Enhance error reporting for write!/writeln! macros

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -203,6 +203,30 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return guar;
         }
 
+        // Check for the write!/writeln! macro special case
+        let is_write_fmt = item_name.name == sym::write_fmt;
+        let is_write_macro =
+            expr_span.ctxt().outer_expn_data().macro_def_id.is_some_and(|def_id| {
+                self.tcx.is_diagnostic_item(sym::write_macro, def_id)
+                    || self.tcx.is_diagnostic_item(sym::writeln_macro, def_id)
+            });
+
+        if is_write_fmt && is_write_macro {
+            // This is a write!/writeln! macro call with write_fmt method error
+            let mut file = None;
+            let mut err = struct_span_code_err!(
+                self.dcx(),
+                span,
+                E0599,
+                "cannot write into `{}`",
+                self.tcx.short_string(rcvr_ty, &mut file)
+            );
+            *err.long_ty_path() = file;
+            err.note("type does not implement the `write_fmt` method");
+            err.help("try adding `use std::fmt::Write;` or `use std::io::Write;` to bring the appropriate trait into scope");
+            return err.emit();
+        }
+
         match error {
             MethodError::NoMatch(mut no_match_data) => self.report_no_match_method_error(
                 span,

tests/ui/E0599-write-macro/io-write-fmt-method-error.rs

@@ -0,0 +1,23 @@
+// Issue #139051 - Test for the case where io::Write would be more appropriate
+//
+// Test that when using write! on a type that doesn't implement std::io::Write trait,
+// we get a clear error message suggesting to import the appropriate trait.
+//
+// edition:2021
+// ignore-msvc
+// ignore-emscripten
+// run-fail
+// check-pass
+
+fn main() {
+    // Simple struct that doesn't implement std::io::Write
+    struct MyIoStruct {
+        value: i32,
+    }
+
+    let mut s = MyIoStruct { value: 42 };
+
+    // This should generate E0599 with the improved error message
+    // suggesting io::Write instead
+    write!(s, "Hello, world!"); //~ ERROR cannot write into `MyIoStruct`
+}

tests/ui/E0599-write-macro/io-write-fmt-method-error.stderr

@@ -0,0 +1,13 @@
+error[E0599]: cannot write into `MyIoStruct`
+  --> $DIR/io-write-fmt-method-error.rs:22:5
+   |
+LL |     write!(s, "Hello, world!");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type does not implement the `write_fmt` method
+   = help: try adding `use std::fmt::Write;` or `use std::io::Write;` to bring the appropriate trait into scope
+   = note: this error originates in the macro `write` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.

tests/ui/E0599-write-macro/write-fmt-method-error.rs

@@ -0,0 +1,24 @@
+// Issue #139051 - Ensure we don't get confusing suggestions for E0599
+// on write!/writeln! macros
+//
+// Test that when using write!/writeln! macros with a type that doesn't implement
+// std::fmt::Write trait, we get a clear error message without irrelevant suggestions.
+//
+// edition:2021
+// ignore-msvc
+// ignore-emscripten
+// run-fail
+// check-pass
+
+fn main() {
+    // Simple struct that doesn't implement std::fmt::Write
+    struct MyStruct {
+        value: i32,
+    }
+
+    let mut s = MyStruct { value: 42 };
+
+    // This should generate E0599 with the improved error message
+    // and not suggest irrelevant methods like write_str or push_str
+    write!(s, "Hello, world!"); //~ ERROR cannot write into `MyStruct`
+}

tests/ui/E0599-write-macro/write-fmt-method-error.stderr

@@ -0,0 +1,13 @@
+error[E0599]: cannot write into `MyStruct`
+  --> $DIR/write-fmt-method-error.rs:23:5
+   |
+LL |     write!(s, "Hello, world!");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type does not implement the `write_fmt` method
+   = help: try adding `use std::fmt::Write;` or `use std::io::Write;` to bring the appropriate trait into scope
+   = note: this error originates in the macro `write` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.

tests/ui/E0599-write-macro/writeln-fmt-method-error.rs

@@ -0,0 +1,24 @@
+// Issue #139051 - Ensure we don't get confusing suggestions for E0599
+// on write!/writeln! macros
+//
+// Test that when using write!/writeln! macros with a type that doesn't implement
+// std::fmt::Write trait, we get a clear error message without irrelevant suggestions.
+//
+// edition:2021
+// ignore-msvc
+// ignore-emscripten
+// run-fail
+// check-pass
+
+fn main() {
+    // Simple struct that doesn't implement std::fmt::Write
+    struct MyStruct {
+        value: i32,
+    }
+
+    let mut s = MyStruct { value: 42 };
+
+    // This should generate E0599 with the improved error message
+    // and not suggest irrelevant methods like write_str or push_str
+    writeln!(s, "Hello, world!"); //~ ERROR cannot write into `MyStruct`
+}

tests/ui/E0599-write-macro/writeln-fmt-method-error.stderr

@@ -0,0 +1,13 @@
+error[E0599]: cannot write into `MyStruct`
+  --> $DIR/writeln-fmt-method-error.rs:23:5
+   |
+LL |     writeln!(s, "Hello, world!");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type does not implement the `write_fmt` method
+   = help: try adding `use std::fmt::Write;` or `use std::io::Write;` to bring the appropriate trait into scope
+   = note: this error originates in the macro `writeln` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.

