Summary
--

This is a preparatory PR in support of #19919. It moves our `Diff` rendering
code from `ruff_linter` to `ruff_db`, where we have direct access to the
`DiagnosticStylesheet` used by our other diagnostic rendering code. As shown by
the tests, this shouldn't cause any visible changes. The colors aren't exactly
the same, as I note in a TODO comment, but I don't think there's any existing
way to see those, even in tests.

The `Diff` implementation is mostly unchanged. I just switched from a
Ruff-specific `SourceFile` to a `DiagnosticSource` (removing an
`expect_ruff_source_file` call) and updated the `LineStyle` struct and other
styling calls to use `fmt_styled` and our existing stylesheet.

In support of these changes, I added three styles to our stylesheet: `insertion`
and `deletion` for the corresponding diff operations, and `underline`, which
apparently we _can_ use, as I hoped on Discord. This isn't supported in all
terminals, though. It worked in ghostty but not in st for me.

I moved the `calculate_print_width` function from the now-deleted `diff.rs` to a
method on `OneIndexed`, where it was available everywhere we needed it. I'm not
sure if that's desirable, or if my other changes to the function are
either (using `ilog10` instead of a loop). This does make it `const` and
slightly simplifies things in my opinion, but I'm happy to revert it if
preferred.

I also inlined a version of `show_nonprinting` from the `ShowNonprinting` trait
in `ruff_linter`:

https://github.com/astral-sh/ruff/blob/f4be05a83be770c8c9781088508275170396b411/crates/ruff_linter/src/text_helpers.rs#L3-L5

This trait is now only used in `source_kind.rs`, so I'm not sure it's worth
having the trait or the macro-generated implementation (which is only called
once). This is obviously closely related to our unprintable character handling
in diagnostic rendering, but the usage seems different enough not to try to
combine them.

https://github.com/astral-sh/ruff/blob/f4be05a83be770c8c9781088508275170396b411/crates/ruff_db/src/diagnostic/render.rs#L990-L998

We could also move the trait to another crate where we can use it in `ruff_db`
instead of inlining here, of course.

Finally, this PR makes `TextEmitter` a very thin wrapper around a
`DisplayDiagnosticsConfig`. It's still used in a few places, though, unlike the
other emitters we've replaced, so I figured it was worth keeping around. It's a
pretty nice API for setting all of the options on the config and then passing
that along to a `DisplayDiagnostics`.

Test Plan
--

Existing snapshot tests with diffs

Summary
--

This is a preparatory PR in support of #19919. It moves our `Diff` rendering
code from `ruff_linter` to `ruff_db`, where we have direct access to the
`DiagnosticStylesheet` used by our other diagnostic rendering code. As shown by
the tests, this shouldn't cause any visible changes. The colors aren't exactly
the same, as I note in a TODO comment, but I don't think there's any existing
way to see those, even in tests.

The `Diff` implementation is mostly unchanged. I just switched from a
Ruff-specific `SourceFile` to a `DiagnosticSource` (removing an
`expect_ruff_source_file` call) and updated the `LineStyle` struct and other
styling calls to use `fmt_styled` and our existing stylesheet.

In support of these changes, I added three styles to our stylesheet: `insertion`
and `deletion` for the corresponding diff operations, and `underline`, which
apparently we _can_ use, as I hoped on Discord. This isn't supported in all
terminals, though. It worked in ghostty but not in st for me.

I moved the `calculate_print_width` function from the now-deleted `diff.rs` to a
method on `OneIndexed`, where it was available everywhere we needed it. I'm not
sure if that's desirable, or if my other changes to the function are
either (using `ilog10` instead of a loop). This does make it `const` and
slightly simplifies things in my opinion, but I'm happy to revert it if
preferred.

I also inlined a version of `show_nonprinting` from the `ShowNonprinting` trait
in `ruff_linter`:

https://github.com/astral-sh/ruff/blob/f4be05a83be770c8c9781088508275170396b411/crates/ruff_linter/src/text_helpers.rs#L3-L5

This trait is now only used in `source_kind.rs`, so I'm not sure it's worth
having the trait or the macro-generated implementation (which is only called
once). This is obviously closely related to our unprintable character handling
in diagnostic rendering, but the usage seems different enough not to try to
combine them.

https://github.com/astral-sh/ruff/blob/f4be05a83be770c8c9781088508275170396b411/crates/ruff_db/src/diagnostic/render.rs#L990-L998

We could also move the trait to another crate where we can use it in `ruff_db`
instead of inlining here, of course.

