check-cfg does not appear to honor target-spec json

[ehuss]

--check-cfg=values() doesn't seem to know that custom targets can extend the set of values.

Steps

The following creates a custom spec with a target_os="ericos".

1. cargo new --lib foo
2. cd foo
3. rm src/lib.rs

4. cat <<EOF > src/lib.rs
   #![no_std]
   
   #[cfg(target_os = "macos")]
   pub fn macos() {}
   
   #[cfg(target_os = "ericos")]
   pub fn eric() {}
   EOF

5. cat << EOF > custom.json
   {
      "llvm-target": "x86_64-unknown-none-gnu",
      "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
      "arch": "x86_64",
      "target-endian": "little",
      "target-pointer-width": "64",
      "target-c-int-width": "32",
      "os": "ericos",
      "linker-flavor": "ld.lld",
      "linker": "rust-lld",
      "executables": true
   }

6. cargo check --target custom.json -Zbuild-std=core -Zcheck-cfg=values

Expected

No warning

Actual

Got the warning:

warning: unexpected `cfg` condition value
 --> src/lib.rs:6:7
  |
6 | #[cfg(target_os = "ericos")]
  |       ^^^^^^^^^^^^^^^^^^^^
  |
  = note: expected values for `target_os` are: aix, android, cuda, dragonfly, emscripten, espidf, freebsd, fuchsia, haiku, hermit, horizon, illumos, ios, l4re, linux, macos, netbsd, none, nto, openbsd, psp, redox, solaris, solid_asp3, tvos, uefi, unknown, vita, vxworks, wasi, watchos, windows, xous
  = note: `#[warn(unexpected_cfgs)]` on by default

warning: unexpected condition value `ericos` for condition name `target_os`
  |
  = help: was set with `--cfg` but isn't in the `--check-cfg` expected values

Meta

rustc 1.69.0-nightly (7281249a1 2023-02-27)
binary: rustc
commit-hash: 7281249a19a9755e9d889ee251ec323629caadab
commit-date: 2023-02-27
host: aarch64-apple-darwin
release: 1.69.0-nightly
LLVM version: 15.0.7

cc @Urgau

[Urgau]

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