Rollup merge of #37613 - DanielKeep:eww-you-got-printf-in-your-format, r=alexcrichton

Add foreign formatting directive detection.

This teaches `format_args!` how to interpret format printf- and
shell-style format directives.  This is used in cases where there are
unused formatting arguments, and the reason for that *might* be because
the programmer is trying to use the wrong kind of formatting string.

This was prompted by an issue encountered by simulacrum on the #rust IRC
channel.  In short: although `println!` told them that they weren't using
all of the conversion arguments, the problem was in using printf-syle
directives rather than ones `println!` would undertand.

Where possible, `format_args!` will tell the programmer what they should
use instead.  For example, it will suggest replacing `%05d` with `{:0>5}`,
or `%2$.*3$s` with `{1:.3$}`.  Even if it cannot suggest a replacement,
it will explicitly note that Rust does not support that style of directive,
and direct the user to the `std::fmt` documentation.

-----

**Example**: given:

```rust
fn main() {
    println!("%.*3$s %s!\n", "Hello,", "World", 4);
    println!("%1$*2$.*3$f", 123.456);
}
```

The compiler outputs the following:

```text
error: multiple unused formatting arguments
 --> local/fmt.rs:2:5
  |
2 |     println!("%.*3$s %s!\n", "Hello,", "World", 4);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: argument never used
 --> local/fmt.rs:2:30
  |
2 |     println!("%.*3$s %s!\n", "Hello,", "World", 4);
  |                              ^^^^^^^^
note: argument never used
 --> local/fmt.rs:2:40
  |
2 |     println!("%.*3$s %s!\n", "Hello,", "World", 4);
  |                                        ^^^^^^^
note: argument never used
 --> local/fmt.rs:2:49
  |
2 |     println!("%.*3$s %s!\n", "Hello,", "World", 4);
  |                                                 ^
  = help: `%.*3$s` should be written as `{:.2$}`
  = help: `%s` should be written as `{}`
  = note: printf formatting not supported; see the documentation for `std::fmt`
  = note: this error originates in a macro outside of the current crate

error: argument never used
 --> local/fmt.rs:6:29
  |
6 |     println!("%1$*2$.*3$f", 123.456);
  |                             ^^^^^^^
  |
  = help: `%1$*2$.*3$f` should be written as `{0:1$.2$}`
  = note: printf formatting not supported; see the documentation for `std::fmt`
```

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libsyntax_ext/lib.rs b/src/libsyntax_ext/lib.rs
index e1542c9e466..1ebac19b4f0 100644
--- a/src/libsyntax_ext/lib.rs
+++ b/src/libsyntax_ext/lib.rs
@@ -40,6 +40,7 @@ mod concat;
 mod concat_idents;
 mod env;
 mod format;
+mod format_foreign;
 mod log_syntax;
 mod trace_macros;