new uninlined_format_args lint to inline explicit arguments

Implement https://github.com/rust-lang/rust-clippy/issues/8368 - a new
lint to inline format arguments such as `print!("{}", var)` into
`print!("{var}")`.

code | suggestion | comment
---|---|---
`print!("{}", var)` | `print!("{var}")` |  simple variables
`print!("{0}", var)` | `print!("{var}")` |  positional variables
`print!("{v}", v=var)` | `print!("{var}")` |  named variables
`print!("{0} {0}", var)` | `print!("{var} {var}")` |  aliased variables
`print!("{0:1$}", var, width)` | `print!("{var:width$}")` |  width
support
`print!("{0:.1$}", var, prec)` | `print!("{var:.prec$}")` |  precision
support
`print!("{:.*}", prec, var)` | `print!("{var:.prec$}")` |  asterisk
support

code | suggestion | comment
---|---|---
`print!("{0}={1}", var, 1+2)` | `print!("{var}={0}", 1+2)` | Format
string uses an indexed argument that cannot be inlined.  Supporting this
case requires re-indexing of the format string.

changelog: [`uninlined_format_args`]: A new lint to inline format
arguments, i.e. `print!("{}", var)` into `print!("{var}")`

2 files changed, 37 insertions, 0 deletions

diff --git a/src/docs.rs b/src/docs.rs
index 6c89b4dde37..a501b56ffbe 100644
--- a/src/docs.rs
+++ b/src/docs.rs
@@ -521,6 +521,7 @@ docs! {
     "unimplemented",
     "uninit_assumed_init",
     "uninit_vec",
+    "uninlined_format_args",
     "unit_arg",
     "unit_cmp",
     "unit_hash",
diff --git a/src/docs/uninlined_format_args.txt b/src/docs/uninlined_format_args.txt
new file mode 100644
index 00000000000..3d2966c84db
--- /dev/null
+++ b/src/docs/uninlined_format_args.txt
@@ -0,0 +1,36 @@
+### What it does
+Detect when a variable is not inlined in a format string,
+and suggests to inline it.
+
+### Why is this bad?
+Non-inlined code is slightly more difficult to read and understand,
+as it requires arguments to be matched against the format string.
+The inlined syntax, where allowed, is simpler.
+
+### Example
+```
+format!("{}", var);
+format!("{v:?}", v = var);
+format!("{0} {0}", var);
+format!("{0:1$}", var, width);
+format!("{:.*}", prec, var);
+```
+Use instead:
+```
+format!("{var}");
+format!("{var:?}");
+format!("{var} {var}");
+format!("{var:width$}");
+format!("{var:.prec$}");
+```
+
+### Known Problems
+
+There may be a false positive if the format string is expanded from certain proc macros:
+
+```
+println!(indoc!("{}"), var);
+```
+
+If a format string contains a numbered argument that cannot be inlined
+nothing will be suggested, e.g. `println!("{0}={1}", var, 1+2)`.
\ No newline at end of file