cycle detected when evaluating type-level constant on invalid return type

[asquared31415]

Code

// Does not repro without the braces.
#[allow(unused_braces)]
fn uwu() -> [(); { () }] {
    loop {}
}

Current output

error[[E0391]](https://doc.rust-lang.org/stable/error_codes/E0391.html): cycle detected when evaluating type-level constant
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
  |
note: ...which requires const-evaluating + checking `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires const-evaluating + checking `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires caching mir of `uwu::{constant#0}` for CTFE...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires elaborating drops for `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires borrow-checking `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires promoting constants in MIR for `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires const checking `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires preparing `uwu::{constant#0}` for borrow checking...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires unsafety-checking `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires building MIR for `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires match-checking `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires building THIR for `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
note: ...which requires type-checking `uwu::{constant#0}`...
 --> src/lib.rs:3:18
  |
3 | fn uwu() -> [(); { () }] {
  |                  ^^^^^^
  = note: ...which again requires evaluating type-level constant, completing the cycle
note: cycle used when checking that `uwu` is well-formed
 --> src/lib.rs:3:1
  |
3 | fn uwu() -> [(); { () }] {
  | ^^^^^^^^^^^^^^^^^^^^^^^^

Desired output

error[[E0308]](https://doc.rust-lang.org/stable/error_codes/E0308.html): mismatched types
 --> src/lib.rs:3:20
  |
3 | fn uwu() -> [(); { () }] {
  |                    ^^ expected `usize`, found `()`

[gurry]

Looks more like a type check bug than a diagnostic issue per se. Output of rustc -Z treat-err-as-bug:

<snip>
query stack during panic:
#0 [typeck] type-checking `uwu::{constant#0}`
#1 [thir_body] building THIR for `uwu::{constant#0}`
#2 [check_match] match-checking `uwu::{constant#0}`
#3 [mir_built] building MIR for `uwu::{constant#0}`
#4 [unsafety_check_result] unsafety-checking `uwu::{constant#0}`
#5 [mir_const] preparing `uwu::{constant#0}` for borrow checking
#6 [mir_const_qualif] const checking `uwu::{constant#0}`
#7 [mir_promoted] promoting constants in MIR for `uwu::{constant#0}`
#8 [mir_borrowck] borrow-checking `uwu::{constant#0}`
#9 [mir_drops_elaborated_and_const_checked] elaborating drops for `uwu::{constant#0}`
#10 [mir_for_ctfe] caching mir of `uwu::{constant#0}` for CTFE
#11 [eval_to_allocation_raw] const-evaluating + checking `uwu::{constant#0}`
#12 [eval_to_allocation_raw] const-evaluating + checking `uwu::{constant#0}`
#13 [eval_to_valtree] evaluating type-level constant
#14 [check_well_formed] checking that `uwu` is well-formed
#15 [check_mod_type_wf] checking that types are well-formed in top-level module
#16 [analysis] running analysis passes on this crate
end of query stack

[gurry]

@rustbot claim

[gurry]

@rustbot claim

[gurry]

Contrary to my comment above, the issue is related to diagnostics (I know better now).

It occurs because we mistake the block in the return type ([(); { () }]) for the block that contains the function's body, which triggers the suggestion to change the return type and during that suggestion we run into the cycle.