Auto merge of rust-lang#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.

Author: bors

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.
 //!
@@ -470,13 +470,15 @@
 //! ```
 //! 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:
 //!
 //! ```text
 //! Hello, `1234.560` has 3 fractional digits
 //! Hello, `123` has 3 characters
+//! Hello, `     123` has 3 right-aligned characters
 //! ```
 //!
 //! # Escaping

src/libcore/fmt/mod.rs

@@ -980,15 +980,19 @@ impl<'a> Formatter<'a> {
             return self.buf.write_str(s);
         }
         // The `precision` field can be interpreted as a `max-width` for the
-        // string being formatted
-        if let Some(max) = self.precision {
-            // If there's a maximum width and our string is longer than
-            // that, then we must always have truncation. This is the only
-            // case where the maximum length will matter.
+        // string being formatted.
+        let s = if let Some(max) = self.precision {
+            // If our string is longer that the precision, then we must have
+            // truncation. However other flags like `fill`, `width` and `align`
+            // must act as always.
             if let Some((i, _)) = s.char_indices().skip(max).next() {
-                return self.buf.write_str(&s[..i])
+                &s[..i]
+            } else {
+                &s
             }
-        }
+        } else {
+            &s
+        };
         // The `width` field is more of a `min-width` parameter at this point.
         match self.width {
             // If we're under the maximum length, and there's no minimum length

src/test/run-pass/ifmt.rs

@@ -125,7 +125,7 @@ pub fn main() {
     t!(format!("{:<4.4}", "aaaaaaaaaaaaaaaaaa"), "aaaa");
     t!(format!("{:>4.4}", "aaaaaaaaaaaaaaaaaa"), "aaaa");
     t!(format!("{:^4.4}", "aaaaaaaaaaaaaaaaaa"), "aaaa");
-    t!(format!("{:>10.4}", "aaaaaaaaaaaaaaaaaa"), "aaaa");
+    t!(format!("{:>10.4}", "aaaaaaaaaaaaaaaaaa"), "      aaaa");
     t!(format!("{:2.4}", "aaaaa"), "aaaa");
     t!(format!("{:2.4}", "aaaa"), "aaaa");
     t!(format!("{:2.4}", "aaa"), "aaa");
@@ -140,6 +140,7 @@ pub fn main() {
     t!(format!("{:a$}", "a", a=4), "a   ");
     t!(format!("{:-#}", "a"), "a");
     t!(format!("{:+#}", "a"), "a");
+    t!(format!("{:/^10.8}", "1234567890"), "/12345678/");
 
     // Some float stuff
     t!(format!("{:}", 1.0f32), "1");