Upgrade Rust toolchain to nightly-2026-04-01

Updates rust-toolchain.toml and REQUIRED_RUST_TOOLCHAIN in build.rs
to nightly-2026-04-01 (commit 48cc71ee88cd0f11217eced958b9930970da998b).

Author: LegNeato

crates/rustc_codegen_spirv/build.rs

@@ -19,9 +19,9 @@ use std::{env, fs, mem};
 /// `cargo publish`. We need to figure out a way to do this properly, but let's hardcode it for now :/
 //const REQUIRED_RUST_TOOLCHAIN: &str = include_str!("../../rust-toolchain.toml");
 const REQUIRED_RUST_TOOLCHAIN: &str = r#"[toolchain]
-channel = "nightly-2026-03-14"
+channel = "nightly-2026-04-01"
 components = ["rust-src", "rustc-dev", "llvm-tools"]
-# commit_hash = 1d8897a4e88051480b36c501e66809425c6e08ad"#;
+# commit_hash = 48cc71ee88cd0f11217eced958b9930970da998b"#;
 
 fn rustc_output(arg: &str) -> Result<String, Box<dyn Error>> {
     let rustc = env::var("RUSTC").unwrap_or_else(|_| "rustc".into());

rust-toolchain.toml

@@ -1,7 +1,7 @@
 [toolchain]
-channel = "nightly-2026-03-14"
+channel = "nightly-2026-04-01"
 components = ["rust-src", "rustc-dev", "llvm-tools"]
-# commit_hash = 1d8897a4e88051480b36c501e66809425c6e08ad
+# commit_hash = 48cc71ee88cd0f11217eced958b9930970da998b
 
 # Whenever changing the nightly channel, update the commit hash above, and
 # change `REQUIRED_RUST_TOOLCHAIN` in `crates/rustc_codegen_spirv/build.rs` too.