From badb81a6129b84a9f01223ab73a56ffe52bf33ce Mon Sep 17 00:00:00 2001
From: George Bateman <george.bateman16@gmail.com>
Date: Tue, 25 Jan 2022 23:19:00 +0000
Subject: [PATCH] #91939: integer to char cast error, make more targeted

---
 compiler/rustc_typeck/src/check/cast.rs       | 21 ++++++++++++++-----
 .../const-eval/const-eval-overflow-4b.stderr  |  6 ++++++
 src/test/ui/error-codes/E0604.stderr          |  6 ++++++
 src/test/ui/error-festival.stderr             |  6 ++++++
 .../ui/mismatched_types/cast-rfc0401.stderr   |  6 ++++++
 5 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/compiler/rustc_typeck/src/check/cast.rs b/compiler/rustc_typeck/src/check/cast.rs
index f6b5e0084d1..be0b7733579 100644
--- a/compiler/rustc_typeck/src/check/cast.rs
+++ b/compiler/rustc_typeck/src/check/cast.rs
@@ -328,17 +328,28 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 err.emit();
             }
             CastError::CastToChar => {
-                type_error_struct!(
+                let mut err = type_error_struct!(
                     fcx.tcx.sess,
                     self.span,
                     self.expr_ty,
                     E0604,
                     "only `u8` can be cast as `char`, not `{}`",
                     self.expr_ty
-                )
-                .span_label(self.span, "invalid cast")
-                .span_help(self.span, "try `char::from_u32` instead")
-                .emit();
+                );
+                err.span_label(self.span, "invalid cast");
+                if self.expr_ty.is_numeric() {
+                    err.span_help(
+                        self.span,
+                        if self.expr_ty == fcx.tcx.types.i8 {
+                            "try casting from `u8` instead"
+                        } else if self.expr_ty == fcx.tcx.types.u32 {
+                            "try `char::from_u32` instead"
+                        } else {
+                            "try `char::from_u32` instead (via a `u32`)"
+                        },
+                    );
+                }
+                err.emit();
             }
             CastError::NonScalar => {
                 let mut err = type_error_struct!(
diff --git a/src/test/ui/consts/const-eval/const-eval-overflow-4b.stderr b/src/test/ui/consts/const-eval/const-eval-overflow-4b.stderr
index 1e181c465db..f59ff329d18 100644
--- a/src/test/ui/consts/const-eval/const-eval-overflow-4b.stderr
+++ b/src/test/ui/consts/const-eval/const-eval-overflow-4b.stderr
@@ -17,6 +17,12 @@ error[E0604]: only `u8` can be cast as `char`, not `i8`
    |
 LL |     : [u32; 5i8 as char as usize]
    |             ^^^^^^^^^^^ invalid cast
+   |
+help: try casting from `u8` instead
+  --> $DIR/const-eval-overflow-4b.rs:22:13
+   |
+LL |     : [u32; 5i8 as char as usize]
+   |             ^^^^^^^^^^^
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/error-codes/E0604.stderr b/src/test/ui/error-codes/E0604.stderr
index 18835310bd5..d715d28b73c 100644
--- a/src/test/ui/error-codes/E0604.stderr
+++ b/src/test/ui/error-codes/E0604.stderr
@@ -3,6 +3,12 @@ error[E0604]: only `u8` can be cast as `char`, not `u32`
    |
 LL |     1u32 as char;
    |     ^^^^^^^^^^^^ invalid cast
+   |
+help: try `char::from_u32` instead
+  --> $DIR/E0604.rs:2:5
+   |
+LL |     1u32 as char;
+   |     ^^^^^^^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/error-festival.stderr b/src/test/ui/error-festival.stderr
index b8cd7b7464a..0ddb6fc99b0 100644
--- a/src/test/ui/error-festival.stderr
+++ b/src/test/ui/error-festival.stderr
@@ -58,6 +58,12 @@ error[E0604]: only `u8` can be cast as `char`, not `u32`
    |
 LL |     0u32 as char;
    |     ^^^^^^^^^^^^ invalid cast
+   |
+help: try `char::from_u32` instead
+  --> $DIR/error-festival.rs:25:5
+   |
+LL |     0u32 as char;
+   |     ^^^^^^^^^^^^
 
 error[E0605]: non-primitive cast: `u8` as `Vec<u8>`
   --> $DIR/error-festival.rs:29:5
diff --git a/src/test/ui/mismatched_types/cast-rfc0401.stderr b/src/test/ui/mismatched_types/cast-rfc0401.stderr
index 7f91d5ed42c..6dbf24baf23 100644
--- a/src/test/ui/mismatched_types/cast-rfc0401.stderr
+++ b/src/test/ui/mismatched_types/cast-rfc0401.stderr
@@ -99,6 +99,12 @@ error[E0604]: only `u8` can be cast as `char`, not `u32`
    |
 LL |     let _ = 0x61u32 as char;
    |             ^^^^^^^^^^^^^^^ invalid cast
+   |
+help: try `char::from_u32` instead
+  --> $DIR/cast-rfc0401.rs:41:13
+   |
+LL |     let _ = 0x61u32 as char;
+   |             ^^^^^^^^^^^^^^^
 
 error[E0606]: casting `bool` as `f32` is invalid
   --> $DIR/cast-rfc0401.rs:43:13