Fix rustfmt giving up on formatting when multi-line strings exceed max_width

[thomasqueirozb]

Summary

Fix rustfmt giving up on formatting when multi-line strings exceed max_width.

rustfmt would refuse to format code containing multi-line string literals that exceed max_width. The problem occurred because filtered_str_fits() in src/utils.rs would reject valid formatting attempts when string content (which cannot be shortened) exceeded width limits.

Added test fixtures for both issues:

• tests/source/issue-6769.rs / tests/target/issue-6769.rs - byte string literals in array tuples
• tests/source/issue-6778.rs / tests/target/issue-6778.rs - multi-line strings in struct fields

References

• Fixes bug: rustfmt silently breaks on this sample #6769
• Fixes rustfmt gives up if second part of multiline string is >= 100 chars long #6778 (duplicate)

---

Note that reverting commit fa5b1e4 "breaks" a bunch of fixtures for the better, allowing formatting of rust code even after very long strings.

Example:

Mismatch at tests/source/string_punctuation.rs:5:
         "ThisIsAReallyLongStringWithNoSpaces.It_should_prefer_to_break_onpunctuation:\
          Likethisssssssssssss"
     );
-    format!("{}__{}__{}ItShouldOnlyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyNoticeSemicolonsPeriodsColonsAndCommasAndResortToMid-CharBreaksAfterPunctuation{}{}",x,y,z,a,b);
+    format!(
+        "{}__{}__{}ItShouldOnlyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyNoticeSemicolonsPeriodsColonsAndCommasAndResortToMid-CharBreaksAfterPunctuation{}{}",
+        x, y, z, a, b
+    );

and

Mismatch at tests/source/issue-2179/one.rs:27:
         } else {
             for package in &opts.package {
                 if let None = ws.members().find(|x| x.name() == package) {
-                    warn!("cargo - couldn't find member package `{}` specified in `analyze_package` configuration", package);
+                    warn!(
+                        "cargo - couldn't find member package `{}` specified in `analyze_package` configuration",
+                        package
+                    );
                 }
             }
         }

It does, however, break tests/source/issue-1210/c.rs in a not so desirable way:

Mismatch at tests/source/issue-1210/c.rs:1:
 // rustfmt-format_strings: true
 // rustfmt-max_width: 50

-const foo: String =
-    "trailing_spaces!!
+const foo: String = "trailing_spaces!!
                 keep them! Amet neque. Praesent \
-     rhoncus eros non velit.";
+                     rhoncus eros non velit.";

I'd encourage someone to take a look at this and follow up if possible.

---

Note: This PR was generated with assistance from an LLM (Claude). While I'm happy to address minor feedback, I may not have bandwidth to drive major revisions or extensive back-and-forth due to time constraints. If this approach looks promising but needs significant changes, feel free to take over the branch or use this as a starting point. I mainly wanted to get a potential fix out there since these issues were affecting my workflow but since the issue was actually fixed I thought it was worth sharing these changes.

[ytmimi]

Just ran the Diff-Check, and its already started finding breaking formatting changes. There's no way we can move forward with the PR as is.

[ytmimi]

These changes would need to be style_edition gated, but even then I'm not sure if the formatting changes that they introduce are worth it.

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[thomasqueirozb]

This is unfortunate. All checks passed locally so I assumed this was fine but I wasn't aware of diff-check. Thanks for looking into this. I think I'll close this PR then, but someone can probably use this as a starting point.