Add `unused_format_specs` lint

2 files changed, 25 insertions, 0 deletions

diff --git a/src/docs.rs b/src/docs.rs
index 41c31f91bca..33ea8488375 100644
--- a/src/docs.rs
+++ b/src/docs.rs
@@ -557,6 +557,7 @@ docs! {
     "unseparated_literal_suffix",
     "unsound_collection_transmute",
     "unused_async",
+    "unused_format_specs",
     "unused_io_amount",
     "unused_peekable",
     "unused_rounding",
diff --git a/src/docs/unused_format_specs.txt b/src/docs/unused_format_specs.txt
new file mode 100644
index 00000000000..77be3a2fb17
--- /dev/null
+++ b/src/docs/unused_format_specs.txt
@@ -0,0 +1,24 @@
+### What it does
+Detects [formatting parameters] that have no effect on the output of
+`format!()`, `println!()` or similar macros.
+
+### Why is this bad?
+Shorter format specifiers are easier to read, it may also indicate that
+an expected formatting operation such as adding padding isn't happening.
+
+### Example
+```
+println!("{:.}", 1.0);
+
+println!("not padded: {:5}", format_args!("..."));
+```
+Use instead:
+```
+println!("{}", 1.0);
+
+println!("not padded: {}", format_args!("..."));
+// OR
+println!("padded: {:5}", format!("..."));
+```
+
+[formatting parameters]: https://doc.rust-lang.org/std/fmt/index.html#formatting-parameters
\ No newline at end of file