Rollup merge of rust-lang#127221 - Urgau:check-cfg-std-diag, r=pnkfelix

Improve well known value check-cfg diagnostic for the standard library

This PR adjust the current logic for hidding the rustc/Cargo suggestion to add a value to a well-known name to exclude the standard library and rustc crates.

This is done in order to improve the contributor experience, in particular when adding a new target, which often requires adding some cfgs like `target_os` which may not be available yet in stage0.

<details>

The diagnostic code would look like this.

```text
error: unexpected `cfg` condition value: `blable`
   --> library/core/src/lib.rs:369:7
    |
369 | #[cfg(target_os = "blable")]
    |       ^^^^^^^^^^^^^^^^^^^^
    |
    = note: expected values for `target_os` are: `aix`, `android`, `cuda`, `dragonfly`, `emscripten`, `espidf`, `freebsd`, `fuchsia`, `haiku`, `hermit`, `horizon`, `hurd`, `illumos`, `ios`, `l4re`, `linux`, `macos`, `netbsd`, `none`, `nto`, `openbsd`, `psp`, `redox`, `solaris`, `solid_asp3`, `teeos`, `tvos`, `uefi`, `unknown`, `visionos`, `vita`, `vxworks`, `wasi`, `watchos`, and `windows` and 2 more
    = help: consider using a Cargo feature instead
    = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
             [lints.rust]
             unexpected_cfgs = { level = "warn", check-cfg = ['cfg(target_os, values("blable"))'] }
    = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(target_os, values(\"blable\"))");` to the top of the `build.rs`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
    = note: `-D unexpected-cfgs` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unexpected_cfgs)]`
```

</details>

Author: compiler-errors

compiler/rustc_lint/src/context/diagnostics/check_cfg.rs

@@ -261,10 +261,16 @@ pub(super) fn unexpected_cfg_value(
         lints::unexpected_cfg_value::CodeSuggestion::RemoveCondition { suggestion, name }
     };
 
-    // We don't want to suggest adding values to well known names
-    // since those are defined by rustc it-self. Users can still
-    // do it if they want, but should not encourage them.
-    let is_cfg_a_well_know_name = sess.psess.check_config.well_known_names.contains(&name);
+    // We don't want to encourage people to add values to a well-known names, as these are
+    // defined by rustc/Rust itself. Users can still do this if they wish, but should not be
+    // encouraged to do so.
+    let can_suggest_adding_value = !sess.psess.check_config.well_known_names.contains(&name)
+        // Except when working on rustc or the standard library itself, in which case we want to
+        // suggest adding these cfgs to the "normal" place because of bootstraping reasons. As a
+        // basic heuristic, we use the "cheat" unstable feature enable method and the
+        // non-ui-testing enabled option.
+        || (matches!(sess.psess.unstable_features, rustc_feature::UnstableFeatures::Cheat)
+            && !sess.opts.unstable_opts.ui_testing);
 
     let inst = |escape_quotes| to_check_cfg_arg(name, value.map(|(v, _s)| v), escape_quotes);
 
@@ -275,14 +281,14 @@ pub(super) fn unexpected_cfg_value(
             } else {
                 Some(lints::unexpected_cfg_value::CargoHelp::DefineFeatures)
             }
-        } else if !is_cfg_a_well_know_name {
+        } else if can_suggest_adding_value {
             Some(lints::unexpected_cfg_value::CargoHelp::Other(cargo_help_sub(sess, &inst)))
         } else {
             None
         };
         lints::unexpected_cfg_value::InvocationHelp::Cargo(help)
     } else {
-        let help = if !is_cfg_a_well_know_name {
+        let help = if can_suggest_adding_value {
             Some(lints::UnexpectedCfgRustcHelp::new(&inst(EscapeQuotes::No)))
         } else {
             None