fix: get Heltec V3 (ESP32-S3) building with esp toolchain

Use nightly cargo + esp rustc (RUSTC override) + -Zbuild-std=core for
Xtensa targets. This avoids the esp cargo's inability to parse modern
crate manifests (edition 2024) while still using esp's LLVM with full
Xtensa support.

Pin proc-macro-crate to 3.2.0 to avoid transitive deps requiring
rustc 1.85+.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -1362,11 +1362,11 @@ dependencies = [
 
 [[package]]
 name = "proc-macro-crate"
-version = "3.5.0"
+version = "3.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e67ba7e9b2b56446f1d419b1d807906278ffa1a658a8a5d8a39dcb1f5a78614f"
+checksum = "8ecf48c7ca261d60b74ab1a7b20da18bede46776b2e55535cb958eb595c5fa7b"
 dependencies = [
- "toml_edit 0.25.8+spec-1.1.0",
+ "toml_edit",
 ]
 
 [[package]]
@@ -1720,8 +1720,8 @@ checksum = "dc1beb996b9d83529a9e75c17a1686767d148d70663143c7854d8b4a09ced362"
 dependencies = [
  "serde",
  "serde_spanned",
- "toml_datetime 0.6.11",
- "toml_edit 0.22.27",
+ "toml_datetime",
+ "toml_edit",
 ]
 
 [[package]]
@@ -1733,15 +1733,6 @@ dependencies = [
  "serde",
 ]
 
-[[package]]
-name = "toml_datetime"
-version = "1.1.0+spec-1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "97251a7c317e03ad83774a8752a7e81fb6067740609f75ea2b585b569a59198f"
-dependencies = [
- "serde_core",
-]
-
 [[package]]
 name = "toml_edit"
 version = "0.22.27"
@@ -1751,30 +1742,9 @@ dependencies = [
  "indexmap",
  "serde",
  "serde_spanned",
- "toml_datetime 0.6.11",
+ "toml_datetime",
  "toml_write",
- "winnow 0.7.15",
-]
-
-[[package]]
-name = "toml_edit"
-version = "0.25.8+spec-1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "16bff38f1d86c47f9ff0647e6838d7bb362522bdf44006c7068c2b1e606f1f3c"
-dependencies = [
- "indexmap",
- "toml_datetime 1.1.0+spec-1.1.0",
- "toml_parser",
- "winnow 1.0.0",
-]
-
-[[package]]
-name = "toml_parser"
-version = "1.1.0+spec-1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2334f11ee363607eb04df9b8fc8a13ca1715a72ba8662a26ac285c98aabb4011"
-dependencies = [
- "winnow 1.0.0",
+ "winnow",
 ]
 
 [[package]]
@@ -1914,15 +1884,6 @@ dependencies = [
  "memchr",
 ]
 
-[[package]]
-name = "winnow"
-version = "1.0.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a90e88e4667264a994d34e6d1ab2d26d398dcdca8b7f52bec8668957517fc7d8"
-dependencies = [
- "memchr",
-]
-
 [[package]]
 name = "xtensa-lx"
 version = "0.9.0"

Justfile

@@ -1,8 +1,8 @@
 set shell := ["bash", "-c"]
 
 # Board definitions: feature target chip
-# Xtensa targets require the esp toolchain (espup); check-all gracefully skips
-# boards whose toolchain isn't installed.
+# Xtensa targets use nightly cargo + esp rustc + -Zbuild-std=core.
+# check-all gracefully skips boards whose toolchain isn't installed.
 heltec_v3 := "heltec_v3 xtensa-esp32s3-none-elf esp32s3"
 rak_4631  := "rak_4631 thumbv7em-none-eabihf nRF52840_xxAA"
 
@@ -29,9 +29,9 @@ check board:
 # Run clippy on a single board
 clippy board:
     @read -r feat target chip <<< "$(just _info {{board}})"; \
-    tc=""; \
-    case "$target" in xtensa-*) tc="+esp" ;; esac; \
-    cargo $tc clippy --target $target --features $feat -- -D warnings
+    env=""; extra=""; \
+    case "$target" in xtensa-*) env="RUSTC=$(rustup which rustc --toolchain esp)"; extra="+nightly -Zbuild-std=core";; esac; \
+    eval $env cargo $extra clippy --target $target --features $feat -- -D warnings
 
 # Build release firmware and copy to firmware/ with a readable name
 build board profile="release":
@@ -50,13 +50,14 @@ size board:
 
 # ── Private helpers ───────────────────────────────────────────────────
 
-# Run a cargo command for a board, using +esp toolchain for Xtensa targets
+# Run a cargo command for a board.
+# Xtensa: nightly cargo + esp rustc (via RUSTC override) + -Zbuild-std=core
 [private]
 _cargo board cmd:
     @read -r feat target chip <<< "$(just _info {{board}})"; \
-    tc=""; \
-    case "$target" in xtensa-*) tc="+esp" ;; esac; \
-    cargo $tc {{cmd}} --target $target --features $feat
+    env=""; extra=""; \
+    case "$target" in xtensa-*) env="RUSTC=$(rustup which rustc --toolchain esp)"; extra="+nightly -Zbuild-std=core";; esac; \
+    eval $env cargo $extra {{cmd}} --target $target --features $feat
 
 # Check if a board's toolchain is available
 [private]