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}")`

1 files changed, 10 insertions, 0 deletions

diff --git a/clippy_utils/src/source.rs b/clippy_utils/src/source.rs
index 64c3f70efa5..d28bd92d708 100644
--- a/clippy_utils/src/source.rs
+++ b/clippy_utils/src/source.rs
@@ -392,6 +392,16 @@ pub fn trim_span(sm: &SourceMap, span: Span) -> Span {
     .span()
 }
 
+/// Expand a span to include a preceding comma
+/// ```rust,ignore
+/// writeln!(o, "")   ->   writeln!(o, "")
+///             ^^                   ^^^^
+/// ```
+pub fn expand_past_previous_comma(cx: &LateContext<'_>, span: Span) -> Span {
+    let extended = cx.sess().source_map().span_extend_to_prev_char(span, ',', true);
+    extended.with_lo(extended.lo() - BytePos(1))
+}
+
 #[cfg(test)]
 mod test {
     use super::{reindent_multiline, without_block_comments};