        match self {
+        let mut s = match self.cause {
             RetagCause::Normal => "retag",
             RetagCause::FnEntry => "function-entry retag",
             RetagCause::InPlaceFnPassing => "in-place function argument/return passing protection",
             RetagCause::TwoPhase => "two-phase retag",
         }
-        .to_string()
+        .to_string();
+        if self.in_field {
+            s.push_str(" (of a reference/box inside this compound value)");
+        }
+        s
     }
 }
diff --git a/src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs b/src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs
index 7e89d3a0e8c..1aed436e88d 100644
--- a/src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs
+++ b/src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs
@@ -5,9 +5,11 @@ pub mod diagnostics;
 mod item;
 mod stack;
 
-use log::trace;
 use std::cmp;
 use std::fmt::Write;
+use std::mem;
+
+use log::trace;
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::mir::{Mutability, RetagKind};
@@ -19,12 +21,12 @@ use rustc_middle::ty::{
 use rustc_target::abi::{Abi, Size};
 
 use crate::borrow_tracker::{
-    stacked_borrows::diagnostics::{AllocHistory, DiagnosticCx, DiagnosticCxBuilder, TagHistory},
+    stacked_borrows::diagnostics::{AllocHistory, DiagnosticCx, DiagnosticCxBuilder},
     AccessKind, GlobalStateInner, ProtectorKind, RetagFields,
 };
 use crate::*;
 
-use diagnostics::RetagCause;
+use diagnostics::{RetagCause, RetagInfo};
 pub use item::{Item, Permission};
 pub use stack::Stack;
 
@@ -168,15 +170,6 @@ impl NewPermission {
     }
 }
 
-/// Error reporting
-pub fn err_sb_ub<'tcx>(
-    msg: String,
-    help: Option<String>,
-    history: Option<TagHistory>,
-) -> InterpError<'tcx> {
-    err_machine_stop!(TerminationInfo::StackedBorrowsUb { msg, help, history })
-}
-
 // # Stacked Borrows Core Begin
 
 /// We need to make at least the following things true:
@@ -623,7 +616,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         size: Size,
         new_perm: NewPermission,
         new_tag: BorTag,
-        retag_cause: RetagCause, // What caused this retag, for diagnostics only
+        retag_info: RetagInfo, // diagnostics info about this retag
     ) -> InterpResult<'tcx, Option<AllocId>> {
         let this = self.eval_context_mut();
 
@@ -670,7 +663,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
                     // FIXME: can this be done cleaner?
                     let dcx = DiagnosticCxBuilder::retag(
                         &this.machine,
-                        retag_cause,
+                        retag_info,
                         new_tag,
                         orig_tag,
                         alloc_range(base_offset, size),
@@ -761,7 +754,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
                 let global = machine.borrow_tracker.as_ref().unwrap().borrow();
                 let dcx = DiagnosticCxBuilder::retag(
                     machine,
-                    retag_cause,
+                    retag_info,
                     new_tag,
                     orig_tag,
                     alloc_range(base_offset, size),
@@ -804,7 +797,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
                     let global = this.machine.borrow_tracker.as_ref().unwrap().borrow();
                     let dcx = DiagnosticCxBuilder::retag(
                         &this.machine,
-                        retag_cause,
+                        retag_info,
                         new_tag,
                         orig_tag,
                         alloc_range(base_offset, size),
@@ -834,7 +827,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         &mut self,
         val: &ImmTy<'tcx, Provenance>,
         new_perm: NewPermission,
-        cause: RetagCause, // What caused this retag, for diagnostics only
+        info: RetagInfo, // diagnostics info about this retag
     ) -> InterpResult<'tcx, ImmTy<'tcx, Provenance>> {
         let this = self.eval_context_mut();
         // We want a place for where the ptr *points to*, so we get one.
@@ -852,7 +845,7 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         let new_tag = this.machine.borrow_tracker.as_mut().unwrap().get_mut().new_ptr();
 
