sess: users cannot set target modifier cfgs

As with all other builtin cfgs, target modifier cfgs cannot be set by
users.

compiler/rustc_session/src/config/cfg.rs

@@ -20,6 +20,7 @@
 //!  - 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 std::borrow::Cow;
 use std::hash::Hash;
 use std::iter;
 
@@ -30,7 +31,7 @@ use rustc_lint_defs::builtin::EXPLICIT_BUILTIN_CFGS_IN_FLAGS;
 use rustc_span::{Symbol, sym};
 use rustc_target::spec::{PanicStrategy, RelocModel, SanitizerSet, Target};
 
-use crate::config::{CrateType, FmtDebug};
+use crate::config::{CG_OPTIONS, CrateType, FmtDebug, OptionDesc, OptionPrefix, Z_OPTIONS};
 use crate::{Session, errors};
 
 /// The parsed `--cfg` options that define the compilation environment of the
@@ -115,20 +116,20 @@ pub(crate) fn disallow_cfgs(sess: &Session, user_cfgs: &Cfg) {
 
     for cfg in user_cfgs {
         match cfg {
-            (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::contract_checks, None) => disallow(cfg, "-Z contract-checks"),
-            (sym::sanitize, None | Some(_)) => disallow(cfg, "-Z sanitizer"),
+            (sym::overflow_checks, None) => disallow(cfg, Cow::Borrowed("-C overflow-checks")),
+            (sym::debug_assertions, None) => disallow(cfg, Cow::Borrowed("-C debug-assertions")),
+            (sym::ub_checks, None) => disallow(cfg, Cow::Borrowed("-Z ub-checks")),
+            (sym::contract_checks, None) => disallow(cfg, Cow::Borrowed("-Z contract-checks")),
+            (sym::sanitize, None | Some(_)) => disallow(cfg, Cow::Borrowed("-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"),
+            ) => disallow(cfg, Cow::Borrowed("-Z sanitizer=cfi")),
+            (sym::proc_macro, None) => disallow(cfg, Cow::Borrowed("--crate-type proc-macro")),
             (sym::panic, Some(sym::abort | sym::unwind | sym::immediate_abort)) => {
-                disallow(cfg, "-C panic")
+                disallow(cfg, Cow::Borrowed("-C panic"))
             }
-            (sym::target_feature, Some(_)) => disallow(cfg, "-C target-feature"),
+            (sym::target_feature, Some(_)) => disallow(cfg, Cow::Borrowed("-C target-feature")),
             (sym::unix, None)
             | (sym::windows, None)
             | (sym::relocation_model, Some(_))
@@ -147,12 +148,32 @@ pub(crate) fn disallow_cfgs(sess: &Session, user_cfgs: &Cfg) {
             | (sym::target_has_reliable_f16_math, None | Some(_))
             | (sym::target_has_reliable_f128, None | Some(_))
             | (sym::target_has_reliable_f128_math, None | Some(_))
-            | (sym::target_thread_local, None) => disallow(cfg, "--target"),
-            (sym::fmt_debug, None | Some(_)) => disallow(cfg, "-Z fmt-debug"),
-            (sym::emscripten_wasm_eh, None | Some(_)) => disallow(cfg, "-Z emscripten_wasm_eh"),
+            | (sym::target_thread_local, None) => disallow(cfg, Cow::Borrowed("--target")),
+            (sym::fmt_debug, None | Some(_)) => disallow(cfg, Cow::Borrowed("-Z fmt-debug")),
+            (sym::emscripten_wasm_eh, None | Some(_)) => {
+                disallow(cfg, Cow::Borrowed("-Z emscripten_wasm_eh"))
+            }
             _ => {}
         }
     }
+
+    // Also prohibit setting any target modifier cfgs, these all have corresponding flags.
+    fn target_modifiers_with_cfgs(
+        desc: &OptionDesc<impl OptionPrefix>,
+    ) -> Option<(String, Symbol)> {
+        desc.target_modifier_cfg().map(|sym| (desc.flag(), sym))
+    }
+    let check_target_modifier_with_cfg = |flag, flag_cfg| {
+        if let Some(matching_cfg) = user_cfgs.iter().find(|(cfg, _)| *cfg == flag_cfg) {
+            disallow(matching_cfg, Cow::Owned(flag));
+        }
+    };
+    for (flag, cfg) in CG_OPTIONS.iter().filter_map(target_modifiers_with_cfgs) {
+        check_target_modifier_with_cfg(flag, cfg);
+    }
+    for (flag, cfg) in Z_OPTIONS.iter().filter_map(target_modifiers_with_cfgs) {
+        check_target_modifier_with_cfg(flag, cfg);
+    }
 }
 
 /// Generate the default configs for a given session

compiler/rustc_session/src/errors.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::num::{NonZero, ParseIntError};
 
 use rustc_ast::token;
@@ -514,5 +515,5 @@ pub(crate) struct SoftFloatDeprecated;
 pub(crate) struct UnexpectedBuiltinCfg {
     pub(crate) cfg: String,
     pub(crate) cfg_name: Symbol,
-    pub(crate) controlled_by: &'static str,
+    pub(crate) controlled_by: Cow<'static, str>,
 }

tests/ui/cfg/target-modifiers/user-sets.branch_protection.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_modifier_branch_protection="bti"` flag
+   |
+   = note: config `target_modifier_branch_protection` is only supposed to be controlled by `-Z branch-protection`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/target-modifiers/user-sets.fixed_x18.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_modifier_fixed_x18` flag
+   |
+   = note: config `target_modifier_fixed_x18` is only supposed to be controlled by `-Z fixed-x18`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/target-modifiers/user-sets.indirect_branch_cs_prefix.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_modifier_indirect_branch_cs_prefix` flag
+   |
+   = note: config `target_modifier_indirect_branch_cs_prefix` is only supposed to be controlled by `-Z indirect-branch-cs-prefix`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/target-modifiers/user-sets.reg_struct_return.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_modifier_reg_struct_return` flag
+   |
+   = note: config `target_modifier_reg_struct_return` is only supposed to be controlled by `-Z reg-struct-return`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/target-modifiers/user-sets.regparm.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_modifier_regparm="0"` flag
+   |
+   = note: config `target_modifier_regparm` is only supposed to be controlled by `-Z regparm`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/target-modifiers/user-sets.retpoline.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_modifier_retpoline` flag
+   |
+   = note: config `target_modifier_retpoline` is only supposed to be controlled by `-Z retpoline`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/target-modifiers/user-sets.retpoline_external_thunk.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_modifier_retpoline_external_thunk` flag
+   |
+   = note: config `target_modifier_retpoline_external_thunk` is only supposed to be controlled by `-Z retpoline-external-thunk`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/target-modifiers/user-sets.rs

@@ -0,0 +1,14 @@
+//@ check-fail
+//@ compile-flags: --crate-type=lib
+//@ revisions: fixed_x18 indirect_branch_cs_prefix regparm
+//@ revisions: reg_struct_return retpoline retpoline_external_thunk
+//@[fixed_x18] compile-flags: --cfg target_modifier_fixed_x18
+//@[indirect_branch_cs_prefix] compile-flags: --cfg target_modifier_indirect_branch_cs_prefix
+//@[regparm] compile-flags: --cfg target_modifier_regparm="0"
+//@[reg_struct_return] compile-flags: --cfg target_modifier_reg_struct_return
+//@[retpoline] compile-flags: --cfg target_modifier_retpoline
+//@[retpoline_external_thunk] compile-flags: --cfg target_modifier_retpoline_external_thunk
+
+fn main() {}
+
+//~? ERROR unexpected `--cfg