Hint on unknown escape of Unicode quotation marks in string literal

[gilescope]

Fixes #128858

This finishes off @LFS6502's PR #137067, which had been pre-approved by reviewers pending small follow-ups. Two reviewers (@Noratrieb and @estebank) left a short list of concrete requests; the original author indicated they were "working on this now" on 2025-08-28 but never returned. Picking it up so it can land.

The substantive change (the diagnostic itself) is @LFS6502's work, preserved as a single commit in their authorship. The second commit applies the reviewer asks:

• Remove the leftover // lit.replace(c, &ec[1..]) development comment in unescape_error_reporting.rs (Noratrieb, then re-raised by estebank).
• Merge the duplicated help+suggestion. The explanation stays in help; the action moves into span_suggestion_verbose so the same text isn't said twice (estebank).
• Drop redundant line/column numbers from //~^ERROR annotations in the UI test (Noratrieb).
• Re-bless tests/ui/unicode-quote.stderr — picks up the previously-applied ascii → ASCII capitalization fix and the zero-width-diacritic caret-column positions that the rust-log-analyzer flagged on the last CI run.

The first commit attributes the original work to @LFS6502 (and the PR description in their original draft credits #128906 as the prior continuation).

r? compiler

Continuation of #137067 (closes #128858).

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

diff of stderr:

41   --> $DIR/unicode-quote.rs:10:12
42    |
43 LL |     dbg!("\̋"); //U+030B
-    |            ^unknown character escape
+    |            ^ unknown character escape
45    |
46    = help: \u{30b} is not an ASCII quote, but may look like one in some fonts
47 help: consider writing it as `\u{30b}` for clarity

54   --> $DIR/unicode-quote.rs:12:12
55    |
56 LL |     dbg!("\̎"); //U+030E
-    |            ^unknown character escape
+    |            ^ unknown character escape
58    |
59    = help: \u{30e} is not an ASCII quote, but may look like one in some fonts
60 help: consider writing it as `\u{30e}` for clarity

103    |
104 
105 error: aborting due to 8 previous errors
+ 
---
To only update this specific test, also pass `--test-args parser/unicode-quote.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/parser/unicode-quote.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/parser/unicode-quote" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error: unknown character escape: `\u{2033}`
##[error]  --> /checkout/tests/ui/parser/unicode-quote.rs:4:12
   |
LL |     dbg!("\″"); //U+2033
   |            ^ unknown character escape
   |
   = help: \u{2033} is not an ASCII quote, but may look like one in some fonts
help: consider writing it as `\u{2033}` for clarity
   |
LL -     dbg!("\″"); //U+2033
LL +     dbg!("\u{2033}"); //U+2033
   |

error: unknown character escape: `\u{2ba}`
##[error]  --> /checkout/tests/ui/parser/unicode-quote.rs:6:12
   |
LL |     dbg!("\ʺ"); //U+02BA
   |            ^ unknown character escape
   |
   = help: \u{2ba} is not an ASCII quote, but may look like one in some fonts
help: consider writing it as `\u{2ba}` for clarity
   |
LL -     dbg!("\ʺ"); //U+02BA
LL +     dbg!("\u{2ba}"); //U+02BA
   |

error: unknown character escape: `\u{2dd}`
##[error]  --> /checkout/tests/ui/parser/unicode-quote.rs:8:12
   |
LL |     dbg!("\˝"); //U+02DD
   |            ^ unknown character escape
   |
   = help: \u{2dd} is not an ASCII quote, but may look like one in some fonts
help: consider writing it as `\u{2dd}` for clarity
   |
LL -     dbg!("\˝"); //U+02DD
LL +     dbg!("\u{2dd}"); //U+02DD
   |

error: unknown character escape: `\u{30b}`
##[error]  --> /checkout/tests/ui/parser/unicode-quote.rs:10:12
   |
LL |     dbg!("\̋"); //U+030B
   |            ^ unknown character escape
   |
   = help: \u{30b} is not an ASCII quote, but may look like one in some fonts
help: consider writing it as `\u{30b}` for clarity
   |
LL -     dbg!("\̋"); //U+030B
LL +     dbg!("\u{30b}"); //U+030B
   |

error: unknown character escape: `\u{30e}`
##[error]  --> /checkout/tests/ui/parser/unicode-quote.rs:12:12
   |
LL |     dbg!("\̎"); //U+030E
   |            ^ unknown character escape
   |
   = help: \u{30e} is not an ASCII quote, but may look like one in some fonts
help: consider writing it as `\u{30e}` for clarity
   |
LL -     dbg!("\̎"); //U+030E
LL +     dbg!("\u{30e}"); //U+030E
   |

error: unknown character escape: `\u{5f4}`
##[error]  --> /checkout/tests/ui/parser/unicode-quote.rs:14:12
   |
LL |     dbg!("\״"); //U+05F4
   |            ^ unknown character escape
   |
   = help: \u{5f4} is not an ASCII quote, but may look like one in some fonts
help: consider writing it as `\u{5f4}` for clarity
   |
LL -     dbg!("\״"); //U+05F4
LL +     dbg!("\u{5f4}"); //U+05F4
   |

error: unknown character escape: `\u{201c}`
##[error]  --> /checkout/tests/ui/parser/unicode-quote.rs:16:12
   |
LL |     dbg!("\“"); //U+201C
   |            ^ unknown character escape
   |
   = help: \u{201c} is not an ASCII quote, but may look like one in some fonts
help: consider writing it as `\u{201c}` for clarity
   |
LL -     dbg!("\“"); //U+201C
LL +     dbg!("\u{201c}"); //U+201C
   |

error: unknown character escape: `\u{201d}`
##[error]  --> /checkout/tests/ui/parser/unicode-quote.rs:18:12
   |
LL |     dbg!("\”"); //U+201D
   |            ^ unknown character escape
   |
   = help: \u{201d} is not an ASCII quote, but may look like one in some fonts
help: consider writing it as `\u{201d}` for clarity
   |
LL -     dbg!("\”"); //U+201D
LL +     dbg!("\u{201d}"); //U+201D
   |

error: aborting due to 8 previous errors
------------------------------------------