         // Reborrow.
-        let alloc_id = this.sb_reborrow(&place, size, new_perm, new_tag, cause)?;
+        let alloc_id = this.sb_reborrow(&place, size, new_perm, new_tag, info)?;
 
         // Adjust pointer.
         let new_place = place.map_provenance(|p| {
@@ -886,12 +879,12 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
     ) -> InterpResult<'tcx, ImmTy<'tcx, Provenance>> {
         let this = self.eval_context_mut();
         let new_perm = NewPermission::from_ref_ty(val.layout.ty, kind, this);
-        let retag_cause = match kind {
+        let cause = match kind {
             RetagKind::TwoPhase { .. } => RetagCause::TwoPhase,
             RetagKind::FnEntry => unreachable!(),
             RetagKind::Raw | RetagKind::Default => RetagCause::Normal,
         };
-        this.sb_retag_reference(val, new_perm, retag_cause)
+        this.sb_retag_reference(val, new_perm, RetagInfo { cause, in_field: false })
     }
 
     fn sb_retag_place_contents(
@@ -906,7 +899,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             RetagKind::FnEntry => RetagCause::FnEntry,
             RetagKind::Default => RetagCause::Normal,
         };
-        let mut visitor = RetagVisitor { ecx: this, kind, retag_cause, retag_fields };
+        let mut visitor =
+            RetagVisitor { ecx: this, kind, retag_cause, retag_fields, in_field: false };
         return visitor.visit_value(place);
 
         // The actual visitor.
@@ -915,6 +909,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             kind: RetagKind,
             retag_cause: RetagCause,
             retag_fields: RetagFields,
+            in_field: bool,
         }
         impl<'ecx, 'mir, 'tcx> RetagVisitor<'ecx, 'mir, 'tcx> {
             #[inline(always)] // yes this helps in our benchmarks
@@ -922,10 +917,13 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 &mut self,
                 place: &PlaceTy<'tcx, Provenance>,
                 new_perm: NewPermission,
-                retag_cause: RetagCause,
             ) -> InterpResult<'tcx> {
                 let val = self.ecx.read_immediate(&self.ecx.place_to_op(place)?)?;
-                let val = self.ecx.sb_retag_reference(&val, new_perm, retag_cause)?;
+                let val = self.ecx.sb_retag_reference(
+                    &val,
+                    new_perm,
+                    RetagInfo { cause: self.retag_cause, in_field: self.in_field },
+                )?;
                 self.ecx.write_immediate(*val, place)?;
                 Ok(())
             }
@@ -943,7 +941,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             fn visit_box(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx> {
                 // Boxes get a weak protectors, since they may be deallocated.
                 let new_perm = NewPermission::from_box_ty(place.layout.ty, self.kind, self.ecx);
-                self.retag_ptr_inplace(place, new_perm, self.retag_cause)
+                self.retag_ptr_inplace(place, new_perm)
             }
 
             fn visit_value(&mut self, place: &PlaceTy<'tcx, Provenance>) -> InterpResult<'tcx> {
@@ -960,7 +958,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                     ty::Ref(..) => {
                         let new_perm =
                             NewPermission::from_ref_ty(place.layout.ty, self.kind, self.ecx);
-                        self.retag_ptr_inplace(place, new_perm, self.retag_cause)?;
+                        self.retag_ptr_inplace(place, new_perm)?;
                     }
                     ty::RawPtr(..) => {
                         // We do *not* want to recurse into raw pointers -- wide raw pointers have
@@ -984,7 +982,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                             }
                         };
                         if recurse {
+                            let in_field = mem::replace(&mut self.in_field, true); // remember and restore old value
                             self.walk_value(place)?;
+                            self.in_field = in_field;
                         }
                     }
                 }
@@ -1011,7 +1011,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             access: Some(AccessKind::Write),
             protector: Some(ProtectorKind::StrongProtector),
         };
-        let _new_ptr = this.sb_retag_reference(&ptr, new_perm, RetagCause::InPlaceFnPassing)?;
+        let _new_ptr = this.sb_retag_reference(
+            &ptr,
+            new_perm,
+            RetagInfo { cause: RetagCause::InPlaceFnPassing, in_field: false },
+        )?;
         // We just throw away `new_ptr`, so nobody can access this memory while it is protected.
 
