Improve u32 to char diagnostic

Author: gimbles

compiler/rustc_typeck/src/check/cast.rs

@@ -347,16 +347,22 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 );
                 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`)"
-                        },
-                    );
+                    if self.expr_ty == fcx.tcx.types.u32 {
+                        match fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
+                            Ok(snippet) => err.span_suggestion(
+                                self.span,
+                                "try `char::from_u32` instead",
+                                format!("char::from_u32({snippet})"),
+                                Applicability::MachineApplicable,
+                            ),
+
+                            Err(_) => err.span_help(self.span, "try `char::from_u32` instead"),
+                        };
+                    } else if self.expr_ty == fcx.tcx.types.i8 {
+                        err.span_help(self.span, "try casting from `u8` instead");
+                    } else {
+                        err.span_help(self.span, "try `char::from_u32` instead (via a `u32`)");
+                    };
                 }
                 err.emit();
             }

src/test/ui/error-codes/E0604.stderr

@@ -1,14 +1,11 @@
 error[E0604]: only `u8` can be cast as `char`, not `u32`
   --> $DIR/E0604.rs:2:5
    |
-LL |     1u32 as char;
-   |     ^^^^^^^^^^^^ invalid cast
-   |
-help: try `char::from_u32` instead
-  --> $DIR/E0604.rs:2:5
-   |
 LL |     1u32 as char;
    |     ^^^^^^^^^^^^
+   |     |
+   |     invalid cast
+   |     help: try `char::from_u32` instead: `char::from_u32(1u32)`
 
 error: aborting due to previous error
 

src/test/ui/error-festival.stderr

@@ -56,14 +56,11 @@ LL | | }
 error[E0604]: only `u8` can be cast as `char`, not `u32`
   --> $DIR/error-festival.rs:25:5
    |
-LL |     0u32 as char;
-   |     ^^^^^^^^^^^^ invalid cast
-   |
-help: try `char::from_u32` instead
-  --> $DIR/error-festival.rs:25:5
-   |
 LL |     0u32 as char;
    |     ^^^^^^^^^^^^
+   |     |
+   |     invalid cast
+   |     help: try `char::from_u32` instead: `char::from_u32(0u32)`
 
 error[E0605]: non-primitive cast: `u8` as `Vec<u8>`
   --> $DIR/error-festival.rs:29:5

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

@@ -97,14 +97,11 @@ LL |     let _ = E::A as bool;
 error[E0604]: only `u8` can be cast as `char`, not `u32`
   --> $DIR/cast-rfc0401.rs:41:13
    |
-LL |     let _ = 0x61u32 as char;
-   |             ^^^^^^^^^^^^^^^ invalid cast
-   |
-help: try `char::from_u32` instead
-  --> $DIR/cast-rfc0401.rs:41:13
-   |
 LL |     let _ = 0x61u32 as char;
    |             ^^^^^^^^^^^^^^^
+   |             |
+   |             invalid cast
+   |             help: try `char::from_u32` instead: `char::from_u32(0x61u32)`
 
 error[E0606]: casting `bool` as `f32` is invalid
   --> $DIR/cast-rfc0401.rs:43:13