From 2899b58832fec1fd3570e65e231920d2a2e937ae Mon Sep 17 00:00:00 2001
From: Giacomo Stevanato <giaco.stevanato@gmail.com>
Date: Thu, 15 Apr 2021 23:17:15 +0200
Subject: [PATCH 1/2] Add suggestion to borrow when casting T to *const/mut T

---
 compiler/rustc_typeck/src/check/cast.rs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/compiler/rustc_typeck/src/check/cast.rs b/compiler/rustc_typeck/src/check/cast.rs
index 16c344e8e2b..b760a54f08c 100644
--- a/compiler/rustc_typeck/src/check/cast.rs
+++ b/compiler/rustc_typeck/src/check/cast.rs
@@ -359,6 +359,21 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     {
                         sugg = Some(format!("&{}", mutbl.prefix_str()));
                     }
+                } else if let ty::RawPtr(TypeAndMut { mutbl, .. }) = *self.cast_ty.kind() {
+                    if fcx
+                        .try_coerce(
+                            self.expr,
+                            fcx.tcx.mk_ref(
+                                &ty::RegionKind::ReErased,
+                                TypeAndMut { ty: self.expr_ty, mutbl },
+                            ),
+                            self.cast_ty,
+                            AllowTwoPhase::No,
+                        )
+                        .is_ok()
+                    {
+                        sugg = Some(format!("&{}", mutbl.prefix_str()));
+                    }
                 }
                 if let Some(sugg) = sugg {
                     err.span_label(self.span, "invalid cast");

From 791e8001d1368a34fc938ed0e39f600c1b4079e1 Mon Sep 17 00:00:00 2001
From: Giacomo Stevanato <giaco.stevanato@gmail.com>
Date: Thu, 15 Apr 2021 23:17:44 +0200
Subject: [PATCH 2/2] Add regression test

---
 src/test/ui/cast/issue-84213.fixed  | 15 +++++++++++++++
 src/test/ui/cast/issue-84213.rs     | 15 +++++++++++++++
 src/test/ui/cast/issue-84213.stderr | 25 +++++++++++++++++++++++++
 3 files changed, 55 insertions(+)
 create mode 100644 src/test/ui/cast/issue-84213.fixed
 create mode 100644 src/test/ui/cast/issue-84213.rs
 create mode 100644 src/test/ui/cast/issue-84213.stderr

diff --git a/src/test/ui/cast/issue-84213.fixed b/src/test/ui/cast/issue-84213.fixed
new file mode 100644
index 00000000000..e1a60557a20
--- /dev/null
+++ b/src/test/ui/cast/issue-84213.fixed
@@ -0,0 +1,15 @@
+// run-rustfix
+
+struct Something {
+    pub field: u32,
+}
+
+fn main() {
+    let mut something = Something { field: 1337 };
+
+    let _pointer_to_something = &something as *const Something;
+    //~^ ERROR: non-primitive cast
+
+    let _mut_pointer_to_something = &mut something as *mut Something;
+    //~^ ERROR: non-primitive cast
+}
diff --git a/src/test/ui/cast/issue-84213.rs b/src/test/ui/cast/issue-84213.rs
new file mode 100644
index 00000000000..3df264bdffb
--- /dev/null
+++ b/src/test/ui/cast/issue-84213.rs
@@ -0,0 +1,15 @@
+// run-rustfix
+
+struct Something {
+    pub field: u32,
+}
+
+fn main() {
+    let mut something = Something { field: 1337 };
+
+    let _pointer_to_something = something as *const Something;
+    //~^ ERROR: non-primitive cast
+
+    let _mut_pointer_to_something = something as *mut Something;
+    //~^ ERROR: non-primitive cast
+}
diff --git a/src/test/ui/cast/issue-84213.stderr b/src/test/ui/cast/issue-84213.stderr
new file mode 100644
index 00000000000..a76aac58013
--- /dev/null
+++ b/src/test/ui/cast/issue-84213.stderr
@@ -0,0 +1,25 @@
+error[E0605]: non-primitive cast: `Something` as `*const Something`
+  --> $DIR/issue-84213.rs:10:33
+   |
+LL |     let _pointer_to_something = something as *const Something;
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
+   |
+help: borrow the value for the cast to be valid
+   |
+LL |     let _pointer_to_something = &something as *const Something;
+   |                                 ^
+
+error[E0605]: non-primitive cast: `Something` as `*mut Something`
+  --> $DIR/issue-84213.rs:13:37
+   |
+LL |     let _mut_pointer_to_something = something as *mut Something;
+   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
+   |
+help: borrow the value for the cast to be valid
+   |
+LL |     let _mut_pointer_to_something = &mut something as *mut Something;
+   |                                     ^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0605`.