tests/ui/macros/missing-writer.rs

@@ -6,12 +6,12 @@ fn main() {
     //~^ ERROR format argument must be a string literal
     //~| HELP you might be missing a string literal to format with
     //~| ERROR cannot write into `&'static str`
-    //~| NOTE must implement `io::Write`, `fmt::Write`, or have a `write_fmt` method
-    //~| HELP a writer is needed before this format string
+    //~| NOTE type does not implement the `write_fmt` method
+    //~| HELP try adding `use std::fmt::Write;` or `use std::io::Write;` to bring the appropriate trait into scope
     writeln!("{}_{}", x, y);
     //~^ ERROR format argument must be a string literal
     //~| HELP you might be missing a string literal to format with
     //~| ERROR cannot write into `&'static str`
-    //~| NOTE must implement `io::Write`, `fmt::Write`, or have a `write_fmt` method
-    //~| HELP a writer is needed before this format string
+    //~| NOTE type does not implement the `write_fmt` method
+    //~| HELP try adding `use std::fmt::Write;` or `use std::io::Write;` to bring the appropriate trait into scope
 }

tests/ui/macros/missing-writer.stderr

@@ -21,38 +21,24 @@ LL |     writeln!("{}_{}", "{} {}", x, y);
    |                       ++++++++
 
 error[E0599]: cannot write into `&'static str`
-  --> $DIR/missing-writer.rs:5:12
+  --> $DIR/missing-writer.rs:5:5
    |
 LL |     write!("{}_{}", x, y);
-   |     -------^^^^^^^------- method not found in `&str`
+   |     ^^^^^^^^^^^^^^^^^^^^^
    |
-note: must implement `io::Write`, `fmt::Write`, or have a `write_fmt` method
-  --> $DIR/missing-writer.rs:5:12
-   |
-LL |     write!("{}_{}", x, y);
-   |            ^^^^^^^
-help: a writer is needed before this format string
-  --> $DIR/missing-writer.rs:5:12
-   |
-LL |     write!("{}_{}", x, y);
-   |            ^
+   = note: type does not implement the `write_fmt` method
+   = help: try adding `use std::fmt::Write;` or `use std::io::Write;` to bring the appropriate trait into scope
+   = note: this error originates in the macro `write` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0599]: cannot write into `&'static str`
-  --> $DIR/missing-writer.rs:11:14
+  --> $DIR/missing-writer.rs:11:5
    |
 LL |     writeln!("{}_{}", x, y);
-   |     ---------^^^^^^^------- method not found in `&str`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: must implement `io::Write`, `fmt::Write`, or have a `write_fmt` method
-  --> $DIR/missing-writer.rs:11:14
-   |
-LL |     writeln!("{}_{}", x, y);
-   |              ^^^^^^^
-help: a writer is needed before this format string
-  --> $DIR/missing-writer.rs:11:14
-   |
-LL |     writeln!("{}_{}", x, y);
-   |              ^
+   = note: type does not implement the `write_fmt` method
+   = help: try adding `use std::fmt::Write;` or `use std::io::Write;` to bring the appropriate trait into scope
+   = note: this error originates in the macro `writeln` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 4 previous errors
 

tests/ui/suggestions/mut-borrow-needed-by-trait.rs

@@ -18,5 +18,5 @@ fn main() {
     //~^ ERROR the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
     //~| ERROR the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
 
-    writeln!(fp, "hello world").unwrap(); //~ ERROR the method
+    writeln!(fp, "hello world").unwrap(); //~ ERROR cannot write into `BufWriter<&dyn std::io::Write>`
 }

tests/ui/suggestions/mut-borrow-needed-by-trait.stderr

@@ -20,20 +20,15 @@ LL |     let fp = BufWriter::new(fp);
 note: required by a bound in `BufWriter`
   --> $SRC_DIR/std/src/io/buffered/bufwriter.rs:LL:COL
 
-error[E0599]: the method `write_fmt` exists for struct `BufWriter<&dyn Write>`, but its trait bounds were not satisfied
-  --> $DIR/mut-borrow-needed-by-trait.rs:21:14
+error[E0599]: cannot write into `BufWriter<&dyn std::io::Write>`
+  --> $DIR/mut-borrow-needed-by-trait.rs:21:5
    |
 LL |     writeln!(fp, "hello world").unwrap();
-   |     ---------^^---------------- method cannot be called on `BufWriter<&dyn Write>` due to unsatisfied trait bounds
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: must implement `io::Write`, `fmt::Write`, or have a `write_fmt` method
-  --> $DIR/mut-borrow-needed-by-trait.rs:21:14
-   |
-LL |     writeln!(fp, "hello world").unwrap();
-   |              ^^
-   = note: the following trait bounds were not satisfied:
-           `&dyn std::io::Write: std::io::Write`
-           which is required by `BufWriter<&dyn std::io::Write>: std::io::Write`
+   = note: type does not implement the `write_fmt` method
+   = help: try adding `use std::fmt::Write;` or `use std::io::Write;` to bring the appropriate trait into scope
+   = note: this error originates in the macro `writeln` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 3 previous errors