Support user format-like macros (rust-lang#9948)

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!`

---

changelog: Enhancement: [`format_in_format_args`],
[`recursive_format_impl`], [`to_string_in_format_args`],
[`uninlined_format_args`], [`unused_format_specs`]: Recognizes
`#[clippy::format_args]` to support custom 3rs party format macros.

Author: Alexendoo

book/src/SUMMARY.md

@@ -7,6 +7,7 @@
 - [Configuration](configuration.md)
     - [Lint Configuration](lint_configuration.md)
 - [Clippy's Lints](lints.md)
+- [Attributes for Crate Authors](attribs.md)
 - [Continuous Integration](continuous_integration/README.md)
     - [GitHub Actions](continuous_integration/github_actions.md)
     - [GitLab CI](continuous_integration/gitlab.md)

book/src/attribs.md

@@ -0,0 +1,53 @@
+# Attributes for Crate Authors
+
+In some cases it is possible to extend Clippy coverage to 3rd party libraries.
+To do this, Clippy provides attributes that can be applied to items in the 3rd party crate.
+
+## `#[clippy::format_args]`
+
+_Available since Clippy v1.84_
+
+This attribute can be added to a macro that supports `format!`, `println!`, or similar syntax.
+It tells Clippy that the macro is a formatting macro, and that the arguments to the macro
+should be linted as if they were arguments to `format!`. Any lint that would apply to a
+`format!` call will also apply to the macro call. The macro may have additional arguments
+before the format string, and these will be ignored.
+
+### Example
+
+```rust
+/// A macro that prints a message if a condition is true.
+#[macro_export]
+#[clippy::format_args]
+macro_rules! print_if {
+    ($condition:expr, $($args:tt)+) => {{
+        if $condition {
+            println!($($args)+)
+        }
+    }};
+}
+```
+
+## `#[clippy::has_significant_drop]`
+
+_Available since Clippy v1.60_
+
+The `clippy::has_significant_drop` attribute can be added to types whose Drop impls have an important side effect,
+such as unlocking a mutex, making it important for users to be able to accurately understand their lifetimes.
+When a temporary is returned in a function call in a match scrutinee, its lifetime lasts until the end of the match
+block, which may be surprising.
+
+### Example
+
+```rust
+#[clippy::has_significant_drop]
+struct CounterWrapper<'a> {
+    counter: &'a Counter,
+}
+
+impl<'a> Drop for CounterWrapper<'a> {
+    fn drop(&mut self) {
+        self.counter.i.fetch_sub(1, Ordering::Relaxed);
+    }
+}
+```

clippy_utils/src/attrs.rs

@@ -27,7 +27,10 @@ pub const BUILTIN_ATTRIBUTES: &[(&str, DeprecationStatus)] = &[
     ("cyclomatic_complexity", DeprecationStatus::Replaced("cognitive_complexity")),
     ("dump",                  DeprecationStatus::None),
     ("msrv",                  DeprecationStatus::None),
+    // The following attributes are for the 3rd party crate authors.
+    // See book/src/attribs.md
     ("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/format_args_unfixable.rs

@@ -119,3 +119,32 @@ fn test2() {
         format!("something failed at {}", Location::caller())
     );
 }
+
+#[clippy::format_args]
+macro_rules! usr_println {
+    ($target:expr, $($args:tt)*) => {{
+        if $target {
+            println!($($args)*)
+        }
+    }};
+}
+
+fn user_format() {
+    let error = Error::new(ErrorKind::Other, "bad thing");
+    let x = 'x';
+
+    usr_println!(true, "error: {}", format!("boom at {}", Location::caller()));
+    //~^ ERROR: `format!` in `usr_println!` args
+    usr_println!(true, "{}: {}", error, format!("boom at {}", Location::caller()));
+    //~^ ERROR: `format!` in `usr_println!` args
+    usr_println!(true, "{:?}: {}", error, format!("boom at {}", Location::caller()));
+    //~^ ERROR: `format!` in `usr_println!` args
+    usr_println!(true, "{{}}: {}", format!("boom at {}", Location::caller()));
+    //~^ ERROR: `format!` in `usr_println!` args
+    usr_println!(true, r#"error: "{}""#, format!("boom at {}", Location::caller()));
+    //~^ ERROR: `format!` in `usr_println!` args
+    usr_println!(true, "error: {}", format!(r#"boom at "{}""#, Location::caller()));
+    //~^ ERROR: `format!` in `usr_println!` args
+    usr_println!(true, "error: {}", format!("boom at {} {0}", Location::caller()));
+    //~^ ERROR: `format!` in `usr_println!` args
+}

