Auto merge of #95345 - dtolnay:escape0, r=Dylan-DPC

Debug print char 0 as '\0' rather than '\u{0}'

```rust
println!("{:?}", "foo\0");
```

- **Before:** `"foo\u{0}"`
- **After:** `"foo\0"`

```rust
println!("{:?}", '\0');
```

- **Before:** `'\u{0}'`
- **After:** `'\0'`

`'\0'` will be more recognizable to everyone than `'\u{0}'` because it's how we talk about character 0 in all of our docs and example code, such as https://doc.rust-lang.org/std/ffi/index.html, https://doc.rust-lang.org/std/ffi/struct.CStr.html, https://doc.rust-lang.org/std/ffi/struct.CString.html.

library/alloc/tests/fmt.rs

@@ -69,7 +69,7 @@ fn test_format_macro_interface() {
     t!(format!("{:?}", "true"), "\"true\"");
     t!(format!("{:?}", "foo\nbar"), "\"foo\\nbar\"");
     t!(format!("{:?}", "foo\n\"bar\"\r\n\'baz\'\t\\qux\\"), r#""foo\n\"bar\"\r\n'baz'\t\\qux\\""#);
-    t!(format!("{:?}", "foo\0bar\x01baz\u{7f}q\u{75}x"), r#""foo\u{0}bar\u{1}baz\u{7f}qux""#);
+    t!(format!("{:?}", "foo\0bar\x01baz\u{7f}q\u{75}x"), r#""foo\0bar\u{1}baz\u{7f}qux""#);
     t!(format!("{:o}", 10_usize), "12");
     t!(format!("{:x}", 10_usize), "a");
     t!(format!("{:X}", 10_usize), "A");

library/alloc/tests/str.rs

@@ -1116,7 +1116,7 @@ fn test_escape_debug() {
     assert_eq!("abc".escape_debug().to_string(), "abc");
     assert_eq!("a c".escape_debug().to_string(), "a c");
     assert_eq!("éèê".escape_debug().to_string(), "éèê");
-    assert_eq!("\r\n\t".escape_debug().to_string(), "\\r\\n\\t");
+    assert_eq!("\0\r\n\t".escape_debug().to_string(), "\\0\\r\\n\\t");
     assert_eq!("'\"\\".escape_debug().to_string(), "\\'\\\"\\\\");
     assert_eq!("\u{7f}\u{ff}".escape_debug().to_string(), "\\u{7f}\u{ff}");
     assert_eq!("\u{100}\u{ffff}".escape_debug().to_string(), "\u{100}\\u{ffff}");

library/core/src/char/methods.rs

@@ -421,6 +421,7 @@ impl char {
     #[inline]
     pub(crate) fn escape_debug_ext(self, args: EscapeDebugExtArgs) -> EscapeDebug {
         let init_state = match self {
+            '\0' => EscapeDefaultState::Backslash('0'),
             '\t' => EscapeDefaultState::Backslash('t'),
             '\r' => EscapeDefaultState::Backslash('r'),
             '\n' => EscapeDefaultState::Backslash('n'),

library/core/tests/char.rs

@@ -197,7 +197,7 @@ fn test_escape_debug() {
     assert_eq!(string('~'), "~");
     assert_eq!(string('é'), "é");
     assert_eq!(string('文'), "文");
-    assert_eq!(string('\x00'), "\\u{0}");
+    assert_eq!(string('\x00'), "\\0");
     assert_eq!(string('\x1f'), "\\u{1f}");
     assert_eq!(string('\x7f'), "\\u{7f}");
     assert_eq!(string('\u{80}'), "\\u{80}");

src/test/ui/half-open-range-patterns/half-open-range-pats-exhaustive-fail.stderr

@@ -50,17 +50,17 @@ LL ~         match $s { $($t)+ => {}
 LL ~         '\u{10fffe}'..='\u{10ffff}' => todo!() }
    |
 
-error[E0004]: non-exhaustive patterns: `'\u{0}'` not covered
+error[E0004]: non-exhaustive patterns: `'\0'` not covered
   --> $DIR/half-open-range-pats-exhaustive-fail.rs:28:8
    |
 LL |     m!('a', ALMOST_MIN..);
-   |        ^^^ pattern `'\u{0}'` not covered
+   |        ^^^ pattern `'\0'` not covered
    |
    = note: the matched value is of type `char`
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         match $s { $($t)+ => {}
-LL ~         '\u{0}' => todo!() }
+LL ~         '\0' => todo!() }
    |
 
 error[E0004]: non-exhaustive patterns: `'\u{10ffff}'` not covered