uefi: serial: fix core::fmt::Write impl

This is the more expected and natural behavior.

uefi-test-runner/src/main.rs

@@ -10,6 +10,7 @@ extern crate alloc;
 
 use alloc::string::ToString;
 use alloc::vec::Vec;
+use core::fmt::Write;
 use core::time::Duration;
 use uefi::mem::memory_map::MemoryMap;
 use uefi::prelude::*;
@@ -116,7 +117,10 @@ fn send_request_helper(serial: &mut Serial, request: HostRequest) -> Result {
     serial.set_attributes(&io_mode)?;
 
     // Send a screenshot request to the host.
-    serial.write(request.as_bytes()).discard_errdata()?;
+    //serial.write(request.as_bytes()).discard_errdata()?;
+    serial
+        .write_str(request.as_str())
+        .expect("should write all bytes");
 
     // Wait for the host's acknowledgement before moving forward.
     let mut reply = [0; 3];

uefi/CHANGELOG.md

@@ -30,6 +30,7 @@
   return `n` in any case. In the happy path, this always corresponds to the
   length if the provided data/buffer, but helps to cope with non-spec-compliant
   implementations.
+- Fixed potential partial writes in `core::fmt::Write` impl of `Serial` protocol
 
 # uefi - v0.36.1 (2025-11-05)
 

uefi/src/proto/console/serial.rs

@@ -345,6 +345,6 @@ impl Serial {
 
 impl Write for Serial {
     fn write_str(&mut self, s: &str) -> fmt::Result {
-        self.write(s.as_bytes()).map(|_| ()).map_err(|_| fmt::Error)
+        self.write_exact(s.as_bytes()).map_err(|_| fmt::Error)
     }
 }