#[cfg]: suggest alternative target_ name when the value does not match

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

@@ -316,9 +316,27 @@ pub(super) fn unexpected_cfg_value(
     let is_from_cargo = rustc_session::utils::was_invoked_from_cargo();
     let is_from_external_macro = name_span.in_external_macro(sess.source_map());
 
-    // Show the full list if all possible values for a given name, but don't do it
-    // for names as the possibilities could be very long
-    let code_sugg = if !possibilities.is_empty() {
+    let code_sugg = if let Some((value, _)) = value
+        && sess.psess.check_config.well_known_names.contains(&name)
+        && let valid_names = possible_well_known_names_for_cfg_value(sess, value)
+        && !valid_names.is_empty()
+    {
+        // Suggest changing the name to something for which `value` is an expected value.
+        let max_suggestions = 3;
+        let suggestions = valid_names
+            .iter()
+            .take(max_suggestions)
+            .copied()
+            .map(|name| lints::unexpected_cfg_value::ChangeNameSuggestion {
+                span: name_span,
+                name,
+                value,
+            })
+            .collect::<Vec<_>>();
+        lints::unexpected_cfg_value::CodeSuggestion::ChangeName { suggestions }
+    } else if !possibilities.is_empty() {
+        // Show the full list if all possible values for a given name, but don't do it
+        // for names as the possibilities could be very long
         let expected_values = {
             let (possibilities, and_more) = sort_and_truncate_possibilities(
                 sess,
@@ -419,3 +437,22 @@ pub(super) fn unexpected_cfg_value(
         value: value.map_or_else(String::new, |(v, _span)| v.to_string()),
     }
 }
+
+/// Ordering of the output is not stable, use this only in diagnostic code.
+fn possible_well_known_names_for_cfg_value(sess: &Session, value: Symbol) -> Vec<Symbol> {
+    #[allow(rustc::potential_query_instability)]
+    sess.psess
+        .check_config
+        .well_known_names
+        .iter()
+        .filter(|name| {
+            sess.psess
+                .check_config
+                .expecteds
+                .get(*name)
+                .map(|expected_values| expected_values.contains(&Some(value)))
+                .unwrap_or_default()
+        })
+        .copied()
+        .collect()
+}

compiler/rustc_lint/src/lints.rs

@@ -2904,6 +2904,10 @@ pub(crate) enum CodeSuggestion {
 
             name: Symbol,
         },
+        ChangeName {
+            #[subdiagnostic]
+            suggestions: Vec<ChangeNameSuggestion>,
+        },
     }
 
     #[derive(Subdiagnostic)]
@@ -2961,6 +2965,20 @@ pub(crate) struct ExpectedValues {
         pub and_more: usize,
     }
 
+    #[derive(Subdiagnostic)]
+    #[suggestion(
+        "`{$value}` is an expected value for `{$name}`",
+        code = "{name}",
+        applicability = "maybe-incorrect",
+        style = "verbose"
+    )]
+    pub(crate) struct ChangeNameSuggestion {
+        #[primary_span]
+        pub span: Span,
+        pub name: Symbol,
+        pub value: Symbol,
+    }
+
     #[derive(Subdiagnostic)]
     pub(crate) enum InvocationHelp {
         #[note(

compiler/rustc_session/src/config/cfg.rs

@@ -65,6 +65,13 @@ fn insert(&mut self, value: T) -> bool {
             ExpectedValues::Any => false,
         }
     }
+
+    pub fn contains(&self, value: &Option<T>) -> bool {
+        match self {
+            ExpectedValues::Some(expecteds) => expecteds.contains(value),
+            ExpectedValues::Any => false,
+        }
+    }
 }
 
 impl<T: Eq + Hash> Extend<T> for ExpectedValues<T> {

tests/ui/cfg/suggest-alternative-name-on-target.rs

@@ -5,34 +5,35 @@
 #[cfg(target_abi = "arm")]
 //~^ ERROR unexpected `cfg` condition value:
 //~| NOTE see <https://doc.rust-lang.org
-//~| NOTE expected values for `target_abi` are
+//~| HELP `arm` is an expected value for `target_arch`
 struct A;
 
 // target env used in `target_arch`
 #[cfg(target_arch = "gnu")]
 //~^ ERROR unexpected `cfg` condition value:
 //~| NOTE see <https://doc.rust-lang.org
-//~| NOTE expected values for `target_arch` are
+//~| HELP `gnu` is an expected value for `target_env`
 struct B;
 
 // target os used in `target_env`
 #[cfg(target_env = "openbsd")]
 //~^ ERROR unexpected `cfg` condition value:
 //~| NOTE see <https://doc.rust-lang.org
-//~| NOTE expected values for `target_env` are
+//~| HELP `openbsd` is an expected value for `target_os`
 struct C;
 
 // target abi used in `target_os`
 #[cfg(target_os = "eabi")]
 //~^ ERROR unexpected `cfg` condition value:
 //~| NOTE see <https://doc.rust-lang.org
-//~| NOTE expected values for `target_os` are
+//~| HELP `eabi` is an expected value for `target_abi`
 struct D;
 
