diff --git a/src/librustc_mir/borrow_check/mutability_errors.rs b/src/librustc_mir/borrow_check/mutability_errors.rs index ba625fb08c82c..fe24ec382ceb3 100644 --- a/src/librustc_mir/borrow_check/mutability_errors.rs +++ b/src/librustc_mir/borrow_check/mutability_errors.rs @@ -218,6 +218,33 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> { debug!("report_mutability_error: act={:?}, acted_on={:?}", act, acted_on); match the_place_err { + // Suggest making an existing shared borrow in a struct definition a mutable borrow. + // + // This is applicable when we have a deref of a field access to a deref of a local - + // something like `*((*_1).0`. The local that we get will be a reference to the + // struct we've got a field access of (it must be a reference since there's a deref + // after the field access). + Place::Projection(box Projection { + base: Place::Projection(box Projection { + base: Place::Projection(box Projection { + base, + elem: ProjectionElem::Deref, + }), + elem: ProjectionElem::Field(field, _), + }), + elem: ProjectionElem::Deref, + }) => { + err.span_label(span, format!("cannot {ACT}", ACT = act)); + + if let Some((span, message)) = annotate_struct_field( + self.infcx.tcx, + base.ty(self.mir, self.infcx.tcx).to_ty(self.infcx.tcx), + field, + ) { + err.span_label(span, message); + } + }, + // Suggest removing a `&mut` from the use of a mutable reference. Place::Local(local) if { @@ -592,3 +619,56 @@ fn suggest_ampmut<'cx, 'gcx, 'tcx>( fn is_closure_or_generator(ty: ty::Ty) -> bool { ty.is_closure() || ty.is_generator() } + +/// Add a suggestion to a struct definition given a field access to a local. +/// This function expects the local to be a reference to a struct in order to produce a suggestion. +/// +/// ```text +/// LL | s: &'a String +/// | ---------- use `&'a mut String` here to make mutable +/// ``` +fn annotate_struct_field( + tcx: TyCtxt<'cx, 'gcx, 'tcx>, + ty: ty::Ty<'tcx>, + field: &mir::Field, +) -> Option<(Span, String)> { + // Expect our local to be a reference to a struct of some kind. + if let ty::TyKind::Ref(_, ty, _) = ty.sty { + if let ty::TyKind::Adt(def, _) = ty.sty { + let field = def.all_fields().nth(field.index())?; + let span = tcx.def_span(field.did); + + // Use the HIR types to construct the diagnostic message. + let node_id = tcx.hir.as_local_node_id(field.did)?; + let node = tcx.hir.find(node_id)?; + // Now we're dealing with the actual struct that we're going to suggest a change to, + // we can expect a field that is an immutable reference to a type. + if let hir::Node::Field(field) = node { + if let hir::TyKind::Rptr(lifetime, hir::MutTy { + mutbl: hir::Mutability::MutImmutable, + ref ty + }) = field.ty.node { + // Get the snippets in two parts - the named lifetime (if there is one) and + // type being referenced, that way we can reconstruct the snippet without loss + // of detail. + let type_snippet = tcx.sess.source_map().span_to_snippet(ty.span).ok()?; + let lifetime_snippet = if !lifetime.is_elided() { + format!("{} ", tcx.sess.source_map().span_to_snippet(lifetime.span).ok()?) + } else { + String::new() + }; + + return Some(( + span, + format!( + "use `&{}mut {}` here to make mutable", + lifetime_snippet, &*type_snippet, + ), + )); + } + } + } + } + + None +} diff --git a/src/test/ui/did_you_mean/issue-38147-2.nll.stderr b/src/test/ui/did_you_mean/issue-38147-2.nll.stderr index 21fc4079d5b59..91ccef1a32f1f 100644 --- a/src/test/ui/did_you_mean/issue-38147-2.nll.stderr +++ b/src/test/ui/did_you_mean/issue-38147-2.nll.stderr @@ -1,6 +1,9 @@ error[E0596]: cannot borrow `*self.s` as mutable, as it is behind a `&` reference --> $DIR/issue-38147-2.rs:17:9 | +LL | s: &'a String + | ------------- use `&'a mut String` here to make mutable +... LL | self.s.push('x'); | ^^^^^^ cannot borrow as mutable diff --git a/src/test/ui/did_you_mean/issue-38147-3.nll.stderr b/src/test/ui/did_you_mean/issue-38147-3.nll.stderr index d426c1f37fc1c..ecc80a1d4310b 100644 --- a/src/test/ui/did_you_mean/issue-38147-3.nll.stderr +++ b/src/test/ui/did_you_mean/issue-38147-3.nll.stderr @@ -1,6 +1,9 @@ error[E0596]: cannot borrow `*self.s` as mutable, as it is behind a `&` reference --> $DIR/issue-38147-3.rs:17:9 | +LL | s: &'a String + | ------------- use `&'a mut String` here to make mutable +... LL | self.s.push('x'); | ^^^^^^ cannot borrow as mutable