no_std support in the Rust guest bindings.

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

@@ -53,6 +53,10 @@ pub struct Opts {
     /// crate.
     #[cfg_attr(feature = "clap", arg(skip))]
     pub standalone: bool,
+
+    /// If true, code generation should avoid any features that depend on `std`.
+    #[cfg_attr(feature = "clap", arg(long))]
+    pub no_std: bool,
 }
 
 #[derive(Default)]
@@ -92,6 +96,10 @@ impl RustWasm {
 }
 
 impl RustGenerator for RustWasm {
+    fn use_std(&self) -> bool {
+        !self.opts.no_std
+    }
+
     fn default_param_mode(&self) -> TypeMode {
         if self.in_import {
             // We default to borrowing as much as possible to maximize the ability
@@ -167,6 +175,10 @@ impl Generator for RustWasm {
             ));
         }
 
+        self.src.push_str("#[allow(unused_imports)]");
+        self.src
+            .push_str("use wit_bindgen_guest_rust::rt::{alloc, vec::Vec, string::String};");
+
         self.sizes.fill(iface);
     }
 
@@ -758,14 +770,14 @@ 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 {{\nalloc::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
+                        alloc::dealloc(ptr, layout);\n
                     }\n
                 }\n",
             );
@@ -1231,7 +1243,7 @@ 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 +1280,7 @@ 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\")"
                 };
@@ -1400,15 +1412,15 @@ impl Bindgen for FunctionBindgen<'_> {
                 let size = self.gen.sizes.size(element);
                 let align = self.gen.sizes.align(element);
                 self.push_str(&format!(
-                    "let {layout} = core::alloc::Layout::from_size_align_unchecked({vec}.len() * {size}, {align});\n",
+                    "let {layout} = 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{{\nlet ptr = 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{{\nalloc::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/src/lib.rs

@@ -14,6 +14,11 @@ pub enum TypeMode {
 }
 
 pub trait RustGenerator {
+    /// Return true iff the generator can use `std` features in its output.
+    fn use_std(&self) -> bool {
+        true
+    }
+
     fn push_str(&mut self, s: &str);
     fn info(&self, ty: TypeId) -> TypeInfo;
     fn types_mut(&mut self) -> &mut Types;
@@ -828,9 +833,11 @@ 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 self.use_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-macro/src/lib.rs

@@ -52,6 +52,7 @@ mod kw {
     syn::custom_keyword!(paths);
     syn::custom_keyword!(unchecked);
     syn::custom_keyword!(multi_module);
+    syn::custom_keyword!(no_std);
 }
 
 impl Parse for Opts {
@@ -69,6 +70,7 @@ impl Parse for Opts {
                     ConfigField::Unchecked => opts.unchecked = true,
                     ConfigField::MultiModule => opts.multi_module = true,
                     ConfigField::Interfaces(v) => interfaces = v,
+                    ConfigField::NoStd => opts.no_std = true,
                 }
             }
             if interfaces.is_empty() {
@@ -104,6 +106,7 @@ enum ConfigField {
     Interfaces(Vec<Interface>),
     Unchecked,
     MultiModule,
+    NoStd,
 }
 
 impl Parse for ConfigField {
@@ -141,6 +144,9 @@ impl Parse for ConfigField {
         } else if l.peek(kw::multi_module) {
             input.parse::<kw::multi_module>()?;
             Ok(ConfigField::MultiModule)
+        } else if l.peek(kw::no_std) {
+            input.parse::<kw::no_std>()?;
+            Ok(ConfigField::NoStd)
         } else {
             Err(l.error())
         }

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 +128,10 @@ pub unsafe trait LocalHandle: HandleType {
 
 #[doc(hidden)]
 pub mod rt {
-    use std::alloc::{self, Layout};
+    use super::alloc::alloc::Layout;
+
+    // Re-export things from liballoc for convenient use.
+    pub use super::alloc::{alloc, string, vec};
 
     #[no_mangle]
     unsafe extern "C" fn cabi_realloc(