Resolves#10187
<details>
<summary>Old PR description; accurate through commit e86dd7d; probably
best to leave this fold closed</summary>
## Description of change
In the case of a printf-style format string with only one %-placeholder
and a variable at right (e.g. `"%s" % var`):
* The new behavior attempts to dereference the variable and then match
on the bound expression to distinguish between a 1-tuple (fix), n-tuple
(bug 🐛), or a non-tuple (fix). Dereferencing is via
`analyze::typing::find_binding_value`.
* If the variable cannot be dereferenced, then the type-analysis routine
is called to distinguish only tuple (no-fix) or non-tuple (fix). Type
analysis is via `analyze::typing::is_tuple`.
* If any of the above fails, the rule still fires, but no fix is
offered.
## Alternatives
* If the reviewers think that singling out the 1-tuple case is too
complicated, I will remove that.
* The ecosystem results show that no new fixes are detected. So I could
probably delete all the variable dereferencing code and code that tries
to generate fixes, tbh.
## Changes to existing behavior
**All the previous rule-firings and fixes are unchanged except for** the
"false negatives" in
`crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_1.py`. Those
previous "false negatives" are now true positives and so I moved them to
`crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_0.py`.
<details>
<summary>Existing false negatives that are now true positives</summary>
```
crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_0.py:134:1: UP031 Use format specifiers instead of percent format
|
133 | # UP031 (no longer false negatives)
134 | 'Hello %s' % bar
| ^^^^^^^^^^^^^^^^ UP031
135 |
136 | 'Hello %s' % bar.baz
|
= help: Replace with format specifiers
crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_0.py:136:1: UP031 Use format specifiers instead of percent format
|
134 | 'Hello %s' % bar
135 |
136 | 'Hello %s' % bar.baz
| ^^^^^^^^^^^^^^^^^^^^ UP031
137 |
138 | 'Hello %s' % bar['bop']
|
= help: Replace with format specifiers
crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_0.py:138:1: UP031 Use format specifiers instead of percent format
|
136 | 'Hello %s' % bar.baz
137 |
138 | 'Hello %s' % bar['bop']
| ^^^^^^^^^^^^^^^^^^^^^^^ UP031
|
= help: Replace with format specifiers
```
One of them newly offers a fix.
```
# UP031 (no longer false negatives)
-'Hello %s' % bar
+'Hello {}'.format(bar)
```
This fix occurs because the new code dereferences `bar` to where it was
defined earlier in the file as a non-tuple:
```python
bar = {"bar": y}
```
---
</details>
## Behavior requiring new tests
Additionally, we now handle a few cases that we didn't previously test.
These cases are when a string has a single %-placeholder and the
righthand operand to the modulo operator is a variable **which can be
dereferenced.** One of those was shown in the previous section (the
"dereference non-tuple" case).
<details>
<summary>New cases handled</summary>
```
crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_0.py:126:1: UP031 [*] Use format specifiers instead of percent format
|
125 | t1 = (x,)
126 | "%s" % t1
| ^^^^^^^^^ UP031
127 | # UP031: deref t1 to 1-tuple, offer fix
|
= help: Replace with format specifiers
crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_0.py:130:1: UP031 Use format specifiers instead of percent format
|
129 | t2 = (x,y)
130 | "%s" % t2
| ^^^^^^^^^ UP031
131 | # UP031: deref t2 to n-tuple, this is a bug
|
= help: Replace with format specifiers
```
One of these offers a fix.
```
t1 = (x,)
-"%s" % t1
+"{}".format(t1[0])
# UP031: deref t1 to 1-tuple, offer fix
```
The other doesn't offer a fix because it's a bug.
---
</details>
---
</details>
## Changes to existing behavior
In the case of a string with a single %-placeholder and a single
ambiguous righthand argument to the modulo operator, (e.g. `"%s" % var`)
the rule now fires and offers a fix. We explain about this in the "fix
safety" section of the updated documentation.
## Documentation changes
I swapped the order of the "known problems" and the "examples" sections
so that the examples which describe the rule are first, before the
exceptions to the rule are described. I also tweaked the language to be
more explicit, as I had trouble understanding the documentation at
first. The "known problems" section is now "fix safety" but the content
is largely similar.
The diff of the documentation changes looks a little difficult unless
you look at the individual commits.
