Suppress E0621 perpetual borrow suggestion

compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/named_anon_conflict.rs

@@ -2,6 +2,7 @@
 //! where one region is named and the other is anonymous.
 
 use rustc_errors::Diag;
+use rustc_middle::ty;
 use tracing::debug;
 
 use crate::error_reporting::infer::nice_region_error::NiceRegionError;
@@ -72,16 +73,34 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         {
             return None;
         }
+        let orig_ty = anon_param_info.orig_param_ty;
+        // Don't suggest naming the outer lifetime when it already appears in the pointee
+        // (e.g. `&'a mut Buffer<'a>`); point at borrow splitting instead.
+        let suggestion_would_alias_lifetime =
+            if let ty::Ref(_, orig_inner_ty, ty::Mutability::Mut) = orig_ty.kind() {
+                self.tcx().any_free_region_meets(orig_inner_ty, |r| r == named)
+            } else {
+                false
+            };
+
         let named = named.to_string();
+        let new_ty_span = if suggestion_would_alias_lifetime { None } else { Some(new_ty_span) };
         let err = match param.pat.simple_ident() {
             Some(simple_ident) => ExplicitLifetimeRequired::WithIdent {
                 span,
                 simple_ident,
                 named,
                 new_ty_span,
                 new_ty,
+                link_nomicon: suggestion_would_alias_lifetime,
+            },
+            None => ExplicitLifetimeRequired::WithParamType {
+                span,
+                named,
+                new_ty_span,
+                new_ty,
+                link_nomicon: suggestion_would_alias_lifetime,
             },
-            None => ExplicitLifetimeRequired::WithParamType { span, named, new_ty_span, new_ty },
         };
         Some(self.tcx().sess.dcx().create_err(err))
     }

compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/util.rs

@@ -16,6 +16,8 @@ pub struct AnonymousParamInfo<'tcx> {
     pub param: &'tcx hir::Param<'tcx>,
     /// The type corresponding to the anonymous region parameter.
     pub param_ty: Ty<'tcx>,
+    /// The original type before region replacement.
+    pub orig_param_ty: Ty<'tcx>,
     /// The `ty::LateParamRegionKind` corresponding to the anonymous region.
     pub kind: ty::LateParamRegionKind,
     /// The `Span` of the parameter type.
@@ -94,7 +96,14 @@ pub fn find_param_with_region<'tcx>(
                 let ty_hir_id = fn_decl.inputs[index].hir_id;
                 let param_ty_span = tcx.hir_span(ty_hir_id);
                 let is_first = index == 0;
-                AnonymousParamInfo { param, param_ty: new_param_ty, param_ty_span, kind, is_first }
+                AnonymousParamInfo {
+                    param,
+                    param_ty: new_param_ty,
+                    orig_param_ty: ty,
+                    param_ty_span,
+                    kind,
+                    is_first,
+                }
             })
         })
 }

compiler/rustc_trait_selection/src/errors.rs

@@ -868,8 +868,13 @@ pub(crate) enum ExplicitLifetimeRequired<'a> {
             applicability = "unspecified",
             style = "verbose"
         )]
-        new_ty_span: Span,
+        new_ty_span: Option<Span>,
         new_ty: Ty<'a>,
+        #[help(
+            "see <https://doc.rust-lang.org/nomicon/borrow-splitting.html> \
+             for more information about lifetime errors related to mutable references"
+        )]
+        link_nomicon: bool,
     },
     #[diag("explicit lifetime required in parameter type", code = E0621)]
     WithParamType {
@@ -883,8 +888,13 @@ pub(crate) enum ExplicitLifetimeRequired<'a> {
             applicability = "unspecified",
             style = "verbose"
         )]
-        new_ty_span: Span,
+        new_ty_span: Option<Span>,
         new_ty: Ty<'a>,
+        #[help(
+            "see <https://doc.rust-lang.org/nomicon/borrow-splitting.html> \
+             for more information about lifetime errors related to mutable references"
+        )]
+        link_nomicon: bool,
     },
 }
 

tests/ui/lifetimes/lifetime-errors/e0621-mut-ref-aliases-pointee-lifetime-distinct.rs

@@ -0,0 +1,10 @@
+// Regression test for #156682
+// When the suggested type would NOT alias the outer reference's lifetime with one
+// already used inside the pointee, the normal suggestion should still apply.
+
+pub fn push<'a>(x: &mut Vec<&'a u8>, y: &u8) {
+    x.push(y);
+    //~^ ERROR explicit lifetime required in the type of `y`
+}
+
+fn main() {}

tests/ui/lifetimes/lifetime-errors/e0621-mut-ref-aliases-pointee-lifetime-distinct.stderr

@@ -0,0 +1,14 @@
+error[E0621]: explicit lifetime required in the type of `y`
+  --> $DIR/e0621-mut-ref-aliases-pointee-lifetime-distinct.rs:6:5
+   |
+LL |     x.push(y);
+   |     ^^^^^^^^^ lifetime `'a` required
+   |
+help: add explicit lifetime `'a` to the type of `y`
+   |
+LL | pub fn push<'a>(x: &mut Vec<&'a u8>, y: &'a u8) {
+   |                                          ++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0621`.

tests/ui/lifetimes/lifetime-errors/e0621-mut-ref-aliases-pointee-lifetime.rs

@@ -0,0 +1,23 @@
+// Regression test for #156682
+// E0621 should not suggest `&'a mut Buffer<'a>`, which would alias the outer
+// reference's lifetime with one already used inside the pointee.
+
+pub struct Buffer<'a> {
+    buf: &'a mut [u8],
+}
+
+pub fn foo<'a>(buffer: &mut Buffer<'a>) {
+    buffer.buf = &mut buffer.buf[..];
+    //~^ ERROR explicit lifetime required in the type of `buffer`
+}
+
+pub struct Wrapper<'a> {
+    inner: Buffer<'a>,
+}
+
+pub fn baz<'a>(wrapper: &mut Wrapper<'a>) {
+    wrapper.inner.buf = &mut wrapper.inner.buf[..];
+    //~^ ERROR explicit lifetime required in the type of `wrapper`
+}
+
+fn main() {}

tests/ui/lifetimes/lifetime-errors/e0621-mut-ref-aliases-pointee-lifetime.stderr

@@ -0,0 +1,19 @@
+error[E0621]: explicit lifetime required in the type of `buffer`
+  --> $DIR/e0621-mut-ref-aliases-pointee-lifetime.rs:10:5
+   |
+LL |     buffer.buf = &mut buffer.buf[..];
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime `'a` required
+   |
+   = help: see <https://doc.rust-lang.org/nomicon/borrow-splitting.html> for more information about lifetime errors related to mutable references
+
+error[E0621]: explicit lifetime required in the type of `wrapper`
+  --> $DIR/e0621-mut-ref-aliases-pointee-lifetime.rs:19:5
+   |
+LL |     wrapper.inner.buf = &mut wrapper.inner.buf[..];
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime `'a` required
+   |
+   = help: see <https://doc.rust-lang.org/nomicon/borrow-splitting.html> for more information about lifetime errors related to mutable references
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0621`.