Migrate back to std:: stylistically (#554)

* Migrate back to `std::` stylistically

This commit moves away from idioms such as `alloc::` and `core::` as
imports of standard data structures and types. Instead it migrates all
crates to uniformly use `std::` for importing standard data structures
and types. This also removes the `std` and `core` features from all
crates to and removes any conditional checking for `feature = "std"`

All of this support was previously added in #407 in an effort to make
wasmtime/cranelift "`no_std` compatible". Unfortunately though this
change comes at a cost:

* The usage of `alloc` and `core` isn't idiomatic. Especially trying to
  dual between types like `HashMap` from `std` as well as from
  `hashbrown` causes imports to be surprising in some cases.
* Unfortunately there was no CI check that crates were `no_std`, so none
  of them actually were. Many crates still imported from `std` or
  depended on crates that used `std`.

It's important to note, however, that **this does not mean that wasmtime
will not run in embedded environments**. The style of the code today and
idioms aren't ready in Rust to support this degree of multiplexing and
makes it somewhat difficult to keep up with the style of `wasmtime`.
Instead it's intended that embedded runtime support will be added as
necessary. Currently only `std` is necessary to build `wasmtime`, and
platforms that natively need to execute `wasmtime` will need to use a
Rust target that supports `std`. Note though that not all of `std` needs
to be supported, but instead much of it could be configured off to
return errors, and `wasmtime` would be configured to gracefully handle
errors.

The goal of this PR is to move `wasmtime` back to idiomatic usage of
features/`std`/imports/etc and help development in the short-term.
Long-term when platform concerns arise (if any) they can be addressed by
moving back to `no_std` crates (but fixing the issues mentioned above)
or ensuring that the target in Rust has `std` available.

* Start filling out platform support doc

crates/jit/src/action.rs

@@ -2,11 +2,9 @@
 
 use crate::compiler::Compiler;
 use crate::instantiate::SetupError;
-use alloc::string::String;
-use alloc::vec::Vec;
-use core::cmp::max;
-use core::{fmt, mem, ptr, slice};
 use cranelift_codegen::ir;
+use std::cmp::max;
+use std::{fmt, mem, ptr, slice};
 use thiserror::Error;
 use wasmtime_runtime::{wasmtime_call_trampoline, Export, InstanceHandle, VMInvokeArgument};
 

crates/jit/src/code_memory.rs

@@ -1,11 +1,8 @@
 //! Memory management for executable code.
 
 use crate::function_table::FunctionTable;
-use alloc::boxed::Box;
-use alloc::string::String;
-use alloc::vec::Vec;
-use core::{cmp, mem};
 use region;
+use std::{cmp, mem};
 use wasmtime_environ::{Compilation, CompiledFunction};
 use wasmtime_runtime::{Mmap, VMFunctionBody};
 

crates/jit/src/compiler.rs

@@ -1,13 +1,8 @@
 //! JIT compilation.
 
-use super::HashMap;
 use crate::code_memory::CodeMemory;
 use crate::instantiate::SetupError;
 use crate::target_tunables::target_tunables;
-use alloc::boxed::Box;
-use alloc::string::String;
-use alloc::vec::Vec;
-use core::convert::TryFrom;
 use cranelift_codegen::ir::InstBuilder;
 use cranelift_codegen::isa::{TargetFrontendConfig, TargetIsa};
 use cranelift_codegen::print_errors::pretty_error;
@@ -16,6 +11,8 @@ use cranelift_codegen::{binemit, ir};
 use cranelift_entity::{EntityRef, PrimaryMap};
 use cranelift_frontend::{FunctionBuilder, FunctionBuilderContext};
 use cranelift_wasm::{DefinedFuncIndex, DefinedMemoryIndex, ModuleTranslationState};
+use std::collections::HashMap;
+use std::convert::TryFrom;
 use wasmtime_debug::{emit_debugsections_image, DebugInfoData};
 use wasmtime_environ::{
     Compilation, CompileError, CompiledFunction, Compiler as _C, FunctionBodyData, Module,
@@ -202,7 +199,7 @@ impl Compiler {
         signature: &ir::Signature,
         value_size: usize,
     ) -> Result<*const VMFunctionBody, SetupError> {
-        use super::hash_map::Entry::{Occupied, Vacant};
+        use std::collections::hash_map::Entry::{Occupied, Vacant};
         Ok(match self.trampoline_park.entry(callee_address) {
             Occupied(entry) => *entry.get(),
             Vacant(entry) => {

