error causes subsequent spurious error about literal cast

[durka]

See this code:

fn main() {
    error;
    1 as f32;
}

It has an error on the second line, obviously. But it also prints an error about the third line (illegal cast). But the cast is perfectly legal and removing the first error also "fixes" the second one.

[durka]

I suspect this somehow has to do with the constant evaluator. @oli-obk?

[oli-obk]

nope, it's not const eval, it's typeck.

beta/nightly even produce "non-scalar cast: {integer} as f32", so it's not something weird like the error; turning the 1 into another type.

[durka]

But it is a scalar cast. Maybe the first error leaves some kind of boolean flag around that "there was an error", and the casting code picks up on it?

[Detegr]

I've investigated this a bit. Apologies if I've understood something wrong, I'm still a total compiler noob :)

Looks like the reason this happens is when there's an error before the cast, the type inference of 1 differs from the case when there's just the cast. These types are checked in src/librustc_typeck/check/cast.rs in a function called do_check.

In the working version self.expr_ty.sty looks like this (from rust-gdb):

TyS = {sty = TyInt = {I32}, flags = Cell<rustc::ty::TypeFlags> = {
  value = UnsafeCell<rustc::ty::TypeFlags> = {value = TypeFlags = {
    bits = 0}}}, region_depth = 0}

where in the version with the error it looks like this:

TyS = {sty = TyInfer = {IntVar = {IntVid = {index = 0}}},
  flags = Cell<rustc::ty::TypeFlags> = {
    value = UnsafeCell<rustc::ty::TypeFlags> = {value = TypeFlags = {
      bits = 3076}}}, region_depth = 0}

If do_check results in an error (it does if sty is TyInfer, see from_ty) the compiler reports the cast error.

Why the types are resolved differently I have no idea.

EDIT: I dug around a bit more and I think I found the root cause. The reason was what @durka was anticipating, default_type_parameters() treats all uninstantiated type variables as TyError if an error had been discovered earlier. This causes do_check to fail and the code handling the failure does not check if is_tainted_by_errors() is true or not but just outputs the error anyway.

[durka]

Nice detective work @Detegr.

[Detegr]

Thanks, however my initial solution to the problem was too shortsighted and caused more harm than good. Anyway this has been a fun dive to the compiler internals so I'll keep trying.

[estebank]

This is fixed in nightly and beta (probably due to #46732).