rustc_parse_format: improve diagnostics for unsupported debug = syntax

Detect Python-style f-string debug syntax in format strings and emit a
clear diagnostic explaining that it is not supported in Rust. When the
intended operation can be inferred, suggest the corresponding Rust
alternative e.g from `println!("{=}", x)` to `dbg!({x})`.

Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>

compiler/rustc_builtin_macros/src/errors.rs

@@ -678,6 +678,18 @@ pub(crate) enum InvalidFormatStringSuggestion {
         #[primary_span]
         span: Span,
     },
+
+    #[suggestion(
+        "use rust debug printing macro",
+        code = "{replacement}",
+        style = "verbose",
+        applicability = "machine-applicable"
+    )]
+    UseRustDebugPrintingMacro {
+        #[primary_span]
+        macro_span: Span,
+        replacement: String,
+    },
 }
 
 #[derive(Diagnostic)]

compiler/rustc_builtin_macros/src/format.rs

@@ -160,6 +160,7 @@ fn make_format_args(
     ecx: &mut ExtCtxt<'_>,
     input: MacroInput,
     append_newline: bool,
+    macro_span: Span,
 ) -> ExpandResult<Result<FormatArgs, ErrorGuaranteed>, ()> {
     let msg = "format argument must be a string literal";
     let unexpanded_fmt_span = input.fmtstr.span;
@@ -333,6 +334,23 @@ fn make_format_args(
                 let span = fmt_span.from_inner(InnerSpan::new(span.start, span.end));
                 e.sugg_ = Some(errors::InvalidFormatStringSuggestion::AddMissingColon { span });
             }
+            parse::Suggestion::UseRustDebugPrintingMacro => {
+                // This targets `println!("{=}", x);` and `println!("{0=}", x);`
+                if let [arg] = args.all_args() {
+                    let expr_span = arg.expr.span;
+                    if let Ok(expr_snippet) = ecx.source_map().span_to_snippet(expr_span) {
+                        let replacement = format!("{}!({})", "dbg", expr_snippet);
+
+                        let call_span = macro_span.source_callsite();
+                        e.sugg_ = Some(
+                            errors::InvalidFormatStringSuggestion::UseRustDebugPrintingMacro {
+                                macro_span: call_span,
+                                replacement,
+                            },
+                        );
+                    }
+                }
+            }
         }
         let guar = ecx.dcx().emit_err(e);
         return ExpandResult::Ready(Err(guar));
@@ -1048,7 +1066,7 @@ fn expand_format_args_impl<'cx>(
     sp = ecx.with_def_site_ctxt(sp);
     ExpandResult::Ready(match parse_args(ecx, sp, tts) {
         Ok(input) => {
-            let ExpandResult::Ready(mac) = make_format_args(ecx, input, nl) else {
+            let ExpandResult::Ready(mac) = make_format_args(ecx, input, nl, sp) else {
                 return ExpandResult::Retry(());
             };
             match mac {

compiler/rustc_parse_format/src/lib.rs

@@ -187,6 +187,9 @@ pub enum Suggestion {
     /// Add missing colon:
     /// `format!("{foo?}")` -> `format!("{foo:?}")`
     AddMissingColon(Range<usize>),
+    /// Use Rust format string:
+    /// `format!("{x=}")` -> `dbg!(x)`
+    UseRustDebugPrintingMacro,
 }
 
 /// The parser structure for interpreting the input format string. This is
@@ -462,6 +465,7 @@ impl<'input> Parser<'input> {
                 ('?', _) => self.suggest_format_debug(),
                 ('<' | '^' | '>', _) => self.suggest_format_align(c),
                 (',', _) => self.suggest_unsupported_python_numeric_grouping(),
+                ('=', '}') => self.suggest_rust_debug_printing_macro(),
                 _ => self.suggest_positional_arg_instead_of_captured_arg(arg),
             }
         }
@@ -871,6 +875,27 @@ impl<'input> Parser<'input> {
         }
     }
 
+    fn suggest_rust_debug_printing_macro(&mut self) {
+        if let Some((range, _)) = self.consume_pos('=') {
+            self.errors.insert(
+                0,
+                ParseError {
+                    description:
+                        "python's f-string debug `=` is not supported in rust, use `dbg(x)` instead"
+                            .to_owned(),
+                    note: Some(format!("to print `{{`, you can escape it using `{{{{`",)),
+                    label: "expected `}`".to_owned(),
+                    span: range,
+                    secondary_label: self
+                        .last_open_brace
+                        .clone()
+                        .map(|sp| ("because of this opening brace".to_owned(), sp)),
+                    suggestion: Suggestion::UseRustDebugPrintingMacro,
+                },
+            );
+        }
+    }
+
     fn suggest_format_align(&mut self, alignment: char) {
         if let Some((range, _)) = self.consume_pos(alignment) {
             self.errors.insert(

tests/ui/fmt/format-string-error-2.rs

@@ -88,4 +88,7 @@ raw  { \n
     //~^ ERROR invalid format string: expected `}`, found `?`
     println!("{x,}, world!",);
     //~^ ERROR invalid format string: python's numeric grouping `,` is not supported in rust format strings
+
+    println!("{x=}");
+    //~^ ERROR invalid format string: python's f-string debug `=` is not supported in rust, use `dbg(x)` instead
 }

tests/ui/fmt/format-string-error-2.stderr

@@ -198,5 +198,15 @@ LL |     println!("{x,}, world!",);
    |
    = note: to print `{`, you can escape it using `{{`
 
-error: aborting due to 20 previous errors
+error: invalid format string: python's f-string debug `=` is not supported in rust, use `dbg(x)` instead
+  --> $DIR/format-string-error-2.rs:92:17
+   |
+LL |     println!("{x=}");
+   |               - ^ expected `}` in format string
+   |               |
+   |               because of this opening brace
+   |
+   = note: to print `{`, you can escape it using `{{`
+
+error: aborting due to 21 previous errors
 

tests/ui/fmt/format-string-error-3.fixed

@@ -0,0 +1,5 @@
+//@ run-rustfix
+fn main() {
+    let x = 32;
+    dbg!(x); //~ ERROR invalid format string: python's f-string debug
+}

tests/ui/fmt/format-string-error-3.rs

@@ -0,0 +1,5 @@
+//@ run-rustfix
+fn main() {
+    let x = 32;
+    println!("{=}", x); //~ ERROR invalid format string: python's f-string debug
+}

tests/ui/fmt/format-string-error-3.stderr

@@ -0,0 +1,17 @@
+error: invalid format string: python's f-string debug `=` is not supported in rust, use `dbg(x)` instead
+  --> $DIR/format-string-error-3.rs:4:16
+   |
+LL |     println!("{=}", x);
+   |               -^ expected `}` in format string
+   |               |
+   |               because of this opening brace
+   |
+   = note: to print `{`, you can escape it using `{{`
+help: use rust debug printing macro
+   |
+LL -     println!("{=}", x);
+LL +     dbg!(x);
+   |
+
+error: aborting due to 1 previous error
+