Properly allow macro expanded format_args invocations to uses captures

Originally, this was kinda half-allowed. There were some primitive
checks in place that looked at the span to see whether the input was
likely a literal. These "source literal" checks are needed because the
spans created during `format_args` parsing only make sense when it is
indeed a literal that was written in the source code directly.

This is orthogonal to the restriction that the first argument must be a
"direct literal", not being exanpanded from macros. This restriction was
imposed by [RFC 2795] on the basis of being too confusing. But this was
only concerned with the argument of the invocation being a literal, not
whether it was a source literal (maybe in spirit it meant it being a
source literal, this is not clear to me).

Since the original check only really cared about source literals (which
is good enough to deny the `format_args!(concat!())` example), macros
expanding to `format_args` invocations were able to use implicit
captures if they spanned the string in a way that lead back to a source
string.

The "source literal" checks were not strict enough and caused ICEs in
certain cases (see # 106191 (the space is intended to avoid spammy
backreferences)). So I tightened it up in # 106195 to really only work
if it's a direct source literal.

This caused the `indoc` crate to break. `indoc` transformed the source
literal by removing whitespace, which made it not a "source literal"
anymore (which is required to fix the ICE). But since `indoc` spanned
the literal in ways that made the old check think that it's a literal,
it was able to use implicit captures (which is useful and nice for the
users of `indoc`).

This commit properly seperates the previously introduced concepts of
"source literal" and "direct literal" and therefore allows `indoc`
invocations, which don't create "source literals" to use implicit
captures again.

[RFC 2795]: https://rust-lang.github.io/rfcs/2795-format-args-implicit-identifiers.html#macro-hygiene

compiler/rustc_builtin_macros/src/format.rs

@@ -36,6 +36,21 @@ enum PositionUsedAs {
 }
 use PositionUsedAs::*;
 
+struct MacroInput {
+    fmtstr: P<Expr>,
+    args: FormatArguments,
+    /// Whether the first argument was a string literal or a result from eager macro expansion.
+    /// If it's not a string literal, we disallow implicit arugment capturing.
+    ///
+    /// This does not correspond to whether we can treat spans to the literal normally, as the whole
+    /// invocation might be the result of another macro expansion, in which case this flag may still be true.
+    ///
+    /// See [RFC 2795] for more information.
+    ///
+    /// [RFC 2795]: https://rust-lang.github.io/rfcs/2795-format-args-implicit-identifiers.html#macro-hygiene
+    is_direct_literal: bool,
+}
+
 /// Parses the arguments from the given list of tokens, returning the diagnostic
 /// if there's a parse error so we can continue parsing other format!
 /// expressions.
@@ -45,11 +60,7 @@ use PositionUsedAs::*;
 /// ```text
 /// Ok((fmtstr, parsed arguments))
 /// ```
-fn parse_args<'a>(
-    ecx: &mut ExtCtxt<'a>,
-    sp: Span,
-    tts: TokenStream,
-) -> PResult<'a, (P<Expr>, FormatArguments)> {
+fn parse_args<'a>(ecx: &mut ExtCtxt<'a>, sp: Span, tts: TokenStream) -> PResult<'a, MacroInput> {
     let mut args = FormatArguments::new();
 
     let mut p = ecx.new_parser_from_tts(tts);
@@ -59,25 +70,21 @@ fn parse_args<'a>(
     }
 
     let first_token = &p.token;
-    let fmtstr = match first_token.kind {
-        token::TokenKind::Literal(token::Lit {
-            kind: token::LitKind::Str | token::LitKind::StrRaw(_),
-            ..
-        }) => {
-            // If the first token is a string literal, then a format expression
-            // is constructed from it.
-            //
-            // This allows us to properly handle cases when the first comma
-            // after the format string is mistakenly replaced with any operator,
-            // which cause the expression parser to eat too much tokens.
-            p.parse_literal_maybe_minus()?
-        }
-        _ => {
-            // Otherwise, we fall back to the expression parser.
-            p.parse_expr()?
-        }
+
+    let fmtstr = if let token::Literal(lit) = first_token.kind && matches!(lit.kind, token::Str | token::StrRaw(_)) {
+        // This allows us to properly handle cases when the first comma
+        // after the format string is mistakenly replaced with any operator,
+        // which cause the expression parser to eat too much tokens.
+        p.parse_literal_maybe_minus()?
+    } else {
+        // Otherwise, we fall back to the expression parser.
+        p.parse_expr()?
     };
 
+    // Only allow implicit captures to be used when the argument is a direct literal
+    // instead of a macro expanding to one.
+    let is_direct_literal = matches!(fmtstr.kind, ExprKind::Lit(_));
+
     let mut first = true;
 
     while p.token != token::Eof {
@@ -147,17 +154,19 @@ fn parse_args<'a>(
             }
         }
     }
-    Ok((fmtstr, args))
+    Ok(MacroInput { fmtstr, args, is_direct_literal })
 }
 
-pub fn make_format_args(
+fn make_format_args(
     ecx: &mut ExtCtxt<'_>,
-    efmt: P<Expr>,
-    mut args: FormatArguments,
+    input: MacroInput,
     append_newline: bool,
 ) -> Result<FormatArgs, ()> {
     let msg = "format argument must be a string literal";
-    let unexpanded_fmt_span = efmt.span;
+    let unexpanded_fmt_span = input.fmtstr.span;
+
+    let MacroInput { fmtstr: efmt, mut args, is_direct_literal } = input;
+
     let (fmt_str, fmt_style, fmt_span) = match expr_to_spanned_string(ecx, efmt, msg) {
         Ok(mut fmt) if append_newline => {
             fmt.0 = Symbol::intern(&format!("{}\n", fmt.0));
@@ -208,11 +217,11 @@ pub fn make_format_args(
         }
     }
 
-    let is_literal = parser.is_literal;
+    let is_source_literal = parser.is_source_literal;
 
     if !parser.errors.is_empty() {
         let err = parser.errors.remove(0);
-        let sp = if is_literal {
+        let sp = if is_source_literal {
             fmt_span.from_inner(InnerSpan::new(err.span.start, err.span.end))
         } else {
             // The format string could be another macro invocation, e.g.:
@@ -230,7 +239,7 @@ pub fn make_format_args(
         if let Some(note) = err.note {
             e.note(&note);
         }
-        if let Some((label, span)) = err.secondary_label && is_literal {
+        if let Some((label, span)) = err.secondary_label && is_source_literal {
             e.span_label(fmt_span.from_inner(InnerSpan::new(span.start, span.end)), label);
         }
         if err.should_be_replaced_with_positional_argument {
@@ -256,7 +265,7 @@ pub fn make_format_args(
     }
 
     let to_span = |inner_span: rustc_parse_format::InnerSpan| {
-        is_literal.then(|| {
+        is_source_literal.then(|| {
             fmt_span.from_inner(InnerSpan { start: inner_span.start, end: inner_span.end })
         })
     };
@@ -304,7 +313,7 @@ pub fn make_format_args(
                     // Name not found in `args`, so we add it as an implicitly captured argument.
                     let span = span.unwrap_or(fmt_span);
                     let ident = Ident::new(name, span);
-                    let expr = if is_literal {
+                    let expr = if is_direct_literal {
                         ecx.expr_ident(span, ident)
                     } else {
                         // For the moment capturing variables from format strings expanded from macros is
@@ -814,7 +823,7 @@ fn report_invalid_references(
         // for `println!("{7:7$}", 1);`
         indexes.sort();
         indexes.dedup();
-        let span: MultiSpan = if !parser.is_literal || parser.arg_places.is_empty() {
+        let span: MultiSpan = if !parser.is_source_literal || parser.arg_places.is_empty() {
             MultiSpan::from_span(fmt_span)
         } else {
             MultiSpan::from_spans(invalid_refs.iter().filter_map(|&(_, span, _, _)| span).collect())
@@ -855,8 +864,8 @@ fn expand_format_args_impl<'cx>(
 ) -> Box<dyn base::MacResult + 'cx> {
     sp = ecx.with_def_site_ctxt(sp);
     match parse_args(ecx, sp, tts) {
-        Ok((efmt, args)) => {
-            if let Ok(format_args) = make_format_args(ecx, efmt, args, nl) {
+        Ok(input) => {
+            if let Ok(format_args) = make_format_args(ecx, input, nl) {
                 MacEager::expr(ecx.expr(sp, ExprKind::FormatArgs(P(format_args))))
             } else {
                 MacEager::expr(DummyResult::raw_expr(sp, true))

compiler/rustc_parse_format/src/lib.rs

@@ -234,8 +234,10 @@ pub struct Parser<'a> {
     last_opening_brace: Option<InnerSpan>,
     /// Whether the source string is comes from `println!` as opposed to `format!` or `print!`
     append_newline: bool,
-    /// Whether this formatting string is a literal or it comes from a macro.
-    pub is_literal: bool,
+    /// Whether this formatting string was written directly in the source. This controls whether we
+    /// can use spans to refer into it and give better error messages.
+    /// N.B: This does _not_ control whether implicit argument captures can be used.
+    pub is_source_literal: bool,
     /// Start position of the current line.
     cur_line_start: usize,
     /// Start and end byte offset of every line of the format string. Excludes
@@ -262,7 +264,7 @@ impl<'a> Iterator for Parser<'a> {
                     } else {
                         let arg = self.argument(lbrace_end);
                         if let Some(rbrace_pos) = self.must_consume('}') {
-                            if self.is_literal {
+                            if self.is_source_literal {
                                 let lbrace_byte_pos = self.to_span_index(pos);
                                 let rbrace_byte_pos = self.to_span_index(rbrace_pos);
 
@@ -302,7 +304,7 @@ impl<'a> Iterator for Parser<'a> {
                 _ => Some(String(self.string(pos))),
             }
         } else {
-            if self.is_literal {
+            if self.is_source_literal {
                 let span = self.span(self.cur_line_start, self.input.len());
                 if self.line_spans.last() != Some(&span) {
                     self.line_spans.push(span);
@@ -323,7 +325,7 @@ impl<'a> Parser<'a> {
         mode: ParseMode,
     ) -> Parser<'a> {
         let input_string_kind = find_width_map_from_snippet(snippet, style);
-        let (width_map, is_literal) = match input_string_kind {
+        let (width_map, is_source_literal) = match input_string_kind {
             InputStringKind::Literal { width_mappings } => (width_mappings, true),
             InputStringKind::NotALiteral => (Vec::new(), false),
         };
@@ -339,7 +341,7 @@ impl<'a> Parser<'a> {
             width_map,
             last_opening_brace: None,
             append_newline,
-            is_literal,
+            is_source_literal,
             cur_line_start: 0,
             line_spans: vec![],
         }
@@ -532,13 +534,13 @@ impl<'a> Parser<'a> {
                 '{' | '}' => {
                     return &self.input[start..pos];
                 }
-                '\n' if self.is_literal => {
+                '\n' if self.is_source_literal => {
                     self.line_spans.push(self.span(self.cur_line_start, pos));
                     self.cur_line_start = pos + 1;
                     self.cur.next();
                 }
                 _ => {
-                    if self.is_literal && pos == self.cur_line_start && c.is_whitespace() {
+                    if self.is_source_literal && pos == self.cur_line_start && c.is_whitespace() {
                         self.cur_line_start = pos + c.len_utf8();
                     }
                     self.cur.next();

tests/ui/fmt/auxiliary/format-string-proc-macro.rs

@@ -28,25 +28,41 @@ pub fn err_with_input_span(input: TokenStream) -> TokenStream {
     TokenStream::from(TokenTree::Literal(lit))
 }
 
+fn build_format(args: impl Into<TokenStream>) -> TokenStream {
+    TokenStream::from_iter([
+        TokenTree::from(Ident::new("format", Span::call_site())),
+        TokenTree::from(Punct::new('!', Spacing::Alone)),
+        TokenTree::from(Group::new(Delimiter::Parenthesis, args.into())),
+    ])
+}
 
 #[proc_macro]
 pub fn respan_to_invalid_format_literal(input: TokenStream) -> TokenStream {
     let mut s = Literal::string("{");
     s.set_span(input.into_iter().next().unwrap().span());
-    TokenStream::from_iter([
-        TokenTree::from(Ident::new("format", Span::call_site())),
-        TokenTree::from(Punct::new('!', Spacing::Alone)),
-        TokenTree::from(Group::new(Delimiter::Parenthesis, TokenTree::from(s).into())),
-    ])
+
+    build_format(TokenTree::from(s))
 }
 
 #[proc_macro]
 pub fn capture_a_with_prepended_space_preserve_span(input: TokenStream) -> TokenStream {
     let mut s = Literal::string(" {a}");
     s.set_span(input.into_iter().next().unwrap().span());
-    TokenStream::from_iter([
-        TokenTree::from(Ident::new("format", Span::call_site())),
-        TokenTree::from(Punct::new('!', Spacing::Alone)),
-        TokenTree::from(Group::new(Delimiter::Parenthesis, TokenTree::from(s).into())),
-    ])
+
+    build_format(TokenTree::from(s))
+}
+
+#[proc_macro]
+pub fn format_args_captures(_: TokenStream) -> TokenStream {
+    r#"{ let x = 5; format!("{x}") }"#.parse().unwrap()
+}
+
+#[proc_macro]
+pub fn bad_format_args_captures(_: TokenStream) -> TokenStream {
+    r#"{ let x = 5; format!(concat!("{x}")) }"#.parse().unwrap()
+}
+
+#[proc_macro]
+pub fn identity_pm(input: TokenStream) -> TokenStream {
+    input
 }

tests/ui/fmt/format-args-capture-first-literal-is-macro.rs

@@ -0,0 +1,21 @@
+// aux-build:format-string-proc-macro.rs
+
+#[macro_use]
+extern crate format_string_proc_macro;
+
+macro_rules! identity_mbe {
+    ($tt:tt) => {
+        $tt
+        //~^ ERROR there is no argument named `a`
+    };
+}
+
+fn main() {
+    let a = 0;
+
+    format!(identity_pm!("{a}"));
+    //~^ ERROR there is no argument named `a`
+    format!(identity_mbe!("{a}"));
+    format!(concat!("{a}"));
+    //~^ ERROR there is no argument named `a`
+}

tests/ui/fmt/format-args-capture-first-literal-is-macro.stderr

@@ -0,0 +1,30 @@
+error: there is no argument named `a`
+  --> $DIR/format-args-capture-first-literal-is-macro.rs:16:26
+   |
+LL |     format!(identity_pm!("{a}"));
+   |                          ^^^^^
+   |
+   = note: did you intend to capture a variable `a` from the surrounding scope?
+   = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro
+
+error: there is no argument named `a`
+  --> $DIR/format-args-capture-first-literal-is-macro.rs:8:9
+   |
+LL |         $tt
+   |         ^^^
+   |
+   = note: did you intend to capture a variable `a` from the surrounding scope?
+   = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro
+
+error: there is no argument named `a`
+  --> $DIR/format-args-capture-first-literal-is-macro.rs:19:13
+   |
+LL |     format!(concat!("{a}"));
+   |             ^^^^^^^^^^^^^^
+   |
+   = note: did you intend to capture a variable `a` from the surrounding scope?
+   = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro
+   = note: this error originates in the macro `concat` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
+

tests/ui/fmt/format-args-capture-from-pm-first-arg-macro.rs

@@ -0,0 +1,8 @@
+// aux-build:format-string-proc-macro.rs
+
+extern crate format_string_proc_macro;
+
+fn main() {
+    format_string_proc_macro::bad_format_args_captures!();
+    //~^ ERROR there is no argument named `x`
+}

tests/ui/fmt/format-args-capture-from-pm-first-arg-macro.stderr

@@ -0,0 +1,12 @@
+error: there is no argument named `x`
+  --> $DIR/format-args-capture-from-pm-first-arg-macro.rs:6:5
+   |
+LL |     format_string_proc_macro::bad_format_args_captures!();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: did you intend to capture a variable `x` from the surrounding scope?
+   = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro
+   = note: this error originates in the macro `concat` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+

tests/ui/fmt/format-args-capture-issue-106408.rs

@@ -0,0 +1,10 @@
+// check-pass
+// aux-build:format-string-proc-macro.rs
+
+extern crate format_string_proc_macro;
+
+fn main() {
+    // While literal macros like `format_args!(concat!())` are not supposed to work with implicit
+    // captures, it should work if the whole invocation comes from a macro expansion (#106408).
+    format_string_proc_macro::format_args_captures!();
+}

tests/ui/fmt/format-args-capture-macro-hygiene-pass.rs

@@ -0,0 +1,16 @@
+// run-pass
+
+macro_rules! format_mbe {
+    ($tt:tt) => {
+        {
+            #[allow(unused_variables)]
+            let a = 123;
+            format!($tt)
+        }
+    };
+}
+
+fn main() {
+    let a = 0;
+    assert_eq!(format_mbe!("{a}"), "0");
+}

tests/ui/fmt/respanned-literal-issue-106191.rs

@@ -1,15 +1,10 @@
 // aux-build:format-string-proc-macro.rs
-// check-fail
-// known-bug: #106191
-// unset-rustc-env:RUST_BACKTRACE
-// had to be reverted
-// error-pattern:unexpectedly panicked
-// failure-status:101
-// dont-check-compiler-stderr
 
 extern crate format_string_proc_macro;
 
 fn main() {
     format_string_proc_macro::respan_to_invalid_format_literal!("¡");
+    //~^ ERROR invalid format string: expected `'}'` but string was terminated
     format_args!(r#concat!("¡        {"));
+    //~^ ERROR invalid format string: expected `'}'` but string was terminated
 }