[E0054-E0604-E0620-E0606] TypeCasting ErrorCodes

[MahadMuhammad]

Added All Casting ErrorCodes & Updated error message accordingly:

• Added errorcodes according to different conditions and updated error message according to type casting type.

These are all the error codes related to casting in rustc.

• E0054 to bool
• E0604 to char other than u8
• E0605 Invalid cast between standard data structures, needs rust standard library.
• E0606 incompatible cast - only primitive casts.
• E0607 cast of thin & fat pointers - not supported in gccrs.
• E0620 cast to unsized type.
• E0623 A lifetime didn't match what was expected - not supported by gccrs.
• E0641 cast to/from a pointer with an unknown kind - gccrs emits wrong error here. We need to improve this PR gccrs: Fix ICE by adding check for enum candidate's #2490 to fix this issue. See https://godbolt.org/z/xzovd3P36
• E0781 Use of ABI with non-function pointers.

---

Added ErrorCodes:

• E0054
• E0604
• E0620
• E0606

---

Code Tested:

fn main() {
    let x = 5;
    let x_is_nonzero = x as bool; // { dg-error "cannot cast .<integer>. as .bool." }

    0u32 as char; // { dg-error "cannot cast .u32. as .char., only .u8. can be cast as .char." }

    let x = &[1_usize, 2] as [usize]; // { dg-error "cast to unsized type: .& .usize:CAPACITY.. as ..usize.." }

    let a = &0u8; // Here, `x` is a `&u8`.
    let y: u32 = a as u32; // { dg-error "casting .& u8. as .u32. is invalid" }
}

---

Output:

/home/mahad/Desktop/mahad/gccrs/gcc/testsuite/rust/compile/all-cast.rs:3:24: error: cannot cast ‘<integer>’ as ‘bool’ [E0054]
    3 |     let x_is_nonzero = x as bool; // { dg-error "cannot cast .<integer>. as .bool." }
      |                        ^    ~~~~
/home/mahad/Desktop/mahad/gccrs/gcc/testsuite/rust/compile/all-cast.rs:5:5: error: cannot cast ‘u32’ as ‘char’, only ‘u8’ can be cast as ‘char’ [E0604]
    5 |     0u32 as char; // { dg-error "cannot cast .u32. as .char., only .u8. can be cast as .char." }
      |     ^~~~    ~~~~
/home/mahad/Desktop/mahad/gccrs/gcc/testsuite/rust/compile/all-cast.rs:7:13: error: cast to unsized type: ‘& [usize:CAPACITY]’ as ‘[usize]’ [E0620]
    7 |     let x = &[1_usize, 2] as [usize]; // { dg-error "cast to unsized type: .& .usize:CAPACITY.. as ..usize.." }
      |             ^                ~
/home/mahad/Desktop/mahad/gccrs/gcc/testsuite/rust/compile/all-cast.rs:10:18: error: casting ‘& u8’ as ‘u32’ is invalid [E0606]
   10 |     let y: u32 = a as u32; // { dg-error "casting .& u8. as .u32. is invalid" }
      |                  ^    ~~~

---

gcc/rust/ChangeLog:

* typecheck/rust-casts.cc (TypeCastRules::emit_cast_error): Refactored ErrorCodes & error messages.

gcc/testsuite/ChangeLog:

* rust/compile/bad_as_bool_char.rs: Updated comment to pass test case.
* rust/compile/cast1.rs: likewise.
* rust/compile/cast4.rs: likewise.
* rust/compile/cast5.rs: likewise.
* rust/compile/all-cast.rs: New test for all error codes.

[philberty]

LGTM really good job on this one!!! Really nice to see specific errors for specific cases being possible with the new type checking code