Repository: mitsuhiko/minijinja (rust)
Issue: join() filter drops empty entries at start of list which doesn't happen in Python Jinja2
## Description

join() filter drops empty entries at start of list which doesn't happen in Python Jinja2

## Reproduction steps

Template
```
{{ ['1', '', '2', ''] | join(',') }}
{{ ['', '1', '', '2', ''] | join(',') }}
```

MiniJinja output
```
1,,2,
1,,2,
```

Python Jinja2 output
```
1,,2,
,1,,2,
```

Note the leading comma in the second line which is there for Python and missing in MiniJinja.

Additional helpful information:

- Versions of minijinja: 2.9.0, 2.10.2

## What did you expect

I expected MiniJinja and Python Jinja2 to behave the same for this case.
Pull request description: Fix joining of leading undefineds or empty strings
Fixes #791

Patch:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8126d07..d943cc9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,13 @@
 
 All notable changes to MiniJinja are documented here.
 
+## 2.11.0
+
+* Fixed incorrect joining of leading undefineds or empty
+  strings in the `|join` filter.  This was inconsistent with
+  Jinja2 and the filter itself for undefineds in other
+  positions.  #794
+
 ## 2.10.2
 
 * Fixed an issue with the function bounds that caused the
diff --git a/minijinja/src/filters.rs b/minijinja/src/filters.rs
index e2614b1..096e784 100644
--- a/minijinja/src/filters.rs
+++ b/minijinja/src/filters.rs
@@ -409,8 +409,8 @@ mod builtins {
         }));
 
         let mut rv = String::new();
-        for item in iter {
-            if !rv.is_empty() {
+        for (idx, item) in iter.enumerate() {
+            if idx > 0 {
                 rv.push_str(joiner);
             }
             if let Some(s) = item.as_str() {


