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new uninlined_format_args lint to inline explicit arguments

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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}")`
This commit is contained in:
Yuri Astrakhan 2022-09-14 12:25:48 -04:00
parent 57c9daa09b
commit 5a71bbdf3f
15 changed files with 1451 additions and 29 deletions
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36
src/docs/uninlined_format_args.txt Normal file
View file

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### 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)`.