         Ok(())
diff --git a/src/tools/miri/src/diagnostics.rs b/src/tools/miri/src/diagnostics.rs
index 2a06bd871ef..8d9901807ec 100644
--- a/src/tools/miri/src/diagnostics.rs
+++ b/src/tools/miri/src/diagnostics.rs
@@ -22,7 +22,7 @@ pub enum TerminationInfo {
     UnsupportedInIsolation(String),
     StackedBorrowsUb {
         msg: String,
-        help: Option<String>,
+        help: Vec<String>,
         history: Option<TagHistory>,
     },
     TreeBorrowsUb {
@@ -224,11 +224,10 @@ pub fn report_error<'tcx, 'mir>(
                     (None, format!("or pass `-Zmiri-isolation-error=warn` to configure Miri to return an error code from isolated operations (if supported for that operation) and continue with a warning")),
                 ],
             StackedBorrowsUb { help, history, .. } => {
-                let url = "https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md";
                 msg.extend(help.clone());
                 let mut helps = vec![
                     (None, format!("this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental")),
-                    (None, format!("see {url} for further information")),
+                    (None, format!("see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information")),
                 ];
                 if let Some(TagHistory {created, invalidated, protected}) = history.clone() {
                     helps.push((Some(created.1), created.0));
diff --git a/src/tools/miri/src/eval.rs b/src/tools/miri/src/eval.rs
index ed3d741db1c..79a7115f24e 100644
--- a/src/tools/miri/src/eval.rs
+++ b/src/tools/miri/src/eval.rs
@@ -183,7 +183,7 @@ impl Default for MiriConfig {
             mute_stdout_stderr: false,
             preemption_rate: 0.01, // 1%
             report_progress: None,
-            retag_fields: RetagFields::OnlyScalar,
+            retag_fields: RetagFields::Yes,
             external_so_file: None,
             gc_interval: 10_000,
             num_cpus: 1,
diff --git a/src/tools/miri/tests/fail/both_borrows/buggy_split_at_mut.rs b/src/tools/miri/tests/fail/both_borrows/buggy_split_at_mut.rs
index 4dc6eaf7cd3..80ededd2100 100644
--- a/src/tools/miri/tests/fail/both_borrows/buggy_split_at_mut.rs
+++ b/src/tools/miri/tests/fail/both_borrows/buggy_split_at_mut.rs
@@ -14,6 +14,7 @@ mod safe {
                 from_raw_parts_mut(ptr, len - mid), // BUG: should be "mid" instead of "len - mid"
                 from_raw_parts_mut(ptr.offset(mid as isize), len - mid),
             )
+            //~[stack]^^^^ ERROR: /retag .* tag does not exist in the borrow stack/
         }
     }
 }
@@ -21,7 +22,6 @@ mod safe {
 fn main() {
     let mut array = [1, 2, 3, 4];
     let (a, b) = safe::split_at_mut(&mut array, 0);
-    //~[stack]^ ERROR: /retag .* tag does not exist in the borrow stack/
     a[1] = 5;
     b[1] = 6;
     //~[tree]^ ERROR: /write access through .* is forbidden/
diff --git a/src/tools/miri/tests/fail/both_borrows/buggy_split_at_mut.stack.stderr b/src/tools/miri/tests/fail/both_borrows/buggy_split_at_mut.stack.stderr
index c75d8cab3fc..daa4339225d 100644
--- a/src/tools/miri/tests/fail/both_borrows/buggy_split_at_mut.stack.stderr
+++ b/src/tools/miri/tests/fail/both_borrows/buggy_split_at_mut.stack.stderr
@@ -1,11 +1,15 @@
 error: Undefined Behavior: trying to retag from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/buggy_split_at_mut.rs:LL:CC
    |
-LL |     let (a, b) = safe::split_at_mut(&mut array, 0);
-   |          ^
-   |          |
-   |          trying to retag from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
-   |          this error occurs as part of retag at ALLOC[0x0..0x10]
+LL | /             (
+LL | |                 from_raw_parts_mut(ptr, len - mid), // BUG: should be "mid" instead of "len - mid"
+LL | |                 from_raw_parts_mut(ptr.offset(mid as isize), len - mid),
+LL | |             )
+   | |             ^
+   | |             |
+   | |             trying to retag from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   | |_____________this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x0..0x10]
