Repository: sqlfluff/sqlfluff

Issue:
Jinja slice error
### Search before asking

- [X] I searched the [issues](https://github.com/sqlfluff/sqlfluff/issues) and found no similar issues.


### What Happened

I encountered a parsing error while linting a SQL. I simplified the SQL as follows:
```
{% set max_partitions = 5 %}
{% for i in range(max_partitions) %}{% set useless1 = 0 %}{% set useless2 = 0 %}{% set useless3 = 0 %}{% set useless4 = 0 %}{% set useless5 = 0 %}
    SELECT * FROM my_table
    WHERE partition_key = {{ i }};
{% endfor %}
```
There are many sets following the for element.

### Expected Behaviour

jinja can be parsed normally

### Observed Behaviour

WARNING    Length of templated file mismatch with final slice: 286 != 58. 


### How to reproduce

sqlfluff lint test.sql --dialect ansi

### Dialect

ansi

### Version

python 3.9
sqlfluff 3.0.7

### Configuration

default

### Are you willing to work on and submit a PR to address the issue?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/sqlfluff/sqlfluff/blob/main/CODE_OF_CONDUCT.md)


Discussion:
github-actions[bot]: # Coverage Results ✅
```
Name    Stmts   Miss  Cover   Missing
-------------------------------------
TOTAL   18099      0   100%

228 files skipped due to complete coverage.

```


Patch:
diff --git a/src/sqlfluff/core/templaters/slicers/tracer.py b/src/sqlfluff/core/templaters/slicers/tracer.py
index f1ad0bf69..60caec111 100644
--- a/src/sqlfluff/core/templaters/slicers/tracer.py
+++ b/src/sqlfluff/core/templaters/slicers/tracer.py
@@ -192,7 +192,22 @@ class JinjaTracer:
             # Save all the candidates for each step so we can return them later.
             step_candidates[self.program_counter] = candidates
             # Step forward to the best step found.
-            self.program_counter = candidates[0]
+
+            # https://github.com/sqlfluff/sqlfluff/issues/6121
+            next_indices = self.raw_slice_info[current_raw_slice].next_slice_indices
+            if (
+                current_raw_slice.tag == "endfor"  # noqa
+                # Elements of inside_set_macro_or_call have empty next_slice_indices
+                and next_indices
+                # The next_slice_indices[0] of the 'endfor' is the first element of the
+                # loop.If the target is within the current loop, the program_counter
+                # should move to the first element of this loop. Otherwise, it will
+                # exit this loop and lose the content  rendered in the next iteration
+                and next_indices[0] <= target_slice_idx < self.program_counter
+            ):
+                self.program_counter = next_indices[0]
+            else:
+                self.program_counter = candidates[0]
 
         # Return the candidates at each step.
         return step_candidates


Test patch:
diff --git a/test/core/templaters/jinja_test.py b/test/core/templaters/jinja_test.py
index 649702b36..9fccefce2 100644
--- a/test/core/templaters/jinja_test.py
+++ b/test/core/templaters/jinja_test.py
@@ -1607,6 +1607,32 @@ FROM {{ j }}{{ self.table_name() }}
             ],
             DerivedJinjaTemplater,
         ),
+        (
+            # test for issue 6121: The first rendered element
+            # inside the loop is far from the start position of the loop.
+            """
+{% for i in range(2) %}{% set a = 0 %}{% set b = 0 %}{% set c = 0 %}
+SELECT 1;
+{% endfor %}
+""",
+            None,
+            [
+                ("literal", slice(0, 1, None), slice(0, 1, None)),
+                ("block_start", slice(1, 24, None), slice(1, 1, None)),
+                ("templated", slice(24, 39, None), slice(1, 1, None)),
+                ("templated", slice(39, 54, None), slice(1, 1, None)),
+                ("templated", slice(54, 69, None), slice(1, 1, None)),
+                ("literal", slice(69, 80, None), slice(1, 12, None)),
+                ("block_end", slice(80, 92, None), slice(12, 12, None)),
+                ("templated", slice(24, 39, None), slice(12, 12, None)),
+                ("templated", slice(39, 54, None), slice(12, 12, None)),
+                ("templated", slice(54, 69, None), slice(12, 12, None)),
+                ("literal", slice(69, 80, None), slice(12, 23, None)),
+                ("block_end", slice(80, 92, None), slice(23, 23, None)),
+                ("literal", slice(92, 93, None), slice(23, 24, None)),
+            ],
+            JinjaTemplater,
+        ),
     ],
 )
 def test__templater_jinja_slice_file(