tests/ui/format_args_unfixable.stderr

@@ -174,5 +174,68 @@ LL |     panic!("error: {}", format!("something failed at {}", Location::caller(
    = help: combine the `format!(..)` arguments with the outer `panic!(..)` call
    = help: or consider changing `format!` to `format_args!`
 
-error: aborting due to 18 previous errors
+error: `format!` in `usr_println!` args
+  --> tests/ui/format_args_unfixable.rs:136:5
+   |
+LL |     usr_println!(true, "error: {}", format!("boom at {}", Location::caller()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: combine the `format!(..)` arguments with the outer `usr_println!(..)` call
+   = help: or consider changing `format!` to `format_args!`
+
+error: `format!` in `usr_println!` args
+  --> tests/ui/format_args_unfixable.rs:138:5
+   |
+LL |     usr_println!(true, "{}: {}", error, format!("boom at {}", Location::caller()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: combine the `format!(..)` arguments with the outer `usr_println!(..)` call
+   = help: or consider changing `format!` to `format_args!`
+
+error: `format!` in `usr_println!` args
+  --> tests/ui/format_args_unfixable.rs:140:5
+   |
+LL |     usr_println!(true, "{:?}: {}", error, format!("boom at {}", Location::caller()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: combine the `format!(..)` arguments with the outer `usr_println!(..)` call
+   = help: or consider changing `format!` to `format_args!`
+
+error: `format!` in `usr_println!` args
+  --> tests/ui/format_args_unfixable.rs:142:5
+   |
+LL |     usr_println!(true, "{{}}: {}", format!("boom at {}", Location::caller()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: combine the `format!(..)` arguments with the outer `usr_println!(..)` call
+   = help: or consider changing `format!` to `format_args!`
+
+error: `format!` in `usr_println!` args
+  --> tests/ui/format_args_unfixable.rs:144:5
+   |
+LL |     usr_println!(true, r#"error: "{}""#, format!("boom at {}", Location::caller()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: combine the `format!(..)` arguments with the outer `usr_println!(..)` call
+   = help: or consider changing `format!` to `format_args!`
+
+error: `format!` in `usr_println!` args
+  --> tests/ui/format_args_unfixable.rs:146:5
+   |
+LL |     usr_println!(true, "error: {}", format!(r#"boom at "{}""#, Location::caller()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: combine the `format!(..)` arguments with the outer `usr_println!(..)` call
+   = help: or consider changing `format!` to `format_args!`
+
+error: `format!` in `usr_println!` args
+  --> tests/ui/format_args_unfixable.rs:148:5
+   |
+LL |     usr_println!(true, "error: {}", format!("boom at {} {0}", Location::caller()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: combine the `format!(..)` arguments with the outer `usr_println!(..)` call
+   = help: or consider changing `format!` to `format_args!`
+
+error: aborting due to 25 previous errors
 

tests/ui/uninlined_format_args.fixed

@@ -257,8 +257,6 @@ fn tester2() {
     my_concat!("{}", local_i32);
     my_good_macro!("{}", local_i32);
     my_good_macro!("{}", local_i32,);
-
-    // FIXME: Broken false positives, currently unhandled
     my_bad_macro!("{}", local_i32);
     my_bad_macro2!("{}", local_i32);
     used_twice! {
@@ -267,3 +265,22 @@ fn tester2() {
         local_i32,
     };
 }
+
+#[clippy::format_args]
+macro_rules! usr_println {
+    ($target:expr, $($args:tt)*) => {{
+        if $target {
+            println!($($args)*)
+        }
+    }};
+}
+
+fn user_format() {
+    let local_i32 = 1;
+    let local_f64 = 2.0;
+
+    usr_println!(true, "val='{local_i32}'");
+    usr_println!(true, "{local_i32}");
+    usr_println!(true, "{local_i32:#010x}");
+    usr_println!(true, "{local_f64:.1}");
+}