+   |               errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
@@ -20,7 +24,12 @@ help: <TAG> was later invalidated at offsets [0x0..0x10] by a Unique retag
 LL |                 from_raw_parts_mut(ptr.offset(mid as isize), len - mid),
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: BACKTRACE (of the first span):
-   = note: inside `main` at $DIR/buggy_split_at_mut.rs:LL:CC
+   = note: inside `safe::split_at_mut::<i32>` at $DIR/buggy_split_at_mut.rs:LL:CC
+note: inside `main`
+  --> $DIR/buggy_split_at_mut.rs:LL:CC
+   |
+LL |     let (a, b) = safe::split_at_mut(&mut array, 0);
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 
diff --git a/src/tools/miri/tests/fail/both_borrows/pass_invalid_shr_option.stack.stderr b/src/tools/miri/tests/fail/both_borrows/pass_invalid_shr_option.stack.stderr
index e35d7918c03..26d9f38f239 100644
--- a/src/tools/miri/tests/fail/both_borrows/pass_invalid_shr_option.stack.stderr
+++ b/src/tools/miri/tests/fail/both_borrows/pass_invalid_shr_option.stack.stderr
@@ -5,7 +5,8 @@ LL |     foo(some_xref);
    |         ^^^^^^^^^
    |         |
    |         trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
-   |         this error occurs as part of retag at ALLOC[0x0..0x4]
+   |         this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x0..0x4]
+   |         errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
diff --git a/src/tools/miri/tests/fail/both_borrows/pass_invalid_shr_tuple.stack.stderr b/src/tools/miri/tests/fail/both_borrows/pass_invalid_shr_tuple.stack.stderr
index 10c566d0612..5f0fbf12759 100644
--- a/src/tools/miri/tests/fail/both_borrows/pass_invalid_shr_tuple.stack.stderr
+++ b/src/tools/miri/tests/fail/both_borrows/pass_invalid_shr_tuple.stack.stderr
@@ -5,7 +5,8 @@ LL |     foo(pair_xref);
    |         ^^^^^^^^^
    |         |
    |         trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
-   |         this error occurs as part of retag at ALLOC[0x0..0x4]
+   |         this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x0..0x4]
+   |         errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
diff --git a/src/tools/miri/tests/fail/both_borrows/return_invalid_shr_option.stack.stderr b/src/tools/miri/tests/fail/both_borrows/return_invalid_shr_option.stack.stderr
index f14e8b8532f..7a9f061228a 100644
--- a/src/tools/miri/tests/fail/both_borrows/return_invalid_shr_option.stack.stderr
+++ b/src/tools/miri/tests/fail/both_borrows/return_invalid_shr_option.stack.stderr
@@ -5,7 +5,8 @@ LL |     ret
    |     ^^^
    |     |
    |     trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
-   |     this error occurs as part of retag at ALLOC[0x4..0x8]
+   |     this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x4..0x8]
+   |     errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
diff --git a/src/tools/miri/tests/fail/both_borrows/return_invalid_shr_tuple.stack.stderr b/src/tools/miri/tests/fail/both_borrows/return_invalid_shr_tuple.stack.stderr
index 9ddaad4d1be..6a98c9121ef 100644
--- a/src/tools/miri/tests/fail/both_borrows/return_invalid_shr_tuple.stack.stderr
+++ b/src/tools/miri/tests/fail/both_borrows/return_invalid_shr_tuple.stack.stderr
@@ -5,7 +5,8 @@ LL |     ret
    |     ^^^
    |     |
    |     trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
-   |     this error occurs as part of retag at ALLOC[0x4..0x8]
+   |     this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x4..0x8]
+   |     errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
diff --git a/src/tools/miri/tests/fail/generator-pinned-moved.rs b/src/tools/miri/tests/fail/generator-pinned-moved.rs
index 240ae18cc45..29dc6e56f7c 100644
--- a/src/tools/miri/tests/fail/generator-pinned-moved.rs
+++ b/src/tools/miri/tests/fail/generator-pinned-moved.rs
@@ -1,4 +1,4 @@
-//@compile-flags: -Zmiri-disable-validation
+//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
 #![feature(generators, generator_trait)]
 
