Hint on unknown escape of Unicode quotation marks in string literal

[LFS6502]

Fixes #128858
Continuation of #128906

[rustbot]

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[LFS6502]

closes #128906

[LFS6502]

@jieyouxu @compiler-errors @Dylan-DPC If you've got some spare time, I'd love some input from you too.

I will squash at the end.

[LFS6502]

error: unknown character escape: `\u{201c}`
 --> src/main.rs:2:26
  |
2 |     dbg!("since when is \“THIS\” not allowed in a string literal");
  |                          ^ unknown character escape
  |
  = help: \u{201c} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
help: if you meant to use a unicode quote; consider using its escaped form for clarity
  |
2 -     dbg!("since when is \“THIS\” not allowed in a string literal");
2 +     dbg!("since when is \u{201c}THIS\” not allowed in a string literal");
  |

error: unknown character escape: `\u{201d}`
 --> src/main.rs:2:32
  |
2 |     dbg!("since when is \“THIS\” not allowed in a string literal");
  |                                ^ unknown character escape
  |
  = help: \u{201d} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
help: if you meant to use a unicode quote; consider using its escaped form for clarity
  |
2 -     dbg!("since when is \“THIS\” not allowed in a string literal");
2 +     dbg!("since when is \“THIS\u{201d} not allowed in a string literal");
  |

error: could not compile `diag` (bin "diag") due to 2 previous errors

In image form:

[apiraino]

Assigning review to Michael because of this comment in the previous version of a patch.

r? @compiler-errors

[compiler-errors]

kinda busy so re-rolling

r? compiler