bazel: re-land wasmtime 42 upgrade with LTO fix

.bazelrc

@@ -35,6 +35,20 @@ build --@rules_go//go/config:pure
 build:gofips --//bazel:gofips=True --@rules_go//go/toolchain:sdk_name=go_sdk_with_systemcrypto
 build:gofips --@rules_go//go/config:pure=false
 
+#################
+# Rust settings #
+#################
+
+# Use rules_rust's Rust-side allocator_library that defines the
+# mangled `__rustc::*` allocator shim symbols (rust_alloc,
+# rust_dealloc, etc.). The default cc_allocator_library predates the
+# allocator-symbol mangling in rustc 1.87+ and leaves those symbols
+# undefined when a Rust staticlib is linked into a cc_binary via
+# cc_common.link. See bazelbuild/rules_rust#3459.
+build --@rules_rust//rust/settings:experimental_use_allocator_libraries_with_mangled_symbols=True
+
+# end of rust settings #
+
 # Improve caching and readability of filepaths that the compiler uses by not using
 # absolute paths, and instead use paths that are relative to the redpanda repo.
 # https://github.com/bazel-contrib/toolchains_llvm/pull/445#issuecomment-2605443516
@@ -149,7 +163,22 @@ build:san-all  --linkopt=-fsanitize=address,undefined,vptr,function,alignment
 # PGO + release, but should work with any.
 build:lto --copt -flto=thin
 build:lto --linkopt -flto=thin
-build:lto --@rules_rust//rust/settings:lto=fat
+# Use thin LTO for Rust to match the C++ side. With fat LTO, rustc's
+# LLVM Internalize pass downgrades the auto-injected `__rustc::*`
+# allocator shim symbols from GLOBAL to LOCAL binding when emitting the
+# empty staticlib provided by rules_rust's
+# `experimental_use_allocator_libraries_with_mangled_symbols`. Nothing
+# inside the empty crate references those symbols, so LTO assumes
+# they're internal. The rust stdlib rlibs still emit GLOBAL undefined
+# refs to them, and ELF won't bind a GLOBAL ref to a LOCAL def across
+# object files, so the cc_binary link fails with:
+#   ld.lld: error: undefined symbol: __rustc::__rust_no_alloc_shim_is_unstable_v2
+# Thin LTO doesn't run Internalize across the crate the way fat LTO
+# does, so the symbols stay GLOBAL and the link succeeds. We don't
+# lose any wasm performance: every wasmtime crate is an rlib (rules_rust
+# emits `-Clinker-plugin-lto` regardless of -Clto mode) and the actual
+# cross-language LTO happens at the C++ link step, which is thin too.
+build:lto --@rules_rust//rust/settings:lto=thin
 build:lto --@rules_rust//rust/settings:extra_rustc_flag=-Ccodegen-units=1
 # I've seen others claim these flags help with LTO and the final binary, but these also
 # require a specific version of clang and I don't really want to tie rustc version with

MODULE.bazel

@@ -351,7 +351,7 @@ rust.toolchain(
         "aarch64-unknown-linux-gnu",
         "x86_64-unknown-linux-gnu",
     ],
-    versions = ["1.86.0"],
+    versions = ["1.91.0"],
 )
 use_repo(rust, "rust_toolchains")