no_std support in the Rust guest bindings.

This adds no_std support in the Rust guest bindings. Mostly this
involves using `core` and `alloc` instead of `std`, but it also
involves adding `extern crate alloc;` in a few places, and also
adding a `"std"` cargo feature to gen-guest-rust so that the
`impl std::error::Error` can be made conditional.

This will eventually be useful for generating bindings from the
WASI wit files for std itself to use. And, it's useful for
experimenting with generating minimal bindings.

Author: sunfishcode

crates/gen-guest-rust/Cargo.toml

@@ -10,10 +10,14 @@ doctest = false
 
 [dependencies]
 wit-bindgen-core = { workspace = true }
-wit-bindgen-gen-rust-lib = { workspace = true }
+wit-bindgen-gen-rust-lib = { workspace = true, default-features = false }
 heck = { workspace = true }
 structopt = { workspace = true, optional = true }
 
 [dev-dependencies]
 wit-bindgen-guest-rust = { path = '../guest-rust' }
 test-helpers = { path = '../test-helpers', features = ['guest-rust'] }
+
+[features]
+default = ["std"]
+std = ["wit-bindgen-gen-rust-lib/std"]

crates/gen-guest-rust/src/lib.rs

@@ -758,14 +758,15 @@ impl FunctionBindgen<'_> {
     fn emit_cleanup(&mut self) {
         for (ptr, layout) in mem::take(&mut self.cleanup) {
             self.push_str(&format!(
-                "if {layout}.size() != 0 {{\nstd::alloc::dealloc({ptr}, {layout});\n}}\n"
+                "if {layout}.size() != 0 {{\nextern crate alloc;\nalloc::alloc::dealloc({ptr}, {layout});\n}}\n"
             ));
         }
         if self.needs_cleanup_list {
             self.push_str(
                 "for (ptr, layout) in cleanup_list {\n
                     if layout.size() != 0 {\n
-                        std::alloc::dealloc(ptr, layout);\n
+                        extern crate alloc;\n
+                        alloc::alloc::dealloc(ptr, layout);\n
                     }\n
                 }\n",
             );
@@ -1231,7 +1232,7 @@ impl Bindgen for FunctionBindgen<'_> {
                 assert_eq!(none, "()");
                 let operand = &operands[0];
                 let invalid = if unchecked {
-                    "std::hint::unreachable_unchecked()"
+                    "core::hint::unreachable_unchecked()"
                 } else {
                     "panic!(\"invalid enum discriminant\")"
                 };
@@ -1268,7 +1269,7 @@ impl Bindgen for FunctionBindgen<'_> {
                 let ok = self.blocks.pop().unwrap();
                 let operand = &operands[0];
                 let invalid = if unchecked {
-                    "std::hint::unreachable_unchecked()"
+                    "core::hint::unreachable_unchecked()"
                 } else {
                     "panic!(\"invalid enum discriminant\")"
                 };
@@ -1403,12 +1404,12 @@ impl Bindgen for FunctionBindgen<'_> {
                     "let {layout} = core::alloc::Layout::from_size_align_unchecked({vec}.len() * {size}, {align});\n",
                 ));
                 self.push_str(&format!(
-                    "let {result} = if {layout}.size() != 0\n{{\nlet ptr = std::alloc::alloc({layout});\n",
+                    "let {result} = if {layout}.size() != 0\n{{\nextern crate alloc;\nlet ptr = alloc::alloc::alloc({layout});\n",
                 ));
                 self.push_str(&format!(
-                    "if ptr.is_null()\n{{\nstd::alloc::handle_alloc_error({layout});\n}}\nptr\n}}",
+                    "if ptr.is_null()\n{{\nextern crate alloc;\nalloc::alloc::handle_alloc_error({layout});\n}}\nptr\n}}",
                 ));
-                self.push_str(&format!("else {{\nstd::ptr::null_mut()\n}};\n",));
+                self.push_str(&format!("else {{\ncore::ptr::null_mut()\n}};\n",));
                 self.push_str(&format!("for (i, e) in {vec}.into_iter().enumerate() {{\n",));
                 self.push_str(&format!(
                     "let base = {result} as i32 + (i as i32) * {size};\n",

crates/gen-rust-lib/Cargo.toml

@@ -11,3 +11,7 @@ test = false
 [dependencies]
 wit-bindgen-core = { workspace = true }
 heck = { workspace = true }
+
+[features]
+default = ["std"]
+std = []

crates/gen-rust-lib/src/lib.rs

@@ -828,9 +828,11 @@ pub trait RustGenerator {
             self.push_str("}\n");
             self.push_str("}\n");
             self.push_str("\n");
-            self.push_str("impl std::error::Error for ");
-            self.push_str(&name);
-            self.push_str("{}\n");
+            if cfg!(feature = "std") {
+                self.push_str("impl std::error::Error for ");
+                self.push_str(&name);
+                self.push_str("{}\n");
+            }
         } else {
             self.print_rust_enum_debug(
                 id,

crates/guest-rust/src/lib.rs

@@ -1,7 +1,12 @@
-use std::fmt;
-use std::marker;
-use std::mem;
-use std::ops::Deref;
+#![no_std]
+
+extern crate alloc;
+
+use alloc::boxed::Box;
+use core::fmt;
+use core::marker;
+use core::mem;
+use core::ops::Deref;
 
 #[cfg(feature = "macros")]
 pub use wit_bindgen_guest_rust_macro::{export, import};
@@ -123,7 +128,7 @@ pub unsafe trait LocalHandle: HandleType {
 
 #[doc(hidden)]
 pub mod rt {
-    use std::alloc::{self, Layout};
+    use ::alloc::alloc::{self, Layout};
 
     #[no_mangle]
     unsafe extern "C" fn cabi_realloc(