Improve diagnostics on double write!

macros/src/lib.rs

@@ -1008,14 +1008,17 @@ pub fn write(ts: TokenStream) -> TokenStream {
     let fmt = &write.fmt;
     // FIXME: Introduce a new `"write"` tag and decode it in a loop (breaking change).
     let sym = mksym(&ls, "fmt", false);
-    quote!(match (#fmt.inner, #(&(#args)),*) {
-        (_fmt_, #(#pats),*) => {
-            // HACK conditional should not be here; see FIXME in `format`
-            if _fmt_.needs_tag() {
-                _fmt_.istr(&defmt::export::istr(#sym));
+    quote!({
+        let fmt: ::defmt::Formatter<'_> = #fmt;
+        match (fmt.inner, #(&(#args)),*) {
+            (_fmt_, #(#pats),*) => {
+                // HACK conditional should not be here; see FIXME in `format`
+                if _fmt_.needs_tag() {
+                    _fmt_.istr(&defmt::export::istr(#sym));
+                }
+                #(#exprs;)*
+                _fmt_.finalize();
             }
-            #(#exprs;)*
-            _fmt_.finalize();
         }
     })
     .into()

tests/ui/write-for-loop.stderr

@@ -1,12 +1,8 @@
-error[E0382]: use of moved value: `f.inner`
- --> $DIR/write-for-loop.rs:6:13
+error[E0382]: use of moved value: `f`
+ --> $DIR/write-for-loop.rs:6:27
   |
+4 |     fn format(&self, f: defmt::Formatter) {
+  |                      - move occurs because `f` has type `defmt::Formatter<'_>`, which does not implement the `Copy` trait
+5 |         for _ in 0..3 {
 6 |             defmt::write!(f, "hello");
-  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ value moved here, in previous iteration of loop
-  |
-  = note: move occurs because `f.inner` has type `&mut InternalFormatter`, which does not implement the `Copy` trait
-  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
-help: consider creating a fresh reborrow of `f.inner` here
-  |
-6 |             &mut *defmt::write!(f, "hello");
-  |             ^^^^^^
+  |                           ^ value moved here, in previous iteration of loop

tests/ui/write-twice.stderr

@@ -1,10 +1,9 @@
-error[E0382]: use of moved value: `f.inner`
- --> $DIR/write-twice.rs:6:9
+error[E0382]: use of moved value: `f`
+ --> $DIR/write-twice.rs:6:23
   |
+4 |     fn format(&self, f: defmt::Formatter) {
+  |                      - move occurs because `f` has type `defmt::Formatter<'_>`, which does not implement the `Copy` trait
 5 |         defmt::write!(f, "hello");
-  |         -------------------------- value moved here
+  |                       - value moved here
 6 |         defmt::write!(f, "world");
-  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ value used here after move
-  |
-  = note: move occurs because `f.inner` has type `&mut InternalFormatter`, which does not implement the `Copy` trait
-  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+  |                       ^ value used here after move