feat: Add very simplistic ident completion for format_args! macro input

crates/ide_db/src/helpers/format_string.rs

@@ -0,0 +1,31 @@
+//! Tools to work with format string literals for the `format_args!` family of macros.
+use syntax::{ast, AstNode, AstToken};
+
+pub fn is_format_string(string: &ast::String) -> bool {
+    // Check if `string` is a format string argument of a macro invocation.
+    // `string` is a string literal, mapped down into the innermost macro expansion.
+    // Since `format_args!` etc. remove the format string when expanding, but place all arguments
+    // in the expanded output, we know that the string token is (part of) the format string if it
+    // appears in `format_args!` (otherwise it would have been mapped down further).
+    //
+    // This setup lets us correctly highlight the components of `concat!("{}", "bla")` format
+    // strings. It still fails for `concat!("{", "}")`, but that is rare.
+
+    (|| {
+        let macro_call = string.syntax().ancestors().find_map(ast::MacroCall::cast)?;
+        let name = macro_call.path()?.segment()?.name_ref()?;
+
+        if !matches!(
+            name.text().as_str(),
+            "format_args" | "format_args_nl" | "const_format_args" | "panic_2015" | "panic_2021"
+        ) {
+            return None;
+        }
+
+        // NB: we match against `panic_2015`/`panic_2021` here because they have a special-cased arm for
+        // `"{}"`, which otherwise wouldn't get highlighted.
+
+        Some(())
+    })()
+    .is_some()
+}