Enable conditional compilation checking on the Rust codebase

src/bootstrap/builder.rs

@@ -26,6 +26,7 @@ use crate::run;
 use crate::test;
 use crate::tool::{self, SourceType};
 use crate::util::{self, add_dylib_path, add_link_lib_path, exe, libdir};
+use crate::EXTRA_CHECK_CFGS;
 use crate::{Build, CLang, DocTests, GitRepo, Mode};
 
 pub use crate::Compiler;
@@ -1095,6 +1096,33 @@ impl<'a> Builder<'a> {
             rustflags.arg("-Zunstable-options");
         }
 
+        // #[cfg(not(bootstrap)]
+        if stage != 0 {
+            // Enable cfg checking of cargo features
+            // FIXME: De-comment this when cargo beta get support for it
+            // cargo.arg("-Zcheck-cfg-features");
+
+            // Enable cfg checking of rustc well-known names
+            rustflags.arg("-Zunstable-options").arg("--check-cfg=names()");
+
+            // Add extra cfg not defined in rustc
+            for (restricted_mode, name, values) in EXTRA_CHECK_CFGS {
+                if *restricted_mode == None || *restricted_mode == Some(mode) {
+                    // Creating a string of the values by concatenating each value:
+                    // ',"tvos","watchos"' or '' (nothing) when there are no values
+                    let values = match values {
+                        Some(values) => values
+                            .iter()
+                            .map(|val| [",", "\"", val, "\""])
+                            .flatten()
+                            .collect::<String>(),
+                        None => String::new(),
+                    };
+                    rustflags.arg(&format!("--check-cfg=values({name}{values})"));
+                }
+            }
+        }
+
         // FIXME: It might be better to use the same value for both `RUSTFLAGS` and `RUSTDOCFLAGS`,
         // but this breaks CI. At the very least, stage0 `rustdoc` needs `--cfg bootstrap`. See
         // #71458.

src/bootstrap/lib.rs

@@ -186,6 +186,34 @@ const LLVM_TOOLS: &[&str] = &[
 
 pub const VERSION: usize = 2;
 
+/// Extra --check-cfg to add when building
+/// (Mode restriction, config name, config values (if any))
+const EXTRA_CHECK_CFGS: &[(Option<Mode>, &'static str, Option<&[&'static str]>)] = &[
+    (None, "bootstrap", None),
+    (Some(Mode::Rustc), "parallel_compiler", None),
+    (Some(Mode::ToolRustc), "parallel_compiler", None),
+    (Some(Mode::Std), "miri", None),
+    (Some(Mode::Std), "stdarch_intel_sde", None),
+    (Some(Mode::Std), "no_fp_fmt_parse", None),
+    (Some(Mode::Std), "no_global_oom_handling", None),
+    (Some(Mode::Std), "freebsd12", None),
+    (Some(Mode::Std), "backtrace_in_libstd", None),
+    // FIXME: Used by rustfmt is their test but is invalid (neither cargo nor bootstrap ever set
+    // this config) should probably by removed or use a allow attribute.
+    (Some(Mode::ToolRustc), "release", None),
+    // FIXME: Used by stdarch in their test, should use a allow attribute instead.
+    (Some(Mode::Std), "dont_compile_me", None),
+    // FIXME: Used by serde_json, but we should not be triggering on external dependencies.
+    (Some(Mode::Rustc), "no_btreemap_remove_entry", None),
+    (Some(Mode::ToolRustc), "no_btreemap_remove_entry", None),
+    // FIXME: Used by crossbeam-utils, but we should not be triggering on external dependencies.
+    (Some(Mode::Rustc), "crossbeam_loom", None),
+    (Some(Mode::ToolRustc), "crossbeam_loom", None),
+    // FIXME: Used by proc-macro2, but we should not be triggering on external dependencies.
+    (Some(Mode::Rustc), "span_locations", None),
+    (Some(Mode::ToolRustc), "span_locations", None),
+];
+
 /// A structure representing a Rust compiler.
 ///
 /// Each compiler has a `stage` that it is associated with and a `host` that