Option for host managed memory (#1400)

* Option for host managed memory

* Rename Allocator to MemoryCreator

* Create LinearMemory and MemoryCreator traits in api

* Leave only one as_ptr function in LinearMemory trait

* Memory creator test

* Update comments/docs for LinearMemory and MemoryCreator traits

* Add guard page to the custom memory example

* Remove mut from LinearMemory trait as_ptr

* Host_memory_grow test

crates/api/src/runtime.rs

@@ -1,3 +1,5 @@
+use crate::externals::MemoryCreator;
+use crate::trampoline::MemoryCreatorProxy;
 use anyhow::Result;
 use std::cell::RefCell;
 use std::fmt;
@@ -9,6 +11,7 @@ use wasmtime_environ::settings::{self, Configurable};
 use wasmtime_environ::CacheConfig;
 use wasmtime_jit::{native, CompilationStrategy, Compiler};
 use wasmtime_profiling::{JitDumpAgent, NullProfilerAgent, ProfilingAgent, VTuneAgent};
+use wasmtime_runtime::RuntimeMemoryCreator;
 
 // Runtime Environment
 
@@ -27,6 +30,7 @@ pub struct Config {
     pub(crate) strategy: CompilationStrategy,
     pub(crate) cache_config: CacheConfig,
     pub(crate) profiler: Arc<dyn ProfilingAgent>,
+    pub(crate) memory_creator: Option<MemoryCreatorProxy>,
 }
 
 impl Config {
@@ -66,6 +70,7 @@ impl Config {
             strategy: CompilationStrategy::Auto,
             cache_config: CacheConfig::new_cache_disabled(),
             profiler: Arc::new(NullProfilerAgent),
+            memory_creator: None,
         }
     }
 
@@ -326,6 +331,12 @@ impl Config {
         self.cache_config = wasmtime_environ::CacheConfig::from_file(None)?;
         Ok(self)
     }
+
+    /// Sets a custom memory creator
+    pub fn with_host_memory(&mut self, mem_creator: Arc<dyn MemoryCreator>) -> &mut Self {
+        self.memory_creator = Some(MemoryCreatorProxy { mem_creator });
+        self
+    }
 }
 
 impl Default for Config {
@@ -506,6 +517,11 @@ impl Store {
         &self.inner.engine
     }
 
+    /// Returns an optional reference to a ['RuntimeMemoryCreator']
+    pub(crate) fn memory_creator(&self) -> Option<&dyn RuntimeMemoryCreator> {
+        self.engine().config.memory_creator.as_ref().map(|x| x as _)
+    }
+
     pub(crate) fn compiler(&self) -> std::cell::Ref<'_, Compiler> {
         self.inner.compiler.borrow()
     }