Fix incorrect suggestions for E0605

compiler/rustc_typeck/src/check/cast.rs

@@ -35,6 +35,7 @@ use crate::type_error_struct;
 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, ErrorReported};
 use rustc_hir as hir;
 use rustc_hir::lang_items::LangItem;
+use rustc_middle::mir::Mutability;
 use rustc_middle::ty::adjustment::AllowTwoPhase;
 use rustc_middle::ty::cast::{CastKind, CastTy};
 use rustc_middle::ty::error::TypeError;
@@ -347,15 +348,52 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     fcx.ty_to_string(self.cast_ty)
                 );
                 let mut sugg = None;
+                let mut sugg_mutref = false;
                 if let ty::Ref(reg, _, mutbl) = *self.cast_ty.kind() {
-                    if fcx
-                        .try_coerce(
-                            self.expr,
-                            fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
-                            self.cast_ty,
-                            AllowTwoPhase::No,
-                        )
-                        .is_ok()
+                    if let ty::RawPtr(TypeAndMut { ty: expr_ty, .. }) = *self.expr_ty.kind() {
+                        if fcx
+                            .try_coerce(
+                                self.expr,
+                                fcx.tcx.mk_ref(
+                                    &ty::RegionKind::ReErased,
+                                    TypeAndMut { ty: expr_ty, mutbl },
+                                ),
+                                self.cast_ty,
+                                AllowTwoPhase::No,
+                            )
+                            .is_ok()
+                        {
+                            sugg = Some(format!("&{}*", mutbl.prefix_str()));
+                        }
+                    } else if let ty::Ref(expr_reg, expr_ty, expr_mutbl) = *self.expr_ty.kind() {
+                        if expr_mutbl == Mutability::Not
+                            && mutbl == Mutability::Mut
+                            && fcx
+                                .try_coerce(
+                                    self.expr,
+                                    fcx.tcx.mk_ref(
+                                        expr_reg,
+                                        TypeAndMut { ty: expr_ty, mutbl: Mutability::Mut },
+                                    ),
+                                    self.cast_ty,
+                                    AllowTwoPhase::No,
+                                )
+                                .is_ok()
+                        {
+                            sugg_mutref = true;
+                        }
+                    }
+
+                    if !sugg_mutref
+                        && sugg == None
+                        && fcx
+                            .try_coerce(
+                                self.expr,
+                                fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
+                                self.cast_ty,
+                                AllowTwoPhase::No,
+                            )
+                            .is_ok()
                     {
                         sugg = Some(format!("&{}", mutbl.prefix_str()));
                     }
@@ -375,11 +413,15 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                         sugg = Some(format!("&{}", mutbl.prefix_str()));
                     }
                 }
-                if let Some(sugg) = sugg {
+                if sugg_mutref {
+                    err.span_label(self.span, "invalid cast");
+                    err.span_note(self.expr.span, "this reference is immutable");
+                    err.span_note(self.cast_span, "trying to cast to a mutable reference type");
+                } else if let Some(sugg) = sugg {
                     err.span_label(self.span, "invalid cast");
                     err.span_suggestion_verbose(
                         self.expr.span.shrink_to_lo(),
-                        "borrow the value for the cast to be valid",
+                        "consider borrowing the value",
                         sugg,
                         Applicability::MachineApplicable,
                     );

src/test/ui/cast/issue-84213.stderr

@@ -4,7 +4,7 @@ error[E0605]: non-primitive cast: `Something` as `*const Something`
 LL |     let _pointer_to_something = something as *const Something;
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
    |
-help: borrow the value for the cast to be valid
+help: consider borrowing the value
    |
 LL |     let _pointer_to_something = &something as *const Something;
    |                                 ^
@@ -15,7 +15,7 @@ error[E0605]: non-primitive cast: `Something` as `*mut Something`
 LL |     let _mut_pointer_to_something = something as *mut Something;
    |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
    |
-help: borrow the value for the cast to be valid
+help: consider borrowing the value
    |
 LL |     let _mut_pointer_to_something = &mut something as *mut Something;
    |                                     ^^^^

src/test/ui/error-codes/E0605.stderr

@@ -8,7 +8,12 @@ error[E0605]: non-primitive cast: `*const u8` as `&u8`
   --> $DIR/E0605.rs:6:5
    |
 LL |     v as &u8;
-   |     ^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |     ^^^^^^^^ invalid cast
+   |
+help: consider borrowing the value
+   |
+LL |     &*v as &u8;
+   |     ^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-22289.stderr

@@ -4,7 +4,7 @@ error[E0605]: non-primitive cast: `i32` as `&(dyn Any + 'static)`
 LL |     0 as &dyn std::any::Any;
    |     ^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
    |
-help: borrow the value for the cast to be valid
+help: consider borrowing the value
    |
 LL |     &0 as &dyn std::any::Any;
    |     ^

src/test/ui/issues/issue-22312.stderr

@@ -4,7 +4,7 @@ error[E0605]: non-primitive cast: `Self` as `&dyn Index<usize, Output = <Self as
 LL |         let indexer = &(*self as &dyn Index<usize, Output = <Self as Index<usize>>::Output>);
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
    |
-help: borrow the value for the cast to be valid
+help: consider borrowing the value
    |
 LL |         let indexer = &(&*self as &dyn Index<usize, Output = <Self as Index<usize>>::Output>);
    |                         ^

src/test/ui/issues/issue-2995.stderr

@@ -2,7 +2,12 @@ error[E0605]: non-primitive cast: `*const isize` as `&isize`
   --> $DIR/issue-2995.rs:2:22
    |
 LL |     let _q: &isize = p as &isize;
-   |                      ^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |                      ^^^^^^^^^^^ invalid cast
+   |
+help: consider borrowing the value
+   |
+LL |     let _q: &isize = &*p as &isize;
+   |                      ^^
 
 error: aborting due to previous error
 

src/test/ui/mismatched_types/cast-rfc0401.stderr

@@ -24,7 +24,12 @@ error[E0605]: non-primitive cast: `*const u8` as `&u8`
   --> $DIR/cast-rfc0401.rs:29:13
    |
 LL |     let _ = v as &u8;
-   |             ^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |             ^^^^^^^^ invalid cast
+   |
+help: consider borrowing the value
+   |
+LL |     let _ = &*v as &u8;
+   |             ^^
 
 error[E0605]: non-primitive cast: `*const u8` as `E`
   --> $DIR/cast-rfc0401.rs:30:13