Disallow setting built-in cfgs via set the command-line

compiler/rustc_lint/messages.ftl

@@ -741,6 +741,10 @@ lint_undropped_manually_drops = calls to `std::mem::drop` with `std::mem::Manual
     .label = argument has type `{$arg_ty}`
     .suggestion = use `std::mem::ManuallyDrop::into_inner` to get the inner value
 
+lint_unexpected_builtin_cfg = unexpected `--cfg {$cfg}` flag
+    .controlled_by = config `{$cfg_name}` is only supposed to be controlled by `{$controlled_by}`
+    .issue = see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+
 lint_unexpected_cfg_add_build_rs_println = or consider adding `{$build_rs_println}` to the top of the `build.rs`
 lint_unexpected_cfg_add_cargo_feature = consider using a Cargo feature instead
 lint_unexpected_cfg_add_cargo_toml_lint_cfg = or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:{$cargo_toml_lint_cfg}

compiler/rustc_lint/src/context/diagnostics.rs

@@ -424,5 +424,8 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
             lints::InnerAttributeUnstable::CustomInnerAttribute
         }
         .decorate_lint(diag),
+        BuiltinLintDiag::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by } => {
+            lints::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by }.decorate_lint(diag)
+        }
     }
 }

compiler/rustc_lint/src/lints.rs

@@ -2319,6 +2319,16 @@ pub mod unexpected_cfg_value {
     }
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_unexpected_builtin_cfg)]
+#[note(lint_controlled_by)]
+#[note(lint_issue)]
+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]

compiler/rustc_lint_defs/src/builtin.rs

@@ -105,6 +105,7 @@ declare_lint_pass! {
         UNCONDITIONAL_RECURSION,
         UNCOVERED_PARAM_IN_PROJECTION,
         UNDEFINED_NAKED_FUNCTION_ABI,
+        UNEXPECTED_BUILTIN_CFGS,
         UNEXPECTED_CFGS,
         UNFULFILLED_LINT_EXPECTATIONS,
         UNINHABITED_STATIC,
@@ -3268,6 +3269,39 @@ declare_lint! {
     "detects unexpected names and values in `#[cfg]` conditions",
 }
 
+declare_lint! {
+    /// The `unexpected_builtin_cfgs` lint detects builtin cfgs set via the CLI, `--cfg`.
+    ///
+    /// ### 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: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+    ///   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+    /// ```
+    ///
+    /// ### Explanation
+    ///
+    /// Setting builtin cfgs can and does produce incoherent behavior, 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.
+    pub UNEXPECTED_BUILTIN_CFGS,
+    Deny,
+    "detects builtin cfgs set via the `--cfg`"
+}
+
 declare_lint! {
     /// The `repr_transparent_external_private_fields` lint
     /// detects types marked `#[repr(transparent)]` that (transitively)

compiler/rustc_lint_defs/src/lib.rs

@@ -742,6 +742,11 @@ pub enum BuiltinLintDiag {
     InnerAttributeUnstable {
         is_macro: bool,
     },
+    UnexpectedBuiltinCfg {
+        cfg: String,
+        cfg_name: Symbol,
+        controlled_by: &'static str,
+    },
 }
 
 /// Lints that are buffered up early on in the `Session` before the

compiler/rustc_session/src/config.rs

@@ -1292,7 +1292,10 @@ pub(crate) const fn default_lib_output() -> CrateType {
 }
 
 pub fn build_configuration(sess: &Session, mut user_cfg: Cfg) -> Cfg {
-    // Combine the configuration requested by the session (command line) with
+    // First disallow some configuration given on the command line
+    cfg::disallow_cfgs(sess, &user_cfg);
+
+    // Then combine the configuration requested by the session (command line) with
     // some default and generated configuration items.
     user_cfg.extend(cfg::default_configuration(sess));
     user_cfg

compiler/rustc_session/src/config/cfg.rs

@@ -17,9 +17,13 @@
 //!  - Add the activation logic in [`default_configuration`]
 //!  - Add the cfg to [`CheckCfg::fill_well_known`] (and related files),
 //!    so that the compiler can know the cfg is expected
+//!  - Add the cfg in [`disallow_cfgs`] to disallow users from setting it via `--cfg`
 //!  - Add the feature gating in `compiler/rustc_feature/src/builtin_attrs.rs`
 
+use rustc_ast::ast;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
+use rustc_lint_defs::builtin::UNEXPECTED_BUILTIN_CFGS;
+use rustc_lint_defs::BuiltinLintDiag;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_target::abi::Align;
 use rustc_target::spec::{PanicStrategy, RelocModel, SanitizerSet};
@@ -83,6 +87,75 @@ impl<'a, T: Eq + Hash + Copy + 'a> Extend<&'a T> for ExpectedValues<T> {
     }
 }
 