 #[cfg(target_abi = "windows")]
 //~^ ERROR unexpected `cfg` condition value:
 //~| NOTE see <https://doc.rust-lang.org
-//~| NOTE expected values for `target_abi` are
+//~| HELP `windows` is an expected value for `target_os`
+//~| HELP `windows` is an expected value for `target_family`
 struct E;
 
 fn main() {}

tests/ui/cfg/suggest-alternative-name-on-target.stderr

@@ -4,13 +4,17 @@ error: unexpected `cfg` condition value: `arm`
 LL | #[cfg(target_abi = "arm")]
    |       ^^^^^^^^^^^^^^^^^^
    |
-   = note: expected values for `target_abi` are: ``, `abi64`, `abiv2`, `abiv2hf`, `eabi`, `eabihf`, `elfv1`, `elfv2`, `fortanix`, `ilp32`, `ilp32e`, `llvm`, `macabi`, `sim`, `softfloat`, `spe`, `uwp`, `vec-extabi`, and `x32`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 note: the lint level is defined here
   --> $DIR/suggest-alternative-name-on-target.rs:1:9
    |
 LL | #![deny(unexpected_cfgs)]
    |         ^^^^^^^^^^^^^^^
+help: `arm` is an expected value for `target_arch`
+   |
+LL - #[cfg(target_abi = "arm")]
+LL + #[cfg(target_arch = "arm")]
+   |
 
 error: unexpected `cfg` condition value: `gnu`
   --> $DIR/suggest-alternative-name-on-target.rs:12:7
@@ -18,8 +22,12 @@ error: unexpected `cfg` condition value: `gnu`
 LL | #[cfg(target_arch = "gnu")]
    |       ^^^^^^^^^^^^^^^^^^^
    |
-   = note: expected values for `target_arch` are: `aarch64`, `amdgpu`, `arm`, `arm64ec`, `avr`, `bpf`, `csky`, `hexagon`, `loongarch32`, `loongarch64`, `m68k`, `mips`, `mips32r6`, `mips64`, `mips64r6`, `msp430`, `nvptx64`, `powerpc`, `powerpc64`, `riscv32`, `riscv64`, `s390x`, `sparc`, `sparc64`, `wasm32`, `wasm64`, `x86`, `x86_64`, and `xtensa`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+help: `gnu` is an expected value for `target_env`
+   |
+LL - #[cfg(target_arch = "gnu")]
+LL + #[cfg(target_env = "gnu")]
+   |
 
 error: unexpected `cfg` condition value: `openbsd`
   --> $DIR/suggest-alternative-name-on-target.rs:19:7
@@ -27,8 +35,12 @@ error: unexpected `cfg` condition value: `openbsd`
 LL | #[cfg(target_env = "openbsd")]
    |       ^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: expected values for `target_env` are: ``, `gnu`, `macabi`, `mlibc`, `msvc`, `musl`, `newlib`, `nto70`, `nto71`, `nto71_iosock`, `nto80`, `ohos`, `p1`, `p2`, `p3`, `relibc`, `sgx`, `sim`, `uclibc`, and `v5`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+help: `openbsd` is an expected value for `target_os`
+   |
+LL - #[cfg(target_env = "openbsd")]
+LL + #[cfg(target_os = "openbsd")]
+   |
 
 error: unexpected `cfg` condition value: `eabi`
   --> $DIR/suggest-alternative-name-on-target.rs:26:7
@@ -36,8 +48,30 @@ error: unexpected `cfg` condition value: `eabi`
 LL | #[cfg(target_os = "eabi")]
    |       ^^^^^^^^^^^^^^^^^^
    |
-   = note: expected values for `target_os` are: `aix`, `amdhsa`, `android`, `cuda`, `cygwin`, `dragonfly`, `emscripten`, `espidf`, `freebsd`, `fuchsia`, `haiku`, `helenos`, `hermit`, `horizon`, `hurd`, `illumos`, `ios`, `l4re`, `linux`, `lynxos178`, `macos`, `managarm`, `motor`, `netbsd`, `none`, `nto`, `nuttx`, `openbsd`, `psp`, `psx`, `qurt`, `redox`, `rtems`, `solaris`, and `solid_asp3` and 14 more
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+help: `eabi` is an expected value for `target_abi`
+   |
+LL - #[cfg(target_os = "eabi")]
+LL + #[cfg(target_abi = "eabi")]
+   |
 
-error: aborting due to 4 previous errors
+error: unexpected `cfg` condition value: `windows`
+  --> $DIR/suggest-alternative-name-on-target.rs:32:7
+   |
+LL | #[cfg(target_abi = "windows")]
+   |       ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+help: `windows` is an expected value for `target_os`
+   |
+LL - #[cfg(target_abi = "windows")]
+LL + #[cfg(target_os = "windows")]
+   |
+help: `windows` is an expected value for `target_family`
+   |
+LL - #[cfg(target_abi = "windows")]
+LL + #[cfg(target_family = "windows")]
+   |
+
+error: aborting due to 5 previous errors