Auto merge of #34544 - 3Hren:issue/xx/reinterpret-format-precision-for-strings, r=alexcrichton

feat: reinterpret `precision` field for strings

This commit changes the behavior of formatting string arguments with both width and precision fields set.

Documentation says that the `width` field is the "minimum width" that the format should take up. If the value's string does not fill up this many characters, then the padding specified by fill/alignment will be used to take up the required space.

This is true for all formatted types except string, which is truncated down to `precision` number of chars and then all of `fill`, `align` and `width` fields are completely ignored.

For example: `format!("{:/^10.8}", "1234567890);` emits "12345678". In the contrast Python version works as the expected:
```python
>>> '{:/^10.8}'.format('1234567890')
'/12345678/'
```

This commit gives back the `Python` behavior by changing the `precision` field meaning to the truncation and nothing more. The result string *will* be prepended/appended up to the `width` field with the proper `fill` char.

__However, this is the breaking change, I admit.__ Feel free to close it, but otherwise it should be mentioned in the `std::fmt` documentation somewhere near of `fill/align/width` fields description.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/libcollections/fmt.rs b/src/libcollections/fmt.rs
index 15de0dd802d..c5312d0d9bb 100644
--- a/src/libcollections/fmt.rs
+++ b/src/libcollections/fmt.rs
@@ -409,8 +409,8 @@
 //! ## Precision
 //!
 //! For non-numeric types, this can be considered a "maximum width". If the resulting string is
-//! longer than this width, then it is truncated down to this many characters and only those are
-//! emitted.
+//! longer than this width, then it is truncated down to this many characters and that truncated
+//! value is emitted with proper `fill`, `alignment` and `width` if those parameters are set.
 //!
 //! For integral types, this is ignored.
 //!
@@ -470,6 +470,7 @@
 //! ```
 //! println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56);
 //! println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56");
+//! println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56");
 //! ```
 //!
 //! print two significantly different things:
@@ -477,6 +478,7 @@
 //! ```text
 //! Hello, `1234.560` has 3 fractional digits
 //! Hello, `123` has 3 characters
+//! Hello, `     123` has 3 right-aligned characters
 //! ```
 //!
 //! # Escaping