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, 8 insertions, 0 deletions

diff --git a/tests/ui/fmt/format-args-capture-from-pm-first-arg-macro.rs b/tests/ui/fmt/format-args-capture-from-pm-first-arg-macro.rs
new file mode 100644
index 00000000000..f67edf5e167
--- /dev/null
+++ b/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`
+}