 use std::{
@@ -10,9 +10,10 @@ fn firstn() -> impl Generator<Yield = u64, Return = ()> {
     static move || {
         let mut num = 0;
         let num = &mut num;
+        *num += 0;
 
         yield *num;
-        *num += 1; //~ ERROR: dereferenced after this allocation got freed
+        *num += 1; //~ERROR: dereferenced after this allocation got freed
     }
 }
 
diff --git a/src/tools/miri/tests/fail/stacked_borrows/return_invalid_mut_option.stderr b/src/tools/miri/tests/fail/stacked_borrows/return_invalid_mut_option.stderr
index ff00c54570c..d357ab9639b 100644
--- a/src/tools/miri/tests/fail/stacked_borrows/return_invalid_mut_option.stderr
+++ b/src/tools/miri/tests/fail/stacked_borrows/return_invalid_mut_option.stderr
@@ -5,7 +5,8 @@ LL |     ret
    |     ^^^
    |     |
    |     trying to retag from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
-   |     this error occurs as part of retag at ALLOC[0x4..0x8]
+   |     this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x4..0x8]
+   |     errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
diff --git a/src/tools/miri/tests/fail/stacked_borrows/return_invalid_mut_tuple.stderr b/src/tools/miri/tests/fail/stacked_borrows/return_invalid_mut_tuple.stderr
index 61d041a8816..d346e6fa895 100644
--- a/src/tools/miri/tests/fail/stacked_borrows/return_invalid_mut_tuple.stderr
+++ b/src/tools/miri/tests/fail/stacked_borrows/return_invalid_mut_tuple.stderr
@@ -5,7 +5,8 @@ LL |     ret
    |     ^^^
    |     |
    |     trying to retag from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
-   |     this error occurs as part of retag at ALLOC[0x4..0x8]
+   |     this error occurs as part of retag (of a reference/box inside this compound value) at ALLOC[0x4..0x8]
+   |     errors for retagging in fields are fairly new; please reach out to us (e.g. at <https://rust-lang.zulipchat.com/#narrow/stream/269128-miri>) if you find this error troubling
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
diff --git a/src/tools/miri/tests/pass/generator.rs b/src/tools/miri/tests/pass/generator.rs
index e059c7114e3..20099603455 100644
--- a/src/tools/miri/tests/pass/generator.rs
+++ b/src/tools/miri/tests/pass/generator.rs
@@ -13,7 +13,7 @@ use std::ptr;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 fn basic() {
-    fn finish<T>(mut amt: usize, mut t: T) -> T::Return
+    fn finish<T>(mut amt: usize, self_referential: bool, mut t: T) -> T::Return
     where
         T: Generator<Yield = usize>,
     {
@@ -22,7 +22,10 @@ fn basic() {
         loop {
             let state = t.as_mut().resume(());
             // Test if the generator is valid (according to type invariants).
-            let _ = unsafe { ManuallyDrop::new(ptr::read(t.as_mut().get_unchecked_mut())) };
+            // For self-referential generators however this is UB!
+            if !self_referential {
+                let _ = unsafe { ManuallyDrop::new(ptr::read(t.as_mut().get_unchecked_mut())) };
+            }
             match state {
                 GeneratorState::Yielded(y) => {
                     amt -= y;
@@ -40,9 +43,9 @@ fn basic() {
         panic!()
     }
 
