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, 26 insertions, 10 deletions

diff --git a/tests/ui/fmt/auxiliary/format-string-proc-macro.rs b/tests/ui/fmt/auxiliary/format-string-proc-macro.rs
index 1b7ef93f41d..0c39ade721f 100644
--- a/tests/ui/fmt/auxiliary/format-string-proc-macro.rs
+++ b/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
 }