feat(core/rust): introduce StrBuffer

it is a variant of Buffer specifically for accepting python `str`

core/embed/rust/build.rs

@@ -72,6 +72,7 @@ fn generate_micropython_bindings() {
         .allowlist_var("MP_BUFFER_READ")
         .allowlist_var("MP_BUFFER_WRITE")
         .allowlist_var("MP_BUFFER_RW")
+        .allowlist_var("mp_type_str")
         // dict
         .allowlist_type("mp_obj_dict_t")
         .allowlist_function("mp_obj_new_dict")

core/embed/rust/src/micropython/buffer.rs

@@ -1,7 +1,7 @@
 use core::{
     convert::TryFrom,
     ops::{Deref, DerefMut},
-    ptr, slice,
+    ptr, slice, str,
 };
 
 use crate::{error::Error, micropython::obj::Obj};
@@ -132,6 +132,50 @@ impl AsMut<[u8]> for BufferMut {
     }
 }
 
+/// Represents an immutable UTF-8 string stored on the MicroPython heap and
+/// owned by a `str` object.
+///
+/// # Safety
+///
+/// In most cases, it is unsound to store `StrBuffer` values in a GC-unreachable
+/// location, such as static data. It is also unsound to let the contents be
+/// modified while a reference to them is being held.
+#[derive(Default)]
+pub struct StrBuffer(Buffer);
+
+impl TryFrom<Obj> for StrBuffer {
+    type Error = Error;
+
+    fn try_from(obj: Obj) -> Result<Self, Self::Error> {
+        if obj.is_qstr() || unsafe { ffi::mp_type_str.is_type_of(obj) } {
+            Ok(Self(Buffer::try_from(obj)?))
+        } else {
+            Err(Error::TypeError)
+        }
+    }
+}
+
+impl Deref for StrBuffer {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        self.as_ref()
+    }
+}
+
+impl AsRef<str> for StrBuffer {
+    fn as_ref(&self) -> &str {
+        // SAFETY: MicroPython ensures that values of type `str` are UTF-8
+        unsafe { str::from_utf8_unchecked(self.0.as_ref()) }
+    }
+}
+
+impl From<&'static str> for StrBuffer {
+    fn from(val: &'static str) -> Self {
+        Self(Buffer::from(val.as_bytes()))
+    }
+}
+
 fn get_buffer_info(obj: Obj, flags: u32) -> Result<ffi::mp_buffer_info_t, Error> {
     let mut bufinfo = ffi::mp_buffer_info_t {
         buf: ptr::null_mut(),
@@ -183,3 +227,10 @@ impl crate::trace::Trace for Buffer {
         self.as_ref().trace(t)
     }
 }
+
+#[cfg(feature = "ui_debug")]
+impl crate::trace::Trace for StrBuffer {
+    fn trace(&self, t: &mut dyn crate::trace::Tracer) {
+        self.as_ref().trace(t)
+    }
+}