Update no_std support in cranelift-frontend.

lib/frontend/Cargo.toml

@@ -13,11 +13,12 @@ readme = "README.md"
 cranelift-codegen = { path = "../codegen", version = "0.22.0", default-features = false }
 target-lexicon = { version = "0.0.3", default-features = false }
 log = { version = "0.4.4", default-features = false, features = ["release_max_level_warn"] }
+hashmap_core = { version = "0.1.9", optional = true }
 
 [features]
 default = ["std"]
 std = ["cranelift-codegen/std"]
-core = ["cranelift-codegen/core"]
+core = ["hashmap_core", "cranelift-codegen/core"]
 
 [badges]
 maintenance = { status = "experimental" }

lib/frontend/src/frontend.rs

@@ -653,7 +653,6 @@ impl<'a> FunctionBuilder<'a> {
 
 #[cfg(test)]
 mod tests {
-
     use cranelift_codegen::entity::EntityRef;
     use cranelift_codegen::ir::types::*;
     use cranelift_codegen::ir::{AbiParam, ExternalName, Function, InstBuilder, Signature};
@@ -661,6 +660,7 @@ mod tests {
     use cranelift_codegen::settings::CallConv;
     use cranelift_codegen::verifier::verify_function;
     use frontend::{FunctionBuilder, FunctionBuilderContext};
+    use std::string::ToString;
     use Variable;
 
     fn sample_function(lazy_seal: bool) {

lib/frontend/src/lib.rs

@@ -176,6 +176,9 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(not(feature = "std"), feature(alloc))]
 
+#[cfg(not(feature = "std"))]
+#[macro_use]
+extern crate alloc;
 extern crate cranelift_codegen;
 #[cfg(test)]
 extern crate target_lexicon;
@@ -191,10 +194,16 @@ mod ssa;
 mod switch;
 mod variable;
 
+/// This replaces `std` in builds with `core`.
 #[cfg(not(feature = "std"))]
 mod std {
-    extern crate alloc;
-
-    pub use self::alloc::vec;
+    pub use alloc::{string, vec};
     pub use core::*;
+    pub mod collections {
+        #[allow(unused_extern_crates)]
+        extern crate hashmap_core;
+
+        pub use self::hashmap_core::map as hash_map;
+        pub use self::hashmap_core::{HashMap, HashSet};
+    }
 }

lib/frontend/src/switch.rs

@@ -2,6 +2,7 @@ use cranelift_codegen::ir::condcodes::IntCC;
 use cranelift_codegen::ir::*;
 use frontend::FunctionBuilder;
 use std::collections::HashMap;
+use std::vec::Vec;
 
 type EntryIndex = u64;
 
@@ -185,6 +186,7 @@ mod tests {
     use super::*;
     use cranelift_codegen::ir::Function;
     use frontend::FunctionBuilderContext;
+    use std::string::ToString;
 
     macro_rules! setup {
         ($default:expr, [$($index:expr,)*]) => {{