Auto merge of #108920 - matthiaskrgr:rollup-qrr9a0u, r=matthiaskrgr

Rollup of 8 pull requests

Successful merges:

 - #108754 (Retry `pred_known_to_hold_modulo_regions` with fulfillment if ambiguous)
 - #108759 (1.41.1 supported 32-bit Apple targets)
 - #108839 (Canonicalize root var when making response from new solver)
 - #108856 (Remove DropAndReplace terminator)
 - #108882 (Tweak E0740)
 - #108898 (Set `LIBC_CHECK_CFG=1` when building Rust code in bootstrap)
 - #108911 (Improve rustdoc-gui/tester.js code a bit)
 - #108916 (Remove an unused return value in `rustc_hir_typeck`)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

1 files changed, 9 insertions, 0 deletions

diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 9b5e116aa5c..47d742ef796 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -1721,6 +1721,15 @@ impl<'a> Builder<'a> {
 
         cargo.env("RUSTC_VERBOSE", self.verbosity.to_string());
 
+        // Downstream forks of the Rust compiler might want to use a custom libc to add support for
+        // targets that are not yet available upstream. Adding a patch to replace libc with a
+        // custom one would cause compilation errors though, because Cargo would interpret the
+        // custom libc as part of the workspace, and apply the check-cfg lints on it.
+        //
+        // The libc build script emits check-cfg flags only when this environment variable is set,
+        // so this line allows the use of custom libcs.
+        cargo.env("LIBC_CHECK_CFG", "1");
+
         if source_type == SourceType::InTree {
             let mut lint_flags = Vec::new();
             // When extending this list, add the new lints to the RUSTFLAGS of the