better docs

Author: nyurik

clippy_lints/src/format_args.rs

@@ -103,14 +103,20 @@ declare_clippy_lint! {
     /// format!("{var:.prec$}");
     /// ```
     ///
-    /// ### Known Problems
-    ///
-    /// There may be a false positive if the format string is expanded from certain proc macros:
-    ///
-    /// ```ignore
-    /// println!(indoc!("{}"), var);
+    /// If allow-mixed-uninlined-format-args is set to false in clippy.toml,
+    /// the following code will also trigger the lint:
+    /// ```rust
+    /// # let var = 42;
+    /// format!("{} {}", var, 1+2);
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// # let var = 42;
+    /// format!("{var} {}", 1+2);
     /// ```
     ///
+    /// ### Known Problems
+    ///
     /// If a format string contains a numbered argument that cannot be inlined
     /// nothing will be suggested, e.g. `println!("{0}={1}", var, 1+2)`.
     #[clippy::version = "1.65.0"]
@@ -308,6 +314,11 @@ fn check_uninlined_args(
         "variables can be used directly in the `format!` string",
         |diag| {
             diag.multipart_suggestion("change this to", fixes, Applicability::MachineApplicable);
+            if ignore_mixed {
+                // Improve lint config discoverability
+                diag.note_once("this lint can also fix mixed format arg inlining if \
+                `allow-mixed-uninlined-format-args = false` is set in the `clippy.toml` file");
+            }
         },
     );
 }

tests/ui/uninlined_format_args.stderr

@@ -4,6 +4,7 @@ error: variables can be used directly in the `format!` string
 LL |     println!("val='{}'", local_i32);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
+   = note: this lint can also fix mixed format arg inlining if `allow-mixed-uninlined-format-args = false` is set in the `clippy.toml` file
    = note: `-D clippy::uninlined-format-args` implied by `-D warnings`
 help: change this to
    |

tests/ui/uninlined_format_args_panic.edition2018.stderr

@@ -4,6 +4,7 @@ error: variables can be used directly in the `format!` string
 LL |     println!("val='{}'", var);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
+   = note: this lint can also fix mixed format arg inlining if `allow-mixed-uninlined-format-args = false` is set in the `clippy.toml` file
    = note: `-D clippy::uninlined-format-args` implied by `-D warnings`
 help: change this to
    |

tests/ui/uninlined_format_args_panic.edition2021.stderr

@@ -4,6 +4,7 @@ error: variables can be used directly in the `format!` string
 LL |     println!("val='{}'", var);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
+   = note: this lint can also fix mixed format arg inlining if `allow-mixed-uninlined-format-args = false` is set in the `clippy.toml` file
    = note: `-D clippy::uninlined-format-args` implied by `-D warnings`
 help: change this to
    |