Auto merge of #126158 - Urgau:disallow-cfgs, r=petrochenkov

Disallow setting some built-in cfg via set the command-line

This PR disallow users from setting some built-in cfg via set the command-line in order to prevent incoherent state, eg. `windows` cfg active but target is Linux based.

This implements MCP https://github.com/rust-lang/compiler-team/issues/610, with the caveat that we disallow cfgs no matter if they make sense or not, since I don't think it's useful to allow users to set a cfg that will be set anyway. It also complicates the implementation.

------

The `explicit_builtin_cfgs_in_flags` lint detects builtin cfgs set via the `--cfg` flag.

*(deny-by-default)*

### Example

```text
rustc --cfg unix
```

```rust,ignore (needs command line option)
fn main() {}
```

This will produce:

```text
error: unexpected `--cfg unix` flag
  |
  = note: config `unix` is only supposed to be controlled by `--target`
  = note: manually setting a built-in cfg can and does create incoherent behaviours
  = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
```

### Explanation

Setting builtin cfgs can and does produce incoherent behaviour, it's better to the use the appropriate `rustc` flag that controls the config. For example setting the `windows` cfg but on Linux based target.

-----

r? `@petrochenkov`
cc `@jyn514`

try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: x86_64-mingw
try-job: i686-msvc
try-job: i686-mingw
try-job: x86_64-gnu-llvm-17
try-job: dist-various-1

4 files changed, 30 insertions, 3 deletions

diff --git a/compiler/rustc_lint/src/context.rs b/compiler/rustc_lint/src/context.rs
index 11ad1aa0e8d..c9e2eee16b3 100644
--- a/compiler/rustc_lint/src/context.rs
+++ b/compiler/rustc_lint/src/context.rs
@@ -533,7 +533,7 @@ pub struct EarlyContext<'a> {
 }
 
 impl EarlyContext<'_> {
-    /// Emit a lint at the appropriate level, with an optional associated span and an existing
+    /// Emit a lint at the appropriate level, with an associated span and an existing
     /// diagnostic.
     ///
     /// [`lint_level`]: rustc_middle::lint::lint_level#decorate-signature
@@ -544,7 +544,21 @@ impl EarlyContext<'_> {
         span: MultiSpan,
         diagnostic: BuiltinLintDiag,
     ) {
-        self.opt_span_lint(lint, Some(span), |diag| {
+        self.opt_span_lint_with_diagnostics(lint, Some(span), diagnostic);
+    }
+
+    /// Emit a lint at the appropriate level, with an optional associated span and an existing
+    /// diagnostic.
+    ///
+    /// [`lint_level`]: rustc_middle::lint::lint_level#decorate-signature
+    #[rustc_lint_diagnostics]
+    pub fn opt_span_lint_with_diagnostics(
+        &self,
+        lint: &'static Lint,
+        span: Option<MultiSpan>,
+        diagnostic: BuiltinLintDiag,
+    ) {
+        self.opt_span_lint(lint, span, |diag| {
             diagnostics::decorate_lint(self.sess(), diagnostic, diag);
         });
     }
diff --git a/compiler/rustc_lint/src/context/diagnostics.rs b/compiler/rustc_lint/src/context/diagnostics.rs
index a96af076477..f289d4c81b3 100644
--- a/compiler/rustc_lint/src/context/diagnostics.rs
+++ b/compiler/rustc_lint/src/context/diagnostics.rs
@@ -438,5 +438,8 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
         BuiltinLintDiag::OutOfScopeMacroCalls { path } => {
             lints::OutOfScopeMacroCalls { path }.decorate_lint(diag)
         }
+        BuiltinLintDiag::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by } => {
+            lints::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by }.decorate_lint(diag)
+        }
     }
 }
diff --git a/compiler/rustc_lint/src/early.rs b/compiler/rustc_lint/src/early.rs
index 7b04e8c39e6..6fb0a624644 100644
--- a/compiler/rustc_lint/src/early.rs
+++ b/compiler/rustc_lint/src/early.rs
@@ -47,7 +47,7 @@ impl<'a, T: EarlyLintPass> EarlyContextAndPass<'a, T> {
     fn inlined_check_id(&mut self, id: ast::NodeId) {
         for early_lint in self.context.buffered.take(id) {
             let BufferedEarlyLint { span, node_id: _, lint_id, diagnostic } = early_lint;
-            self.context.span_lint_with_diagnostics(lint_id.lint, span, diagnostic);
+            self.context.opt_span_lint_with_diagnostics(lint_id.lint, span, diagnostic);
         }
     }
 
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index 1a657d31865..03962d796f4 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -2380,6 +2380,16 @@ pub mod unexpected_cfg_value {
 }
 
 #[derive(LintDiagnostic)]
+#[diag(lint_unexpected_builtin_cfg)]
+#[note(lint_controlled_by)]
+#[note(lint_incoherent)]
+pub struct UnexpectedBuiltinCfg {
+    pub(crate) cfg: String,
+    pub(crate) cfg_name: Symbol,
+    pub(crate) controlled_by: &'static str,
+}
+
+#[derive(LintDiagnostic)]
 #[diag(lint_macro_use_deprecated)]
 #[help]
 pub struct MacroUseDeprecated;