Exclude well known names from showing a suggestion in check-cfg

compiler/rustc_lint/src/context/diagnostics.rs

@@ -356,6 +356,12 @@ pub(super) fn builtin(
                 }
             }
 
+            // 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.parse_sess.check_config.well_known_names.contains(&name);
+
             let inst = if let Some((value, _value_span)) = value {
                 let pre = if is_from_cargo { "\\" } else { "" };
                 format!("cfg({name}, values({pre}\"{value}{pre}\"))")
@@ -368,12 +374,14 @@ pub(super) fn builtin(
                     if let Some((value, _value_span)) = value {
                         db.help(format!("consider adding `{value}` as a feature in `Cargo.toml`"));
                     }
-                } else {
+                } else if !is_cfg_a_well_know_name {
                     db.help(format!("consider using a Cargo feature instead or adding `println!(\"cargo:rustc-check-cfg={inst}\");` to the top of a `build.rs`"));
                 }
                 db.note("see <https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg> for more information about checking conditional configuration");
             } else {
-                db.help(format!("to expect this configuration use `--check-cfg={inst}`"));
+                if !is_cfg_a_well_know_name {
+                    db.help(format!("to expect this configuration use `--check-cfg={inst}`"));
+                }
                 db.note("see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration");
             }
         }

compiler/rustc_session/src/config.rs

@@ -1379,6 +1379,8 @@ pub struct CheckCfg {
     pub exhaustive_values: bool,
     /// All the expected values for a config name
     pub expecteds: FxHashMap<Symbol, ExpectedValues<Symbol>>,
+    /// Well known names (only used for diagnostics purposes)
+    pub well_known_names: FxHashSet<Symbol>,
 }
 
 pub enum ExpectedValues<T> {
@@ -1431,9 +1433,10 @@ impl CheckCfg {
         };
 
         macro_rules! ins {
-            ($name:expr, $values:expr) => {
+            ($name:expr, $values:expr) => {{
+                self.well_known_names.insert($name);
                 self.expecteds.entry($name).or_insert_with($values)
-            };
+            }};
         }
 
         // Symbols are inserted in alphabetical order as much as possible.

tests/ui/check-cfg/compact-values.stderr

@@ -5,7 +5,6 @@ LL | #[cfg(target(os = "linux", pointer_width = "X"))]
    |                            ^^^^^^^^^^^^^^^^^^^
    |
    = note: expected values for `target_pointer_width` are: `16`, `32`, `64`
-   = help: to expect this configuration use `--check-cfg=cfg(target_pointer_width, values("X"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
 

tests/ui/check-cfg/exhaustive-names-values.empty_cfg.stderr

@@ -18,7 +18,6 @@ LL | #[cfg(test = "value")]
    |           help: remove the value
    |
    = note: no expected value for `test`
-   = help: to expect this configuration use `--check-cfg=cfg(test, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition name: `feature`

tests/ui/check-cfg/exhaustive-names-values.feature.stderr

@@ -18,7 +18,6 @@ LL | #[cfg(test = "value")]
    |           help: remove the value
    |
    = note: no expected value for `test`
-   = help: to expect this configuration use `--check-cfg=cfg(test, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `unk`

tests/ui/check-cfg/exhaustive-names-values.full.stderr

@@ -18,7 +18,6 @@ LL | #[cfg(test = "value")]
    |           help: remove the value
    |
    = note: no expected value for `test`
-   = help: to expect this configuration use `--check-cfg=cfg(test, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `unk`

tests/ui/check-cfg/exhaustive-values.empty_cfg.stderr

@@ -7,7 +7,6 @@ LL | #[cfg(test = "value")]
    |           help: remove the value
    |
    = note: no expected value for `test`
-   = help: to expect this configuration use `--check-cfg=cfg(test, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
 

tests/ui/check-cfg/exhaustive-values.without_names.stderr

@@ -7,7 +7,6 @@ LL | #[cfg(test = "value")]
    |           help: remove the value
    |
    = note: no expected value for `test`
-   = help: to expect this configuration use `--check-cfg=cfg(test, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
 

tests/ui/check-cfg/no-expected-values.empty.stderr

@@ -20,7 +20,6 @@ LL | #[cfg(test = "foo")]
    |           help: remove the value
    |
    = note: no expected value for `test`
-   = help: to expect this configuration use `--check-cfg=cfg(test, values("foo"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: 2 warnings emitted

tests/ui/check-cfg/no-expected-values.mixed.stderr

@@ -20,7 +20,6 @@ LL | #[cfg(test = "foo")]
    |           help: remove the value
    |
    = note: no expected value for `test`
-   = help: to expect this configuration use `--check-cfg=cfg(test, values("foo"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: 2 warnings emitted

tests/ui/check-cfg/no-expected-values.simple.stderr

@@ -20,7 +20,6 @@ LL | #[cfg(test = "foo")]
    |           help: remove the value
    |
    = note: no expected value for `test`
-   = help: to expect this configuration use `--check-cfg=cfg(test, values("foo"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: 2 warnings emitted