Finally, this PR makes `TextEmitter` a very thin wrapper around a
`DisplayDiagnosticsConfig`. It's still used in a few places, though, unlike the
other emitters we've replaced, so I figured it was worth keeping around. It's a
pretty nice API for setting all of the options on the config and then passing
that along to a `DisplayDiagnostics`.

Test Plan
--

Existing snapshot tests with diffs

Summary
--

This is a preparatory PR in support of #19919. It moves our `Diff`
rendering code from `ruff_linter` to `ruff_db`, where we have direct
access to the `DiagnosticStylesheet` used by our other diagnostic
rendering code. As shown by the tests, this shouldn't cause any visible
changes. The colors aren't exactly the same, as I note in a TODO
comment, but I don't think there's any existing way to see those, even
in tests.

The `Diff` implementation is mostly unchanged. I just switched from a
Ruff-specific `SourceFile` to a `DiagnosticSource` (removing an
`expect_ruff_source_file` call) and updated the `LineStyle` struct and
other styling calls to use `fmt_styled` and our existing stylesheet.

In support of these changes, I added three styles to our stylesheet:
`insertion` and `deletion` for the corresponding diff operations, and
`underline`, which apparently we _can_ use, as I hoped on Discord. This
isn't supported in all terminals, though. It worked in ghostty but not
in st for me.

I moved the `calculate_print_width` function from the now-deleted
`diff.rs` to a method on `OneIndexed`, where it was available everywhere
we needed it. I'm not sure if that's desirable, or if my other changes
to the function are either (using `ilog10` instead of a loop). This does
make it `const` and slightly simplifies things in my opinion, but I'm
happy to revert it if preferred.

I also inlined a version of `show_nonprinting` from the
`ShowNonprinting` trait in `ruff_linter`:


https://github.com/astral-sh/ruff/blob/f4be05a83be770c8c9781088508275170396b411/crates/ruff_linter/src/text_helpers.rs#L3-L5

This trait is now only used in `source_kind.rs`, so I'm not sure it's
worth having the trait or the macro-generated implementation (which is
only called once). This is obviously closely related to our unprintable
character handling in diagnostic rendering, but the usage seems
different enough not to try to combine them.


https://github.com/astral-sh/ruff/blob/f4be05a83be770c8c9781088508275170396b411/crates/ruff_db/src/diagnostic/render.rs#L990-L998

We could also move the trait to another crate where we can use it in
`ruff_db` instead of inlining here, of course.

Finally, this PR makes `TextEmitter` a very thin wrapper around a
`DisplayDiagnosticsConfig`. It's still used in a few places, though,
unlike the other emitters we've replaced, so I figured it was worth
keeping around. It's a pretty nice API for setting all of the options on
the config and then passing that along to a `DisplayDiagnostics`.

Test Plan
--

Existing snapshot tests with diffs

## Summary

I spun this off from #19919 to separate the rendering code change and
snapshot updates from the (much smaller) changes to expose this in the
CLI. I grouped all of the `ruff_linter` snapshot changes in the final
commit in an effort to make this easier to review. The code changes are
in [this
range](https://github.com/astral-sh/ruff/pull/20101/files/619395eb417d2030e31fbb0e487a72dac58902db).

I went through all of the snapshots, albeit fairly quickly, and they all
looked correct to me. In the last few commits I was trying to resolve an
existing issue in the alignment of the line number separator:


https://github.com/astral-sh/ruff/blob/73720c73be981df6f71ce837a67ca1167da0265e/crates/ruff_linter/src/rules/flake8_comprehensions/snapshots/ruff_linter__rules__flake8_comprehensions__tests__C409_C409.py.snap#L87-L89

In the snapshot above on `main`, you can see that a double-digit line
number at the end of the context lines for a snippet was causing a
misalignment with the other separators. That's now resolved. The one
downside is that this can lead to a mismatch with the diagnostic above:

```
C409 [*] Unnecessary list literal passed to `tuple()` (rewrite as a tuple literal)
 --> C409.py:4:6
  |
2 |   t2 = tuple([1, 2])
3 |   t3 = tuple((1, 2))
4 |   t4 = tuple([
  |  ______^
5 | |     1,
6 | |     2
7 | | ])
  | |__^
8 |   t5 = tuple(
9 |       (1, 2)
  |
help: Rewrite as a tuple literal
1  | t1 = tuple([])
2  | t2 = tuple([1, 2])
3  | t3 = tuple((1, 2))
   - t4 = tuple([
4  + t4 = (
5  |     1,
6  |     2
   - ])
7  + )
8  | t5 = tuple(
9  |     (1, 2)
10 | )
note: This is an unsafe fix and may remove comments or change runtime behavior
```

But I don't think we can avoid that without really reworking this
rendering to make the diagnostic and diff rendering aware of each other.
Anyway, this should only happen in relatively rare cases where the
diagnostic is near a digit boundary and also near a context boundary.
Most of our diagnostics line up nicely.

