Allow building without std (#1069)

Closes https://github.com/CraneStation/cranelift/issues/1067

cranelift/frontend/Cargo.toml

@@ -14,13 +14,13 @@ edition = "2018"
 cranelift-codegen = { path = "../cranelift-codegen", version = "0.44.0", default-features = false }
 target-lexicon = "0.8.1"
 log = { version = "0.4.6", default-features = false }
-hashmap_core = { version = "0.1.9", optional = true }
+hashbrown = { version = "0.6", optional = true }
 smallvec = { version = "0.6.10" }
 
 [features]
 default = ["std"]
 std = ["cranelift-codegen/std"]
-core = ["hashmap_core", "cranelift-codegen/core"]
+core = ["hashbrown", "cranelift-codegen/core"]
 
 # Temporary feature that enforces basic block semantics.
 basic-blocks = ["cranelift-codegen/basic-blocks"]

cranelift/frontend/src/frontend.rs

@@ -341,7 +341,7 @@ impl<'a> FunctionBuilder<'a> {
     /// This will not do anything unless `func.dfg.collect_debug_info` is called first.
     pub fn set_val_label(&mut self, val: Value, label: ValueLabel) {
         if let Some(values_labels) = self.func.dfg.values_labels.as_mut() {
-            use std::collections::hash_map::Entry;
+            use crate::hash_map::Entry;
 
             let start = ValueLabelStart {
                 from: self.srcloc,

cranelift/frontend/src/lib.rs

@@ -191,9 +191,9 @@ extern crate alloc as std;
 extern crate std;
 
 #[cfg(not(feature = "std"))]
-use hashmap_core::HashMap;
+use hashbrown::{hash_map, HashMap};
 #[cfg(feature = "std")]
-use std::collections::HashMap;
+use std::collections::{hash_map, HashMap};
 
 pub use crate::frontend::{FunctionBuilder, FunctionBuilderContext};
 pub use crate::switch::Switch;