Add basic GuestString support to wiggle (#13)

* Add basic GuestString support to wiggle

This commit adds basic `GuestString` support to `wiggle`. `GuestString`
is a wrapper around `GuestArray<'_, u8>` array type which itself can
be made into either an owned (cloned) Rust `String` or borrowed as
a reference `&str`. In both cases, `GuestString` ensures that the
underlying bytes are valid Unicode code units, throwing a `InvalidUtf8`
error if not.

This commit adds support *only* for passing in strings as arguments
in WASI. Marshalling of the return arg has not yet been implemented.
I'm not even sure it's possible without multi-value return args
feature of Wasm. It's not a major setback especially since the WASI
spec (and this includes even the `ephemeral` snapshot) doesn't
return strings anywhere. They are only ever passed in as arguments
to interface functions.

It should be noted that error returned in case of invalid UTF-8
requires a lot more love as it doesn't include anything besides
flagging an event that the string contained an invalid Unicode code unit.

* Borrow all of string's memory including nul-byte

Borrow all of string's underlying memory including the nul-byte.
This perhaps might not have a tremendous impact on anything, but
since the nul-byte is technically part of the WASI string, we should
include it in the borrow as well.

* Fill in wiggle-generate blanks for strings

* Print to screen passed string in proptest

* Strings are PointerLengthPairs!

* Fix generation of strings in compound types

* Update test with simple string strategy

* Generate better test strings

* Finalise proptest for strings

* Fix formatting

* Update crates/runtime/src/memory/string.rs

Removes unnecessary comment in code

* Apply Pat's suggestion to wrap Utf8Error as error

crates/runtime/src/memory/string.rs

@@ -0,0 +1,124 @@
+use super::array::{GuestArray, GuestArrayRef};
+use crate::GuestError;
+use std::fmt;
+
+pub struct GuestString<'a> {
+    pub(super) array: GuestArray<'a, u8>,
+}
+
+impl<'a> fmt::Debug for GuestString<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "GuestString {{ array: {:?} }}", self.array)
+    }
+}
+
+impl<'a> GuestString<'a> {
+    pub fn as_ref(&self) -> Result<GuestStringRef<'a>, GuestError> {
+        let ref_ = self.array.as_ref()?;
+        Ok(GuestStringRef { ref_ })
+    }
+
+    pub fn to_string(&self) -> Result<String, GuestError> {
+        Ok(self.as_ref()?.as_str()?.to_owned())
+    }
+}
+
+impl<'a> From<GuestArray<'a, u8>> for GuestString<'a> {
+    fn from(array: GuestArray<'a, u8>) -> Self {
+        Self { array }
+    }
+}
+
+pub struct GuestStringRef<'a> {
+    pub(super) ref_: GuestArrayRef<'a, u8>,
+}
+
+impl<'a> fmt::Debug for GuestStringRef<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "GuestStringRef {{ ref_: {:?} }}", self.ref_)
+    }
+}
+
+impl<'a> GuestStringRef<'a> {
+    pub fn as_str(&self) -> Result<&str, GuestError> {
+        std::str::from_utf8(&*self.ref_).map_err(Into::into)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::{
+        super::{
+            ptr::{GuestPtr, GuestPtrMut},
+            GuestError, GuestMemory,
+        },
+        GuestString,
+    };
+
+    #[repr(align(4096))]
+    struct HostMemory {
+        buffer: [u8; 4096],
+    }
+
+    impl HostMemory {
+        pub fn new() -> Self {
+            Self { buffer: [0; 4096] }
+        }
+        pub fn as_mut_ptr(&mut self) -> *mut u8 {
+            self.buffer.as_mut_ptr()
+        }
+        pub fn len(&self) -> usize {
+            self.buffer.len()
+        }
+    }
+
+    #[test]
+    fn valid_utf8() {
+        let mut host_memory = HostMemory::new();
+        let guest_memory = GuestMemory::new(host_memory.as_mut_ptr(), host_memory.len() as u32);
+        // write string into memory
+        let mut ptr: GuestPtrMut<u8> = guest_memory.ptr_mut(0).expect("ptr mut to start of string");
+        let input_str = "cześć WASI!";
+        for byte in input_str.as_bytes() {
+            let mut ref_mut = ptr.as_ref_mut().expect("valid deref");
+            *ref_mut = *byte;
+            ptr = ptr.elem(1).expect("next ptr");
+        }
+        // read the string as GuestString
+        let ptr: GuestPtr<u8> = guest_memory.ptr(0).expect("ptr to start of string");
+        let guest_string: GuestString<'_> = ptr
+            .array(input_str.len() as u32)
+            .expect("valid null-terminated string")
+            .into();
+        let as_ref = guest_string.as_ref().expect("deref");
+        assert_eq!(as_ref.as_str().expect("valid UTF-8"), input_str);
+    }
+
+    #[test]
+    fn invalid_utf8() {
+        let mut host_memory = HostMemory::new();
+        let guest_memory = GuestMemory::new(host_memory.as_mut_ptr(), host_memory.len() as u32);
+        // write string into memory
+        let mut ptr: GuestPtrMut<u8> = guest_memory.ptr_mut(0).expect("ptr mut to start of string");
+        let input_str = "cześć WASI!";
+        let mut bytes = input_str.as_bytes().to_vec();
+        // insert 0xFE which is an invalid UTF-8 byte
+        bytes[5] = 0xfe;
+        for byte in &bytes {
+            let mut ref_mut = ptr.as_ref_mut().expect("valid deref");
+            *ref_mut = *byte;
+            ptr = ptr.elem(1).expect("next ptr");
+        }
+        // read the string as GuestString
+        let ptr: GuestPtr<u8> = guest_memory.ptr(0).expect("ptr to start of string");
+        let guest_string: GuestString<'_> = ptr
+            .array(bytes.len() as u32)
+            .expect("valid null-terminated string")
+            .into();
+        let as_ref = guest_string.as_ref().expect("deref");
+        match as_ref.as_str().expect_err("should fail") {
+            GuestError::InvalidUtf8(_) => {}
+            x => assert!(false, "expected GuestError::InvalidUtf8(_), got {:?}", x),
+        }
+    }
+}