Support user format-like macros

Add support for `#[clippy::format_args]` attribute that can be attached to any macro to indicate that it functions the same as the built-in format macros like `format!`, `println!` and `write!`

Author: nyurik

clippy_utils/src/attrs.rs

@@ -28,6 +28,7 @@ pub const BUILTIN_ATTRIBUTES: &[(&str, DeprecationStatus)] = &[
     ("dump",                  DeprecationStatus::None),
     ("msrv",                  DeprecationStatus::None),
     ("has_significant_drop",  DeprecationStatus::None),
+    ("format_args",           DeprecationStatus::None),
 ];
 
 pub struct LimitStack {

clippy_utils/src/macros.rs

@@ -1,13 +1,14 @@
 #![allow(clippy::similar_names)] // `expr` and `expn`
 
+use crate::get_unique_attr;
 use crate::visitors::{Descend, for_each_expr_without_closures};
 
 use arrayvec::ArrayVec;
 use rustc_ast::{FormatArgs, FormatArgument, FormatPlaceholder};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::{Lrc, OnceLock};
 use rustc_hir::{self as hir, Expr, ExprKind, HirId, Node, QPath};
-use rustc_lint::LateContext;
+use rustc_lint::{LateContext, LintContext};
 use rustc_span::def_id::DefId;
 use rustc_span::hygiene::{self, MacroKind, SyntaxContext};
 use rustc_span::{BytePos, ExpnData, ExpnId, ExpnKind, Span, SpanData, Symbol, sym};
@@ -36,7 +37,9 @@ pub fn is_format_macro(cx: &LateContext<'_>, macro_def_id: DefId) -> bool {
     if let Some(name) = cx.tcx.get_diagnostic_name(macro_def_id) {
         FORMAT_MACRO_DIAG_ITEMS.contains(&name)
     } else {
-        false
+        // Allow users to tag any macro as being format!-like
+        // TODO: consider deleting FORMAT_MACRO_DIAG_ITEMS and using just this method
+        get_unique_attr(cx.sess(), cx.tcx.get_attrs_unchecked(macro_def_id), "format_args").is_some()
     }
 }
 

tests/ui/uninlined_format_args.fixed

@@ -212,6 +212,7 @@ macro_rules! my_concat {
     }
 }
 
+#[clippy::format_args]
 macro_rules! my_good_macro {
     ($fmt:literal $(, $e:expr)* $(,)?) => {
         println!($fmt $(, $e)*)
@@ -255,8 +256,8 @@ fn tester2() {
     my_println2_args!(true, "{}", local_i32);
     my_println2!(true, "{}", local_i32);
     my_concat!("{}", local_i32);
-    my_good_macro!("{}", local_i32);
-    my_good_macro!("{}", local_i32,);
+    my_good_macro!("{local_i32}");
+    my_good_macro!("{local_i32}",);
 
     // FIXME: Broken false positives, currently unhandled
     my_bad_macro!("{}", local_i32);

tests/ui/uninlined_format_args.rs

@@ -217,6 +217,7 @@ macro_rules! my_concat {
     }
 }
 
+#[clippy::format_args]
 macro_rules! my_good_macro {
     ($fmt:literal $(, $e:expr)* $(,)?) => {
         println!($fmt $(, $e)*)

tests/ui/uninlined_format_args.stderr

@@ -845,5 +845,29 @@ LL -     println!("expand='{}'", local_i32);
 LL +     println!("expand='{local_i32}'");
    |
 
-error: aborting due to 71 previous errors
+error: variables can be used directly in the `format!` string
+  --> tests/ui/uninlined_format_args.rs:264:5
+   |
+LL |     my_good_macro!("{}", local_i32);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: change this to
+   |
+LL -     my_good_macro!("{}", local_i32);
+LL +     my_good_macro!("{local_i32}");
+   |
+
+error: variables can be used directly in the `format!` string
+  --> tests/ui/uninlined_format_args.rs:265:5
+   |
+LL |     my_good_macro!("{}", local_i32,);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: change this to
+   |
+LL -     my_good_macro!("{}", local_i32,);
+LL +     my_good_macro!("{local_i32}",);
+   |
+
+error: aborting due to 73 previous errors