tests/ui/uninlined_format_args.rs

@@ -262,8 +262,6 @@ fn tester2() {
     my_concat!("{}", local_i32);
     my_good_macro!("{}", local_i32);
     my_good_macro!("{}", local_i32,);
-
-    // FIXME: Broken false positives, currently unhandled
     my_bad_macro!("{}", local_i32);
     my_bad_macro2!("{}", local_i32);
     used_twice! {
@@ -272,3 +270,22 @@ fn tester2() {
         local_i32,
     };
 }
+
+#[clippy::format_args]
+macro_rules! usr_println {
+    ($target:expr, $($args:tt)*) => {{
+        if $target {
+            println!($($args)*)
+        }
+    }};
+}
+
+fn user_format() {
+    let local_i32 = 1;
+    let local_f64 = 2.0;
+
+    usr_println!(true, "val='{}'", local_i32);
+    usr_println!(true, "{}", local_i32);
+    usr_println!(true, "{:#010x}", local_i32);
+    usr_println!(true, "{:.1}", local_f64);
+}

tests/ui/uninlined_format_args.stderr

@@ -845,5 +845,53 @@ 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:287:5
+   |
+LL |     usr_println!(true, "val='{}'", local_i32);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: change this to
+   |
+LL -     usr_println!(true, "val='{}'", local_i32);
+LL +     usr_println!(true, "val='{local_i32}'");
+   |
+
+error: variables can be used directly in the `format!` string
+  --> tests/ui/uninlined_format_args.rs:288:5
+   |
+LL |     usr_println!(true, "{}", local_i32);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: change this to
+   |
+LL -     usr_println!(true, "{}", local_i32);
+LL +     usr_println!(true, "{local_i32}");
+   |
+
+error: variables can be used directly in the `format!` string
+  --> tests/ui/uninlined_format_args.rs:289:5
+   |
+LL |     usr_println!(true, "{:#010x}", local_i32);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: change this to
+   |
+LL -     usr_println!(true, "{:#010x}", local_i32);
+LL +     usr_println!(true, "{local_i32:#010x}");
+   |
+
+error: variables can be used directly in the `format!` string
+  --> tests/ui/uninlined_format_args.rs:290:5
+   |
+LL |     usr_println!(true, "{:.1}", local_f64);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: change this to
+   |
+LL -     usr_println!(true, "{:.1}", local_f64);
+LL +     usr_println!(true, "{local_f64:.1}");
+   |
+
+error: aborting due to 75 previous errors
 

tests/ui/unused_format_specs.1.fixed

@@ -33,3 +33,38 @@ fn should_not_lint() {
     let args = format_args!("");
     println!("{args}");
 }
+
+#[clippy::format_args]
+macro_rules! usr_println {
+    ($target:expr, $($args:tt)*) => {{
+        if $target {
+            println!($($args)*)
+        }
+    }};
+}
+
+fn should_lint_user() {
+    // prints `.`, not `     .`
+    usr_println!(true, "{:5}.", format!(""));
+    //~^ ERROR: format specifiers have no effect on `format_args!()`
+    //prints `abcde`, not `abc`
+    usr_println!(true, "{:.3}", format!("abcde"));
+    //~^ ERROR: format specifiers have no effect on `format_args!()`
+
+    usr_println!(true, "{}.", format_args_from_macro!());
+    //~^ ERROR: format specifiers have no effect on `format_args!()`
+
+    let args = format_args!("");
+    usr_println!(true, "{args}");
+    //~^ ERROR: format specifiers have no effect on `format_args!()`
+}
+
+fn should_not_lint_user() {
+    usr_println!(true, "{}", format_args!(""));
+    // Technically the same as `{}`, but the `format_args` docs specifically mention that you can use
+    // debug formatting so allow it
+    usr_println!(true, "{:?}", format_args!(""));
+
+    let args = format_args!("");
+    usr_println!(true, "{args}");
+}