Another potential downside of the new rendering format is its handling
of long stretches of `+` or `-` lines:

```
help: Replace with `Literal[...] | None`
21 |     ...
22 |
23 |
   - def func6(arg1: Literal[
   -     "hello",
   -     None  # Comment 1
   -     , "world"
   -     ]):
24 + def func6(arg1: Literal["hello", "world"] | None):
25 |     ...
26 |
27 |
note: This is an unsafe fix and may remove comments or change runtime behavior
```

To me it just seems a little hard to tell what's going on with just a
long streak of `-`-prefixed lines. I saw an even more exaggerated
example at some point, but I think this is also fairly rare. Most of the
snapshots seem more like the examples we looked at on Discord with
plenty of `|` lines and pairs of `+` and `-` lines.

## Test Plan

Existing tests plus one new test in `ruff_db` to isolate a line
separator alignment issue

Summary
--

This is a preparatory PR in support of astral-sh#19919. It moves our `Diff`
rendering code from `ruff_linter` to `ruff_db`, where we have direct
access to the `DiagnosticStylesheet` used by our other diagnostic
rendering code. As shown by the tests, this shouldn't cause any visible
changes. The colors aren't exactly the same, as I note in a TODO
comment, but I don't think there's any existing way to see those, even
in tests.

The `Diff` implementation is mostly unchanged. I just switched from a
Ruff-specific `SourceFile` to a `DiagnosticSource` (removing an
`expect_ruff_source_file` call) and updated the `LineStyle` struct and
other styling calls to use `fmt_styled` and our existing stylesheet.

In support of these changes, I added three styles to our stylesheet:
`insertion` and `deletion` for the corresponding diff operations, and
`underline`, which apparently we _can_ use, as I hoped on Discord. This
isn't supported in all terminals, though. It worked in ghostty but not
in st for me.

I moved the `calculate_print_width` function from the now-deleted
`diff.rs` to a method on `OneIndexed`, where it was available everywhere
we needed it. I'm not sure if that's desirable, or if my other changes
to the function are either (using `ilog10` instead of a loop). This does
make it `const` and slightly simplifies things in my opinion, but I'm
happy to revert it if preferred.

I also inlined a version of `show_nonprinting` from the
`ShowNonprinting` trait in `ruff_linter`:


https://github.com/astral-sh/ruff/blob/f4be05a83be770c8c9781088508275170396b411/crates/ruff_linter/src/text_helpers.rs#L3-L5

This trait is now only used in `source_kind.rs`, so I'm not sure it's
worth having the trait or the macro-generated implementation (which is
only called once). This is obviously closely related to our unprintable
character handling in diagnostic rendering, but the usage seems
different enough not to try to combine them.


https://github.com/astral-sh/ruff/blob/f4be05a83be770c8c9781088508275170396b411/crates/ruff_db/src/diagnostic/render.rs#L990-L998

We could also move the trait to another crate where we can use it in
`ruff_db` instead of inlining here, of course.

Finally, this PR makes `TextEmitter` a very thin wrapper around a
`DisplayDiagnosticsConfig`. It's still used in a few places, though,
unlike the other emitters we've replaced, so I figured it was worth
keeping around. It's a pretty nice API for setting all of the options on
the config and then passing that along to a `DisplayDiagnostics`.

Test Plan
--

Existing snapshot tests with diffs

## Summary

I spun this off from astral-sh#19919 to separate the rendering code change and
snapshot updates from the (much smaller) changes to expose this in the
CLI. I grouped all of the `ruff_linter` snapshot changes in the final
commit in an effort to make this easier to review. The code changes are
in [this
range](https://github.com/astral-sh/ruff/pull/20101/files/619395eb417d2030e31fbb0e487a72dac58902db).