+/// Disallow "builtin" cfgs from the CLI as they produce incoherent behavior
+pub(crate) fn disallow_cfgs(sess: &Session, user_cfgs: &Cfg) {
+    let disallow = |cfg: &(Symbol, Option<Symbol>), controlled_by| {
+        let cfg_name = cfg.0;
+        let cfg = if let Some(value) = cfg.1 {
+            format!(r#"{}="{}""#, cfg_name, value)
+        } else {
+            format!("{}", cfg_name)
+        };
+        sess.psess.opt_span_buffer_lint(
+            UNEXPECTED_BUILTIN_CFGS,
+            None,
+            ast::CRATE_NODE_ID,
+            BuiltinLintDiag::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by },
+        )
+    };
+
+    // We want to restrict setting cfgs that will produce "incoherent" behavior between
+    // the cfg and the "real" flag that sets it.
+    //
+    // The tests are in tests/ui/cfg/disallowed-cli-cfgs.rs.
+    //
+    // Not all "well known cfgs" can be disallowed, as they are set by external tools via the
+    // CLI, we therefore don't lint on them:
+    //
+    //  - clippy
+    //  - doc
+    //  - doctest
+    //  - miri
+    //  - rustfmt
+    //
+    // doc/doctest: https://github.com/rust-lang/rust/blob/d03d6c0fead582c98c6446ec92456ca8fd03ff65/src/librustdoc/doctest.rs#L134-L135
+    // clippy: https://github.com/rust-lang/rust/blob/f1b0d54ca9a5ea43950c279985a6e12dcf387de8/src/tools/clippy/src/driver.rs#L276
+    // miri: https://github.com/rust-lang/rust/blob/d03d6c0fead582c98c6446ec92456ca8fd03ff65/src/tools/miri/src/lib.rs#L161
+
+    for cfg in user_cfgs {
+        match cfg {
+            (sym::test, None) => disallow(cfg, "--test"),
+            (sym::overflow_checks, None) => disallow(cfg, "-C overflow-checks"),
+            (sym::debug_assertions, None) => disallow(cfg, "-C debug-assertions"),
+            (sym::ub_checks, None) => disallow(cfg, "-Z ub-checks"),
+            (sym::sanitize, None | Some(_)) => disallow(cfg, "-Z sanitizer"),
+            (
+                sym::sanitizer_cfi_generalize_pointers | sym::sanitizer_cfi_normalize_integers,
+                None | Some(_),
+            ) => disallow(cfg, "-Z sanitizer=cfi"),
+            (sym::proc_macro, None) => disallow(cfg, "--crate-type proc-macro"),
+            (sym::panic, Some(sym::abort | sym::unwind)) => disallow(cfg, "-C panic"),
+            (sym::target_feature, Some(_)) => disallow(cfg, "-C target-feature"),
+            (sym::unix, None)
+            | (sym::windows, None)
+            | (sym::relocation_model, Some(_))
+            | (sym::target_abi, None | Some(_))
+            | (sym::target_arch, Some(_))
+            | (sym::target_endian, Some(_))
+            | (sym::target_env, None | Some(_))
+            | (sym::target_family, Some(_))
+            | (sym::target_os, Some(_))
+            | (sym::target_pointer_width, Some(_))
+            | (sym::target_vendor, None | Some(_))
+            | (sym::target_has_atomic, Some(_))
+            | (sym::target_has_atomic_equal_alignment, Some(_))
+            | (sym::target_has_atomic_load_store, Some(_))
+            | (sym::target_thread_local, None) => disallow(cfg, "--target"),
+            _ => {}
+        }
+    }
+}
+
 /// Generate the default configs for a given session
 pub(crate) fn default_configuration(sess: &Session) -> Cfg {
     let mut ret = Cfg::default();

tests/ui/cfg/disallowed-cli-cfgs-allow.rs

@@ -0,0 +1,4 @@
+//@ check-pass
+//@ compile-flags: --cfg unix -Aunexpected_builtin_cfgs
+
+fn main() {}

tests/ui/cfg/disallowed-cli-cfgs.debug_assertions_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg debug_assertions` flag
+   |
+   = note: config `debug_assertions` is only supposed to be controlled by `-C debug-assertions`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.overflow_checks_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg overflow_checks` flag
+   |
+   = note: config `overflow_checks` is only supposed to be controlled by `-C overflow-checks`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.panic_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg panic="abort"` flag
+   |
+   = note: config `panic` is only supposed to be controlled by `-C panic`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.proc_macro_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg proc_macro` flag
+   |
+   = note: config `proc_macro` is only supposed to be controlled by `--crate-type proc-macro`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.relocation_model_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg relocation_model="a"` flag
+   |
+   = note: config `relocation_model` is only supposed to be controlled by `--target`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.rs

