From 9af936debb3d8f03bd26b325a8f21b94dd8af767 Mon Sep 17 00:00:00 2001
From: jinmay <britzegs@gmail.com>
Date: Mon, 20 Jul 2026 13:42:33 +0900
Subject: [PATCH] ci: use `build.warnings` instead of `RUSTFLAGS=-D warnings`

---
 .github/workflows/build.yml | 5 ++++-
 noxfile.py                  | 6 +++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 5615f07c4cb..60e91422fec 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -135,5 +135,8 @@ jobs:
     env:
       CARGO_TERM_VERBOSE: ${{ inputs.verbose }}
       RUST_BACKTRACE: 1
-      RUSTFLAGS: "-D warnings"
+      # `build.warnings` avoids the cache-busting from `RUSTFLAGS`; respected on
+      # Cargo 1.97+ only (MSRV jobs fall back to warn). rustdoc isn't covered by
+      # it, so RUSTDOCFLAGS stays.
+      CARGO_BUILD_WARNINGS: deny
       RUSTDOCFLAGS: "-D warnings"
diff --git a/noxfile.py b/noxfile.py
index 8553b24739f..636238a27f3 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -1665,10 +1665,10 @@ def check_feature_powerset(session: nox.Session):
         *abi3t_version_features,
     ]
 
-    # deny warnings
+    # deny warnings; `build.warnings` avoids the cache-busting from `RUSTFLAGS`
+    # and is respected on Cargo 1.97+ only (MSRV falls back to warn).
     env = os.environ.copy()
-    rust_flags = env.get("RUSTFLAGS", "")
-    env["RUSTFLAGS"] = f"{rust_flags} -Dwarnings"
+    env["CARGO_BUILD_WARNINGS"] = "deny"
 
     subcommand = "hack"
     if "minimal-versions" in session.posargs: