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

1 files changed, 10 insertions, 8 deletions

diff --git a/compiler/rustc_parse_format/src/lib.rs b/compiler/rustc_parse_format/src/lib.rs
index 8a3cedfee79..d67239efa80 100644
--- a/compiler/rustc_parse_format/src/lib.rs
+++ b/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();