@@ -0,0 +1,36 @@
+//@ check-fail
+//@ revisions: test_ overflow_checks_ debug_assertions_ ub_checks_ sanitize_
+//@ revisions: sanitizer_cfi_generalize_pointers_ sanitizer_cfi_normalize_integers_
+//@ revisions: proc_macro_ panic_ target_feature_ unix_ windows_ target_abi_
+//@ revisions: target_arch_ target_endian_ target_env_ target_family_ target_os_
+//@ revisions: target_pointer_width_ target_vendor_ target_has_atomic_
+//@ revisions: target_has_atomic_equal_alignment_ target_has_atomic_load_store_
+//@ revisions: target_thread_local_ relocation_model_
+
+//@ [test_]compile-flags: --cfg test
+//@ [overflow_checks_]compile-flags: --cfg overflow_checks
+//@ [debug_assertions_]compile-flags: --cfg debug_assertions
+//@ [ub_checks_]compile-flags: --cfg ub_checks
+//@ [sanitize_]compile-flags: --cfg sanitize="cfi"
+//@ [sanitizer_cfi_generalize_pointers_]compile-flags: --cfg sanitizer_cfi_generalize_pointers
+//@ [sanitizer_cfi_normalize_integers_]compile-flags: --cfg sanitizer_cfi_normalize_integers
+//@ [proc_macro_]compile-flags: --cfg proc_macro
+//@ [panic_]compile-flags: --cfg panic="abort"
+//@ [target_feature_]compile-flags: --cfg target_feature="sse3"
+//@ [unix_]compile-flags: --cfg unix
+//@ [windows_]compile-flags: --cfg windows
+//@ [target_abi_]compile-flags: --cfg target_abi="gnu"
+//@ [target_arch_]compile-flags: --cfg target_arch="arm"
+//@ [target_endian_]compile-flags: --cfg target_endian="little"
+//@ [target_env_]compile-flags: --cfg target_env
+//@ [target_family_]compile-flags: --cfg target_family="unix"
+//@ [target_os_]compile-flags: --cfg target_os="linux"
+//@ [target_pointer_width_]compile-flags: --cfg target_pointer_width="32"
+//@ [target_vendor_]compile-flags: --cfg target_vendor
+//@ [target_has_atomic_]compile-flags: --cfg target_has_atomic="32"
+//@ [target_has_atomic_equal_alignment_]compile-flags: --cfg target_has_atomic_equal_alignment="32"
+//@ [target_has_atomic_load_store_]compile-flags: --cfg target_has_atomic_load_store="32"
+//@ [target_thread_local_]compile-flags: --cfg target_thread_local
+//@ [relocation_model_]compile-flags: --cfg relocation_model="a"
+
+fn main() {}

tests/ui/cfg/disallowed-cli-cfgs.sanitize_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg sanitize="cfi"` flag
+   |
+   = note: config `sanitize` is only supposed to be controlled by `-Z sanitizer`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.sanitizer_cfi_generalize_pointers_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg sanitizer_cfi_generalize_pointers` flag
+   |
+   = note: config `sanitizer_cfi_generalize_pointers` is only supposed to be controlled by `-Z sanitizer=cfi`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.sanitizer_cfi_normalize_integers_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg sanitizer_cfi_normalize_integers` flag
+   |
+   = note: config `sanitizer_cfi_normalize_integers` is only supposed to be controlled by `-Z sanitizer=cfi`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.target_abi_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_abi="gnu"` flag
+   |
+   = note: config `target_abi` is only supposed to be controlled by `--target`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.target_arch_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_arch="arm"` flag
+   |
+   = note: config `target_arch` is only supposed to be controlled by `--target`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.target_endian_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_endian="little"` flag
+   |
+   = note: config `target_endian` is only supposed to be controlled by `--target`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.target_env_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_env` flag
+   |
+   = note: config `target_env` is only supposed to be controlled by `--target`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.target_family_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_family="unix"` flag
+   |
+   = note: config `target_family` is only supposed to be controlled by `--target`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.target_feature_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_feature="sse3"` flag
+   |
+   = note: config `target_feature` is only supposed to be controlled by `-C target-feature`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.target_has_atomic_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_has_atomic="32"` flag
+   |
+   = note: config `target_has_atomic` is only supposed to be controlled by `--target`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.target_has_atomic_equal_alignment_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_has_atomic_equal_alignment="32"` flag
+   |
+   = note: config `target_has_atomic_equal_alignment` is only supposed to be controlled by `--target`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.target_has_atomic_load_store_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_has_atomic_load_store="32"` flag
+   |
+   = note: config `target_has_atomic_load_store` is only supposed to be controlled by `--target`
+   = note: see <https://github.com/rust-lang/rust/issues/xxxxx> for more information
+   = note: `#[deny(unexpected_builtin_cfgs)]` on by default
+
+error: aborting due to 1 previous error
+