fix: target-pointer-width must be an integer in target spec JSON

rustc 1.91 (rust-lang/rust#144443) changed target-pointer-width in
custom target JSON from a string to an integer. Bump the pinned
toolchain to 1.94 to pick up that requirement and fix the template.

zeroos-build now detects the active rustc minor version at runtime and
emits the value as a quoted string for toolchains older than 1.91,
keeping the tool portable.

Also fix a direct function-item-to-integer cast in zeroos-arch-riscv
that became a hard error under -D warnings once the
function_casts_as_integer lint (rust-lang/rust#141470, stable in 1.93)
was active.

Author: sagar-a16z

crates/zeroos-arch-riscv/src/ops.rs

@@ -130,7 +130,7 @@ pub const ARCH_OPS: ArchOps = ArchOps {
     thread_ctx_set_ra: crate::thread_ctx::thread_ctx_set_ra,
     thread_ctx_set_retval: crate::thread_ctx::thread_ctx_set_retval,
     switch_to,
-    ret_from_fork: || ret_from_fork as usize,
+    ret_from_fork: || ret_from_fork as *const () as usize,
     trap_frame_clone,
     trap_frame_init,
     trap_frame_set_retval,

crates/zeroos-build/src/files/generic-linux.json.template

@@ -6,7 +6,7 @@
   "llvm-abiname": "{{ ABI }}",
   "data-layout": "{{ DATA_LAYOUT }}",
   "target-endian": "{{ ENDIAN }}",
-  "target-pointer-width": "{{ POINTER_WIDTH }}",
+  "target-pointer-width": {{ POINTER_WIDTH }},
   "os": "{{ OS }}",
   "env": "{{ ENV }}",
   "vendor": "{{ VENDOR }}",

crates/zeroos-build/src/spec/utils.rs

@@ -4,6 +4,21 @@ use crate::spec::llvm::LLVMConfig;
 use crate::spec::ArchSpec;
 use mini_template as ztpl;
 
+/// Returns the rustc minor version (e.g. 91 for 1.91.0), or None if it cannot be determined.
+/// Respects the RUSTC env var so that callers running inside a Cargo build script use the
+/// correct toolchain.
+fn rustc_minor_version() -> Option<u32> {
+    let rustc = std::env::var("RUSTC").unwrap_or_else(|_| "rustc".to_string());
+    let output = std::process::Command::new(rustc)
+        .arg("--version")
+        .output()
+        .ok()?;
+    // "rustc 1.94.0 (4a4ef493e 2026-03-02)"
+    let stdout = String::from_utf8(output.stdout).ok()?;
+    let version = stdout.split_whitespace().nth(1)?;
+    version.split('.').nth(1)?.parse().ok()
+}
+
 #[derive(Debug, Clone, Copy)]
 pub struct TargetRenderOptions {
     /// Whether to emit DWARF unwind tables (.eh_frame sections)
@@ -54,14 +69,21 @@ impl TargetConfig {
     ) -> Result<String, String> {
         let template = GENERIC_LINUX_TEMPLATE;
 
+        // target-pointer-width changed from a JSON string to a JSON integer in rustc 1.91
+        // (rust-lang/rust#144443). Default to the integer form for unknown/new versions.
+        let pointer_width_json = match rustc_minor_version() {
+            Some(minor) if minor < 91 => format!("\"{}\"", arch_spec.pointer_width),
+            _ => arch_spec.pointer_width.to_string(),
+        };
+
         let ctx = ztpl::Context::new()
             .with_str("ARCH", arch_spec.arch)
             .with_str("CPU", arch_spec.cpu)
             .with_str("FEATURES", &llvm_config.features)
             .with_str("LLVM_TARGET", &llvm_config.llvm_target)
             .with_str("ABI", &llvm_config.abi)
             .with_str("DATA_LAYOUT", &llvm_config.data_layout)
-            .with_str("POINTER_WIDTH", arch_spec.pointer_width)
+            .with_str("POINTER_WIDTH", &pointer_width_json)
             .with_str("ENDIAN", arch_spec.endian)
             .with_str("OS", &self.os)
             .with_str("ENV", &self.abi)

rust-toolchain.toml

@@ -1,5 +1,5 @@
 [toolchain]
-channel = "1.90"
+channel = "1.94"
 targets = [
   "riscv64imac-unknown-none-elf",
   "riscv32imac-unknown-none-elf",