I went through all of the snapshots, albeit fairly quickly, and they all
looked correct to me. In the last few commits I was trying to resolve an
existing issue in the alignment of the line number separator:


https://github.com/astral-sh/ruff/blob/73720c73be981df6f71ce837a67ca1167da0265e/crates/ruff_linter/src/rules/flake8_comprehensions/snapshots/ruff_linter__rules__flake8_comprehensions__tests__C409_C409.py.snap#L87-L89

In the snapshot above on `main`, you can see that a double-digit line
number at the end of the context lines for a snippet was causing a
misalignment with the other separators. That's now resolved. The one
downside is that this can lead to a mismatch with the diagnostic above:

```
C409 [*] Unnecessary list literal passed to `tuple()` (rewrite as a tuple literal)
 --> C409.py:4:6
  |
2 |   t2 = tuple([1, 2])
3 |   t3 = tuple((1, 2))
4 |   t4 = tuple([
  |  ______^
5 | |     1,
6 | |     2
7 | | ])
  | |__^
8 |   t5 = tuple(
9 |       (1, 2)
  |
help: Rewrite as a tuple literal
1  | t1 = tuple([])
2  | t2 = tuple([1, 2])
3  | t3 = tuple((1, 2))
   - t4 = tuple([
4  + t4 = (
5  |     1,
6  |     2
   - ])
7  + )
8  | t5 = tuple(
9  |     (1, 2)
10 | )
note: This is an unsafe fix and may remove comments or change runtime behavior
```

But I don't think we can avoid that without really reworking this
rendering to make the diagnostic and diff rendering aware of each other.
Anyway, this should only happen in relatively rare cases where the
diagnostic is near a digit boundary and also near a context boundary.
Most of our diagnostics line up nicely.

Another potential downside of the new rendering format is its handling
of long stretches of `+` or `-` lines:

```
help: Replace with `Literal[...] | None`
21 |     ...
22 |
23 |
   - def func6(arg1: Literal[
   -     "hello",
   -     None  # Comment 1
   -     , "world"
   -     ]):
24 + def func6(arg1: Literal["hello", "world"] | None):
25 |     ...
26 |
27 |
note: This is an unsafe fix and may remove comments or change runtime behavior
```

To me it just seems a little hard to tell what's going on with just a
long streak of `-`-prefixed lines. I saw an even more exaggerated
example at some point, but I think this is also fairly rare. Most of the
snapshots seem more like the examples we looked at on Discord with
plenty of `|` lines and pairs of `+` and `-` lines.

## Test Plan

Existing tests plus one new test in `ruff_db` to isolate a line
separator alignment issue

## Summary

This PR fixes astral-sh#7352 by exposing the `show_fix_diff` option used in our
snapshot tests in the CLI. As the issue suggests, we plan to make this
the default output format in the future, so this is added to the `full`
output format in preview for now.

This turned out to be pretty straightforward. I just used our existing
`Applicability` settings to determine whether or not to print the diff.

The snapshot differences are because we now set
`Applicability::DisplayOnly` for our snapshot tests. This
`Applicability` is also used to determine whether or not the fix icon
(`[*]`) is rendered, so this is now shown for display-only fixes in our
snapshots. This was already the case previously, but we were only
setting `Applicability::Unsafe` in these tests and ignoring the
`Applicability` when rendering fix diffs. CLI users can't enable
display-only fixes, so this is only a test change for now, but this
should work smoothly if we decide to expose a `--display-only-fixes`
flag or similar in the future.

I also deleted the `PrinterFlags::SHOW_FIX_DIFF` flag. This was
completely unused before, and it seemed less confusing just to delete it
than to enable it in the right place and check it along with the
`OutputFormat` and `preview`.

## Test Plan

I only added one CLI test for now. I'm kind of assuming that we have
decent coverage of the cases where this shouldn't be firing, especially
the `output_format` CLI test, which shows that this definitely doesn't
affect non-preview `full` output. I'm happy to add more tests with
different combinations of options, if we're worried about any in
particular. I did try `--diff` and `--preview` and a few other
combinations manually.

And here's a screenshot using our trusty UP049 example from the design
discussion confirming that all the colors and other formatting still
look as expected:

<img width="786" height="629" alt="image"
src="https://github.com/user-attachments/assets/94e408bc-af7b-4573-b546-a5ceac2620f2"
/>

And one with an unsafe fix to see the footer:

<img width="782" height="367" alt="image"
src="https://github.com/user-attachments/assets/bbb29e47-310b-4293-b2c2-cc7aee3baff4"
/>


## Related issues and PR
- astral-sh#7352
- astral-sh#12595
- astral-sh#12598
- astral-sh#12599
- astral-sh#12600

I think we could probably close all of these issues now. I think we've
either resolved or avoided most of them, and if we encounter them again
with the new output format, it would probably make sense to open new
ones anyway.