Rollup merge of rust-lang#70264 - tirr-c:issue-69789-mut-suggestion, …

…r=estebank

Fix invalid suggestion on `&mut` iterators yielding `&` references

Fixes rust-lang#69789.

rustc suggested an invalid code when `&` reference from `&mut` iterator is mutated. The compiler knew we're mutating a value behind `&` reference, but as the assignment RHS is from desugaring, it could only see the iterator expression from source and inserted `mut` there.

r? @estebank

src/librustc_mir/borrow_check/diagnostics/mutability_errors.rs

@@ -1,9 +1,10 @@
-use rustc::mir::{self, ClearCrossCrate, Local, LocalInfo, Location, ReadOnlyBodyAndCache};
+use rustc::mir::{self, ClearCrossCrate, Local, LocalInfo, Location};
 use rustc::mir::{Mutability, Place, PlaceRef, ProjectionElem};
 use rustc::ty::{self, Ty, TyCtxt};
 use rustc_hir as hir;
 use rustc_hir::Node;
 use rustc_index::vec::Idx;
+use rustc_span::source_map::DesugaringKind;
 use rustc_span::symbol::kw;
 use rustc_span::Span;
 
@@ -338,24 +339,53 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
  match self.local_names[local] {
  Some(name) if !local_decl.from_compiler_desugaring() => {
- let suggestion = match local_decl.local_info {
+ let label = match local_decl.local_info {
  LocalInfo::User(ClearCrossCrate::Set(
  mir::BindingForm::ImplicitSelf(_),
- )) => Some(suggest_ampmut_self(self.infcx.tcx, local_decl)),
+ )) => {
+ let (span, suggestion) =
+ suggest_ampmut_self(self.infcx.tcx, local_decl);
+ Some((true, span, suggestion))
+ }
 
  LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::Var(
  mir::VarBindingForm {
  binding_mode: ty::BindingMode::BindByValue(_),
  opt_ty_info,
  ..
  },
- ))) => Some(suggest_ampmut(
- self.infcx.tcx,
- self.body,
- local,
- local_decl,
- opt_ty_info,
- )),
+ ))) => {
+ // check if the RHS is from desugaring
+ let locations = self.body.find_assignments(local);
+ let opt_assignment_rhs_span = locations
+ .first()
+ .map(|&location| self.body.source_info(location).span);
+ let opt_desugaring_kind =
+ opt_assignment_rhs_span.and_then(|span| span.desugaring_kind());
+ match opt_desugaring_kind {
+ // on for loops, RHS points to the iterator part
+ Some(DesugaringKind::ForLoop) => Some((
+ false,
+ opt_assignment_rhs_span.unwrap(),
+ format!(
+ "this iterator yields `{SIGIL}` {DESC}s",
+ SIGIL = pointer_sigil,
+ DESC = pointer_desc
+ ),
+ )),
+ // don't create labels for compiler-generated spans
+ Some(_) => None,
+ None => {
+ let (span, suggestion) = suggest_ampmut(
+ self.infcx.tcx,
+ local_decl,
+ opt_assignment_rhs_span,
+ opt_ty_info,
+ );
+ Some((true, span, suggestion))
+ }
+ }
+ }
 
  LocalInfo::User(ClearCrossCrate::Set(mir::BindingForm::Var(
  mir::VarBindingForm {
@@ -365,7 +395,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
  ))) => {
  let pattern_span = local_decl.source_info.span;
  suggest_ref_mut(self.infcx.tcx, pattern_span)
- .map(|replacement| (pattern_span, replacement))
+ .map(|replacement| (true, pattern_span, replacement))
  }
 
  LocalInfo::User(ClearCrossCrate::Clear) => {
@@ -375,13 +405,22 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
  _ => unreachable!(),
  };
 
- if let Some((err_help_span, suggested_code)) = suggestion {
- err.span_suggestion(
- err_help_span,
- &format!("consider changing this to be a mutable {}", pointer_desc),
- suggested_code,
- Applicability::MachineApplicable,
- );
+ match label {
+ Some((true, err_help_span, suggested_code)) => {
+ err.span_suggestion(
+ err_help_span,
+ &format!(
+ "consider changing this to be a mutable {}",
+ pointer_desc
+ ),
+ suggested_code,
+ Applicability::MachineApplicable,
+ );
+ }
+ Some((false, err_label_span, message)) => {
+ err.span_label(err_label_span, &message);
+ }
+ None => {}
  }
  err.span_label(
  span,
@@ -581,14 +620,11 @@ fn suggest_ampmut_self<'tcx>(
 // by trying (3.), then (2.) and finally falling back on (1.).
 fn suggest_ampmut<'tcx>(
  tcx: TyCtxt<'tcx>,
- body: ReadOnlyBodyAndCache<'_, 'tcx>,
- local: Local,
  local_decl: &mir::LocalDecl<'tcx>,
+ opt_assignment_rhs_span: Option<Span>,
  opt_ty_info: Option<Span>,
 ) -> (Span, String) {
- let locations = body.find_assignments(local);
- if !locations.is_empty() {
- let assignment_rhs_span = body.source_info(locations[0]).span;
+ if let Some(assignment_rhs_span) = opt_assignment_rhs_span {
  if let Ok(src) = tcx.sess.source_map().span_to_snippet(assignment_rhs_span) {
  if let (true, Some(ws_pos)) =
  (src.starts_with("&'"), src.find(|c: char| -> bool { c.is_whitespace() }))

src/test/ui/borrowck/issue-69789-iterator-mut-suggestion.rs

@@ -0,0 +1,11 @@
+// Regression test for #69789: rustc generated an invalid suggestion
+// when `&` reference from `&mut` iterator is mutated.
+
+fn main() {
+ for item in &mut std::iter::empty::<&'static ()>() {
+ //~^ NOTE this iterator yields `&` references
+ *item = ();
+ //~^ ERROR cannot assign
+ //~| NOTE cannot be written
+ }
+}

src/test/ui/borrowck/issue-69789-iterator-mut-suggestion.stderr

@@ -0,0 +1,12 @@
+error[E0594]: cannot assign to `*item` which is behind a `&` reference
+ --> $DIR/issue-69789-iterator-mut-suggestion.rs:7:9
+ |
+LL | for item in &mut std::iter::empty::<&'static ()>() {
+ | -------------------------------------- this iterator yields `&` references
+LL |
+LL | *item = ();
+ | ^^^^^^^^^^ `item` is a `&` reference, so the data it refers to cannot be written
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.