Clarify error on casting larger integers to char

[GKFX]

Closes #91836 with changes to E0604.md and a span_help.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @cjgillot (or someone else) soon.

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[camelid]

This should only be suggested if self.expr_ty is u32. Pseudocode:

Suggested change

	.span_help(self.span, "try `char::from_u32` instead")
	let err = type_error_struct!(...).span_label(...);
	if self.expr_ty == fcx.tcx.types.u32 {
	err.span_help(self.span, "try `char::from_u32` instead");
	}
	err.emit();

[camelid]

Once you do that, you'll need to update test output with ./x.py test --bless src/test/ui.

[JohnCSimon]

Ping from triage:
@GKFX can you post your status on this PR?

[GKFX]

Apologies for the late reply. I have one comment on the feedback. If someone tries to cast (e.g.) u16 to char, I think they still should be using char::from_u32. The alternative would be to cast from u16 to u8 and onto char, which obviously fails for input greater than 255.
Of course, if someone is trying to cast a wider value like i64 to char, they need to be a bit thoughtful about their approach, as char::from_u32(0xF_0000_00061i64 as u32) incorrectly returns Some('a'), but in general I think the message should be to use from_u32 for any integer except i8.

                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`)"
                    });
                }

[GKFX]

@rustbot label -S-waiting-on-author +S-waiting-on-review

[cjgillot]

Thanks @GKFX.
@bors r+ rollup

[bors]

📌 Commit badb81a has been approved by cjgillot