alacritty/vte fails to build on rustc master (mutable references are not allowed in constant functions)

[matthiaskrgr]

Upstream ticket: alacritty/vte#68

Code

https://github.com/alacritty/vte/
I tried this code:

<code>

I expected to see this happen: explanation
The code compiles on

   stable-x86_64-unknown-linux-gnu unchanged - rustc 1.47.0 (18bf6b4f0 2020-10-07)
     beta-x86_64-unknown-linux-gnu unchanged - rustc 1.48.0-beta.10 (8236dc07b 2020-11-15)
  nightly-x86_64-unknown-linux-gnu unchanged - rustc 1.50.0-nightly (f5230fbf7 2020-11-16)

but fails to build on rustc master rustc 1.50.0-nightly (c919f490b 2020-11-17)
which I assumes makes this a regression:

    Checking vte v0.9.0 (/tmp/vte)
error[E0658]: mutable references are not allowed in constant functions
   --> src/table.rs:9:1
    |
9   | / generate_state_changes!(state_changes, {
10  | |     Anywhere {
11  | |         0x18 => (Ground, Execute),
12  | |         0x1a => (Ground, Execute),
...   |
170 | |     }
171 | | });
    | |___^
    |
    = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
   --> src/table.rs:9:1
    |
9   | / generate_state_changes!(state_changes, {
10  | |     Anywhere {
11  | |         0x18 => (Ground, Execute),
12  | |         0x1a => (Ground, Execute),
...   |
170 | |     }
171 | | });
    | |___^
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors

Version it worked on

It most recently worked on:

Version with regression

rustc --version --verbose:

<version>

Backtrace

Backtrace

<backtrace>

[matthiaskrgr]

cargo-bisect-rustc says the regression is caused by c6a6105

[matthiaskrgr]

Maybe #79032 ?

[jyn514]

cc @lcnr

[matthiaskrgr]

I assume this is the cause of the error: https://github.com/alacritty/vte/blob/master/vte_generate_state_changes/src/lib.rs#L28

[SNCPlay42]

MCVE:

const fn foo() {
    let mut array = [[0; 1]; 1];
    array[0][0] = 1;
}

[SNCPlay42]

Could be #74989.

[SNCPlay42]

Reverting #74989 fixes the regression.

The regression also appears in the crater report for #74989, but it appears either it was overlooked or the decision was made to merge anyway? cc @KodrAus

[spastorino]

Assigning P-critical as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

[lcnr]

I don't think we should use the Index impl here but instead keep using builtin indexing

we probably have to update something here:

rust/compiler/rustc_typeck/src/check/writeback.rs

Lines 204 to 236 in 8d2d001

	if let Some(ty::Ref(_, base_ty, _)) = base_ty {
	let index_ty = typeck_results.expr_ty_adjusted_opt(&index).unwrap_or_else(|| {
	// When encountering `return [0][0]` outside of a `fn` body we would attempt
	// to access an unexistend index. We assume that more relevant errors will
	// already have been emitted, so we only gate on this with an ICE if no
	// error has been emitted. (#64638)
	self.fcx.tcx.ty_error_with_message(
	e.span,
	&format!("bad index {:?} for base: `{:?}`", index, base),
	)
	});
	let index_ty = self.fcx.resolve_vars_if_possible(index_ty);
	if base_ty.builtin_index().is_some() && index_ty == self.fcx.tcx.types.usize {
	// Remove the method call record
	typeck_results.type_dependent_defs_mut().remove(e.hir_id);
	typeck_results.node_substs_mut().remove(e.hir_id);
	if let Some(a) = typeck_results.adjustments_mut().get_mut(base.hir_id) {
	// Discard the need for a mutable borrow
	// Extra adjustment made when indexing causes a drop
	// of size information - we need to get rid of it
	// Since this is "after" the other adjustment to be
	// discarded, we do an extra `pop()`
	if let Some(Adjustment {
	kind: Adjust::Pointer(PointerCast::Unsize), ..
	}) = a.pop()
	{
	// So the borrow discard actually happens here
	a.pop();
	}
	}

[matthiaskrgr]

@rustbot label E-needs-bisection E-needs-mcve