crates/jit/src/context.rs

@@ -1,14 +1,12 @@
 use crate::action::{get, inspect_memory, invoke};
-use crate::HashMap;
 use crate::{
     instantiate, ActionError, ActionOutcome, CompilationStrategy, CompiledModule, Compiler,
     InstanceHandle, Namespace, RuntimeValue, SetupError,
 };
-use alloc::boxed::Box;
-use alloc::rc::Rc;
-use alloc::string::{String, ToString};
-use core::cell::RefCell;
 use cranelift_codegen::isa::TargetIsa;
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::rc::Rc;
 use thiserror::Error;
 use wasmparser::{validate, OperatorValidatorConfig, ValidatingParserConfig};
 

crates/jit/src/instantiate.rs

@@ -3,19 +3,15 @@
 //! `CompiledModule` to allow compiling and instantiating to be done as separate
 //! steps.
 
-use super::HashMap;
 use crate::compiler::Compiler;
 use crate::link::link_module;
 use crate::resolver::Resolver;
-use alloc::boxed::Box;
-use alloc::rc::Rc;
-use alloc::string::String;
-use alloc::vec::Vec;
-use core::cell::RefCell;
 use cranelift_entity::{BoxedSlice, PrimaryMap};
 use cranelift_wasm::{DefinedFuncIndex, SignatureIndex};
-#[cfg(feature = "std")]
+use std::cell::RefCell;
+use std::collections::HashMap;
 use std::io::Write;
+use std::rc::Rc;
 use thiserror::Error;
 use wasmtime_debug::read_debuginfo;
 use wasmtime_environ::{
@@ -120,7 +116,6 @@ impl<'data> RawCompiledModule<'data> {
         // Make all code compiled thus far executable.
         compiler.publish_compiled_code();
 
-        #[cfg(feature = "std")]
         let dbg_jit_registration = if let Some(img) = dbg_image {
             let mut bytes = Vec::new();
             bytes.write_all(&img).expect("all written");

crates/jit/src/lib.rs

@@ -2,7 +2,6 @@
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces)]
-#![cfg_attr(feature = "std", deny(unstable_features))]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(
     feature = "cargo-clippy",
@@ -22,13 +21,6 @@
     )
 )]
 
-extern crate alloc;
-
-#[cfg(not(feature = "std"))]
-use hashbrown::{hash_map, HashMap, HashSet};
-#[cfg(feature = "std")]
-use std::collections::{hash_map, HashMap, HashSet};
-
 mod action;
 mod code_memory;
 mod compiler;

crates/jit/src/link.rs

@@ -1,14 +1,13 @@
 //! Linking for JIT-compiled code.
 
 use crate::resolver::Resolver;
-use crate::HashSet;
-use alloc::vec::Vec;
-use core::ptr::write_unaligned;
 use cranelift_codegen::binemit::Reloc;
 use cranelift_codegen::ir::JumpTableOffsets;
 use cranelift_entity::PrimaryMap;
 use cranelift_wasm::{DefinedFuncIndex, Global, GlobalInit, Memory, Table, TableElementType};
 use more_asserts::assert_ge;
+use std::collections::HashSet;
+use std::ptr::write_unaligned;
 use wasmtime_environ::{
     MemoryPlan, MemoryStyle, Module, Relocation, RelocationTarget, Relocations, TablePlan,
 };

crates/jit/src/namespace.rs

@@ -2,9 +2,8 @@
 //! to exports. This file provides one possible way to manage multiple instances
 //! and resolve imports to exports among them.
 
-use super::HashMap;
 use crate::resolver::Resolver;
-use alloc::string::String;
+use std::collections::HashMap;
 use wasmtime_runtime::{Export, InstanceHandle};
 
 /// A namespace containing instances keyed by name.

crates/jit/src/target_tunables.rs

@@ -1,4 +1,4 @@
-use core::cmp::min;
+use std::cmp::min;
 use target_lexicon::{OperatingSystem, Triple};
 use wasmtime_environ::Tunables;