Replace use of static mut in cortex-m-semihosting.

Will avoid a bunch of clippy warnings that are hard to get rid of in both
the MSRV and latest stable.

Author: thejpster

cortex-m-semihosting/src/export.rs

@@ -1,53 +1,68 @@
 //! IMPLEMENTATION DETAILS USED BY MACROS
 
-// This must be replaced by a different solution before rust edition 2024
-// https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-references.html
-#![allow(static_mut_refs)]
-
+use core::cell::RefCell;
 use core::fmt::{self, Write};
 
 use crate::hio::{self, HostStream};
 
-static mut HSTDOUT: Option<HostStream> = None;
+static HSTDOUT: critical_section::Mutex<RefCell<Option<HostStream>>> =
+    critical_section::Mutex::new(RefCell::new(None));
 
 pub fn hstdout_str(s: &str) {
-    let _result = critical_section::with(|_| unsafe {
-        if HSTDOUT.is_none() {
-            HSTDOUT = Some(hio::hstdout()?);
+    critical_section::with(|cs| {
+        let mut hstdout_opt = HSTDOUT.borrow_ref_mut(cs);
+        if hstdout_opt.is_none() {
+            if let Ok(hstdout) = hio::hstdout() {
+                hstdout_opt.replace(hstdout);
+            } else {
+                return;
+            }
         }
-
-        HSTDOUT.as_mut().unwrap().write_str(s).map_err(drop)
+        let hstdout = hstdout_opt.as_mut().unwrap();
+        let _ = hstdout.write_str(s);
     });
 }
 
 pub fn hstdout_fmt(args: fmt::Arguments) {
-    let _result = critical_section::with(|_| unsafe {
-        if HSTDOUT.is_none() {
-            HSTDOUT = Some(hio::hstdout()?);
+    critical_section::with(|cs| {
+        let mut hstdout_opt = HSTDOUT.borrow_ref_mut(cs);
+        if hstdout_opt.is_none() {
+            if let Ok(hstdout) = hio::hstdout() {
+                hstdout_opt.replace(hstdout);
+            } else {
+                return;
+            }
         }
-
-        HSTDOUT.as_mut().unwrap().write_fmt(args).map_err(drop)
+        let hstdout = hstdout_opt.as_mut().unwrap();
+        let _ = hstdout.write_fmt(args);
     });
 }
 
-static mut HSTDERR: Option<HostStream> = None;
+static HSTDERR: critical_section::Mutex<RefCell<Option<HostStream>>> =
+    critical_section::Mutex::new(RefCell::new(None));
 
 pub fn hstderr_str(s: &str) {
-    let _result = critical_section::with(|_| unsafe {
-        if HSTDERR.is_none() {
-            HSTDERR = Some(hio::hstderr()?);
+    critical_section::with(|cs| {
+        let mut hstderr_opt = HSTDERR.borrow_ref_mut(cs);
+        if let Ok(hstderr) = hio::hstderr() {
+            hstderr_opt.replace(hstderr);
+        } else {
+            return;
         }
-
-        HSTDERR.as_mut().unwrap().write_str(s).map_err(drop)
+        let hstderr = hstderr_opt.as_mut().unwrap();
+        let _ = hstderr.write_str(s);
     });
 }
 
 pub fn hstderr_fmt(args: fmt::Arguments) {
-    let _result = critical_section::with(|_| unsafe {
-        if HSTDERR.is_none() {
-            HSTDERR = Some(hio::hstderr()?);
+    critical_section::with(|cs| {
+        let mut hstderr_opt = HSTDERR.borrow_ref_mut(cs);
+        if let Ok(hstderr) = hio::hstderr() {
+            hstderr_opt.replace(hstderr);
+        } else {
+            return;
         }
-
-        HSTDERR.as_mut().unwrap().write_fmt(args).map_err(drop)
+        let hstderr = hstderr_opt.as_mut().unwrap();
+        let _ = hstderr.write_fmt(args);
     });
 }