make CastError::NeedsDeref create a MachineApplicable suggestion

compiler/rustc_hir/src/hir.rs

@@ -1787,6 +1787,14 @@ impl Expr<'_> {
         expr
     }
 
+    pub fn peel_borrows(&self) -> &Self {
+        let mut expr = self;
+        while let ExprKind::AddrOf(.., inner) = &expr.kind {
+            expr = inner;
+        }
+        expr
+    }
+
     pub fn can_have_side_effects(&self) -> bool {
         match self.peel_drop_temps().kind {
             ExprKind::Path(_) | ExprKind::Lit(_) => false,

compiler/rustc_hir_typeck/src/cast.rs

@@ -31,6 +31,7 @@
 use super::FnCtxt;
 
 use crate::type_error_struct;
+use hir::ExprKind;
 use rustc_errors::{
     struct_span_err, Applicability, DelayDm, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
 };
@@ -151,7 +152,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
 #[derive(Copy, Clone)]
 pub enum CastError {
-    ErrorGuaranteed,
+    ErrorGuaranteed(ErrorGuaranteed),
 
     CastToBool,
     CastToChar,
@@ -176,8 +177,8 @@ pub enum CastError {
 }
 
 impl From<ErrorGuaranteed> for CastError {
-    fn from(_: ErrorGuaranteed) -> Self {
-        CastError::ErrorGuaranteed
+    fn from(err: ErrorGuaranteed) -> Self {
+        CastError::ErrorGuaranteed(err)
     }
 }
 
@@ -225,33 +226,36 @@ impl<'a, 'tcx> CastCheck<'tcx> {
 
     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'tcx>, e: CastError) {
         match e {
-            CastError::ErrorGuaranteed => {
+            CastError::ErrorGuaranteed(_) => {
                 // an error has already been reported
             }
             CastError::NeedDeref => {
-                let error_span = self.span;
                 let mut err = make_invalid_casting_error(
                     fcx.tcx.sess,
                     self.span,
                     self.expr_ty,
                     self.cast_ty,
                     fcx,
                 );
-                let cast_ty = fcx.ty_to_string(self.cast_ty);
-                err.span_label(
-                    error_span,
-                    format!("cannot cast `{}` as `{}`", fcx.ty_to_string(self.expr_ty), cast_ty),
-                );
-                if let Ok(snippet) = fcx.sess().source_map().span_to_snippet(self.expr_span) {
-                    err.span_suggestion(
-                        self.expr_span,
-                        "dereference the expression",
-                        format!("*{}", snippet),
-                        Applicability::MaybeIncorrect,
+
+                if matches!(self.expr.kind, ExprKind::AddrOf(..)) {
+                    // get just the borrow part of the expression
+                    let span = self.expr_span.with_hi(self.expr.peel_borrows().span.lo());
+                    err.span_suggestion_verbose(
+                        span,
+                        "remove the unneeded borrow",
+                        "",
+                        Applicability::MachineApplicable,
                     );
                 } else {
-                    err.span_help(self.expr_span, "dereference the expression with `*`");
+                    err.span_suggestion_verbose(
+                        self.expr_span.shrink_to_lo(),
+                        "dereference the expression",
+                        "*",
+                        Applicability::MachineApplicable,
+                    );
                 }
+
                 err.emit();
             }
             CastError::NeedViaThinPtr | CastError::NeedViaPtr => {

tests/ui/error-codes/E0606.rs

@@ -1,3 +1,4 @@
 fn main() {
-    &0u8 as u8; //~ ERROR E0606
+    let x = &(&0u8 as u8); //~ ERROR E0606
+    x as u8; //~ casting `&u8` as `u8` is invalid [E0606]
 }

tests/ui/error-codes/E0606.stderr

@@ -1,12 +1,26 @@
 error[E0606]: casting `&u8` as `u8` is invalid
-  --> $DIR/E0606.rs:2:5
+  --> $DIR/E0606.rs:2:14
    |
-LL |     &0u8 as u8;
-   |     ----^^^^^^
-   |     |
-   |     cannot cast `&u8` as `u8`
-   |     help: dereference the expression: `*&0u8`
+LL |     let x = &(&0u8 as u8);
+   |              ^^^^^^^^^^^^
+   |
+help: remove the unneeded borrow
+   |
+LL -     let x = &(&0u8 as u8);
+LL +     let x = &(0u8 as u8);
+   |
+
+error[E0606]: casting `&u8` as `u8` is invalid
+  --> $DIR/E0606.rs:3:5
+   |
+LL |     x as u8;
+   |     ^^^^^^^
+   |
+help: dereference the expression
+   |
+LL |     *x as u8;
+   |     +
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0606`.

tests/ui/error-festival.stderr

@@ -69,10 +69,12 @@ error[E0606]: casting `&u8` as `u32` is invalid
   --> $DIR/error-festival.rs:37:18
    |
 LL |     let y: u32 = x as u32;
-   |                  -^^^^^^^
-   |                  |
-   |                  cannot cast `&u8` as `u32`
-   |                  help: dereference the expression: `*x`
+   |                  ^^^^^^^^
+   |
+help: dereference the expression
+   |
+LL |     let y: u32 = *x as u32;
+   |                  +
 
 error[E0607]: cannot cast thin pointer `*const u8` to fat pointer `*const [u8]`
   --> $DIR/error-festival.rs:41:5

tests/ui/mismatched_types/cast-rfc0401.stderr

@@ -243,10 +243,12 @@ error[E0606]: casting `&{float}` as `f32` is invalid
   --> $DIR/cast-rfc0401.rs:71:30
    |
 LL |     vec![0.0].iter().map(|s| s as f32).collect::<Vec<f32>>();
-   |                              -^^^^^^^
-   |                              |
-   |                              cannot cast `&{float}` as `f32`
-   |                              help: dereference the expression: `*s`
+   |                              ^^^^^^^^
+   |
+help: dereference the expression
+   |
+LL |     vec![0.0].iter().map(|s| *s as f32).collect::<Vec<f32>>();
+   |                              +
 
 error: aborting due to 34 previous errors