-    finish(1, || yield 1);
+    finish(1, false, || yield 1);
 
-    finish(3, || {
+    finish(3, false, || {
         let mut x = 0;
         yield 1;
         x += 1;
@@ -52,27 +55,27 @@ fn basic() {
         assert_eq!(x, 2);
     });
 
-    finish(7 * 8 / 2, || {
+    finish(7 * 8 / 2, false, || {
         for i in 0..8 {
             yield i;
         }
     });
 
-    finish(1, || {
+    finish(1, false, || {
         if true {
             yield 1;
         } else {
         }
     });
 
-    finish(1, || {
+    finish(1, false, || {
         if false {
         } else {
             yield 1;
         }
     });
 
-    finish(2, || {
+    finish(2, false, || {
         if {
             yield 1;
             false
@@ -83,9 +86,20 @@ fn basic() {
         yield 1;
     });
 
-    // also test a self-referential generator
+    // also test self-referential generators
+    assert_eq!(
+        finish(5, true, static || {
+            let mut x = 5;
+            let y = &mut x;
+            *y = 5;
+            yield *y;
+            *y = 10;
+            x
+        }),
+        10
+    );
     assert_eq!(
-        finish(5, || {
+        finish(5, true, || {
             let mut x = Box::new(5);
             let y = &mut *x;
             *y = 5;
@@ -97,7 +111,7 @@ fn basic() {
     );
 
     let b = true;
-    finish(1, || {
+    finish(1, false, || {
         yield 1;
         if b {
             return;
@@ -109,7 +123,7 @@ fn basic() {
         drop(x);
     });
 
-    finish(3, || {
+    finish(3, false, || {
         yield 1;
         #[allow(unreachable_code)]
         let _x: (String, !) = (String::new(), {
diff --git a/src/tools/miri/tests/pass/stacked-borrows/interior_mutability.rs b/src/tools/miri/tests/pass/stacked-borrows/interior_mutability.rs
index c6373a7eaf1..830e9c33847 100644
--- a/src/tools/miri/tests/pass/stacked-borrows/interior_mutability.rs
+++ b/src/tools/miri/tests/pass/stacked-borrows/interior_mutability.rs
@@ -1,4 +1,3 @@
-//@compile-flags: -Zmiri-retag-fields
 use std::cell::{Cell, Ref, RefCell, RefMut, UnsafeCell};
 use std::mem::{self, MaybeUninit};
 
diff --git a/src/tools/miri/tests/pass/stacked-borrows/stacked-borrows.rs b/src/tools/miri/tests/pass/stacked-borrows/stacked-borrows.rs
index d7d7d1f97d6..43ae7d6f522 100644
--- a/src/tools/miri/tests/pass/stacked-borrows/stacked-borrows.rs
+++ b/src/tools/miri/tests/pass/stacked-borrows/stacked-borrows.rs
@@ -1,4 +1,3 @@
-//@compile-flags: -Zmiri-retag-fields
 #![feature(allocator_api)]
 use std::ptr;
 
diff --git a/src/tools/miri/tests/pass/stacked-borrows/zst-field-retagging-terminates.rs b/src/tools/miri/tests/pass/stacked-borrows/zst-field-retagging-terminates.rs
index 6e13a9ea836..4faf6004ad6 100644
--- a/src/tools/miri/tests/pass/stacked-borrows/zst-field-retagging-terminates.rs
+++ b/src/tools/miri/tests/pass/stacked-borrows/zst-field-retagging-terminates.rs
@@ -1,4 +1,3 @@
-//@compile-flags: -Zmiri-retag-fields
 // Checks that the test does not run forever (which relies on a fast path).
 
 #![allow(dropping_copy_types)]