Test patch:
diff --git a/minijinja/tests/inputs/filters.txt b/minijinja/tests/inputs/filters.txt
index ac72599..087511e 100644
--- a/minijinja/tests/inputs/filters.txt
+++ b/minijinja/tests/inputs/filters.txt
@@ -51,6 +51,7 @@ trim-bird: {{ word|trim("Bd") }}
 join-default: {{ list|join }}
 join-pipe: {{ list|join("|") }}
 join_string: {{ word|join('-') }}
+join-empty-strings: {{ [undefined, "", "blub", "", undefined, 42]|join('|') }}
 default: {{ undefined|default == "" }}
 default-value: {{ undefined|default(42) }}
 default-value-falsy: {{ ""|default("b", true) }}
diff --git a/minijinja/tests/snapshots/test_templates__vm@filters.txt.snap b/minijinja/tests/snapshots/test_templates__vm@filters.txt.snap
index b4194c6..35971d9 100644
--- a/minijinja/tests/snapshots/test_templates__vm@filters.txt.snap
+++ b/minijinja/tests/snapshots/test_templates__vm@filters.txt.snap
@@ -1,6 +1,6 @@
 ---
 source: minijinja/tests/test_templates.rs
-description: "lower: {{ word|lower }}\nupper: {{ word|upper }}\ntitle: {{ word|title }}\ntitle-sentence: {{ \"the bIrd, is The:word\"|title }}\ntitle-three-words: {{ three_words|title }}\ncapitalize: {{ word|capitalize }}\ncapitalize-three-words: {{ three_words|capitalize }}\nreplace: {{ word|replace(\"B\", \"th\") }}\nescape: {{ \"<\"|escape }}\ne: {{ \"<\"|e }}\ndouble-escape: {{ \"<\"|escape|escape }}\nsafe: {{ \"<\"|safe|escape }}\nlist-length: {{ list|length }}\nlist-from-list: {{ list|list }}\nlist-from-map: {{ map|list }}\nlist-from-word: {{ word|list }}\nlist-from-undefined: {{ undefined|list }}\nbool-empty-string: {{ \"\"|bool }}\nbool-non-empty-string: {{ \"hello\"|bool }}\nbool-empty-list: {{ []|bool }}\nbool-non-empty-list: {{ [42]|bool }}\nbool-undefined: {{ undefined|bool }}\nmap-length: {{ map|length }}\nstring-length: {{ word|length }}\nstring-count: {{ word|count }}\nreverse-list: {{ list|reverse }}\nreverse-string: {{ word|reverse }}\ntrim: |{{ word_with_spaces|trim }}|\ntrim-bird: {{ word|trim(\"Bd\") }}\njoin-default: {{ list|join }}\njoin-pipe: {{ list|join(\"|\") }}\njoin_string: {{ word|join('-') }}\ndefault: {{ undefined|default == \"\" }}\ndefault-value: {{ undefined|default(42) }}\ndefault-value-falsy: {{ \"\"|default(\"b\", true) }}\nd: {{ undefined|d == \"\" }}\nd-value: {{ undefined|d(42) }}\nfirst-list: {{ list|first }}\nfirst-word: {{ word|first }}\nfirst-undefined: {{ []|first is undefined }}\nlast-list: {{ list|last }}\nlast-word: {{ word|last }}\nlast-undefined: {{ []|first is undefined }}\nmin: {{ other_list|min }}\nmax: {{ other_list|max }}\nsort: {{ other_list|sort }}\nsort-reverse: {{ other_list|sort(reverse=true) }}\nsort-case-insensitive: {{ [\"B\", \"a\", \"C\", \"z\"]|sort }}\nsort-case-sensitive: {{ [\"B\", \"a\", \"C\", \"z\"]|sort(case_sensitive=true) }}\nsort-case-insensitive-mixed: {{ [0, 1, \"true\", \"false\", \"True\", \"False\", true, false]|sort }}\nsort-case-sensitive-mixed: {{ [0, 1, \"true\", \"false\", \"True\", \"False\", true, false]|sort(case_sensitive=true) }}\nsort-attribute {{ objects|sort(attribute=\"name\") }}\nd: {{ undefined|d == \"\" }}\njson: {{ map|tojson }}\njson-pretty: {{ map|tojson(true) }}\njson-scary-html: {{ scary_html|tojson }}\nurlencode: {{ \"hello world/foo-bar_baz.txt\"|urlencode }}\nurlencode-kv: {{ dict(a=\"x y\", b=2, c=3, d=None)|urlencode }}\nbatch: {{ range(10)|batch(3) }}\nbatch-fill: {{ range(10)|batch(3, '-') }}\nslice: {{ range(10)|slice(3) }}\nslice-fill: {{ range(10)|slice(3, '-') }}\nitems: {{ dict(a=1)|items }}\nindent: {{ \"foo\\nbar\\nbaz\"|indent(2)|tojson }}\nindent-first-line: {{ \"foo\\nbar\\nbaz\"|indent(2, true)|tojson }}\nint-abs: {{ -42|abs }}\nfloat-abs: {{ -42.5|abs }}\nint-round: {{ 42|round }}\nfloat-round: {{ 42.5|round }}\nfloat-round-prec2: {{ 42.512345|round(2) }}\nselect-odd: {{ [1, 2, 3, 4, 5, 6]|select(\"odd\") }}\nselect-truthy: {{ [undefined, null, 0, 42, 23, \"\", \"aha\"]|select }}\nreject-truthy: {{ [undefined, null, 0, 42, 23, \"\", \"aha\"]|reject }}\nreject-odd: {{ [1, 2, 3, 4, 5, 6]|reject(\"odd\") }}\nselect-attr: {{ [dict(active=true, key=1), dict(active=false, key=2)]|selectattr(\"active\") }}\nreject-attr: {{ [dict(active=true, key=1), dict(active=false, key=2)]|rejectattr(\"active\") }}\nselect-attr: {{ [dict(active=true, key=1), dict(active=false, key=2)]|selectattr(\"key\", \"even\") }}\nreject-attr: {{ [dict(active=true, key=1), dict(active=false, key=2)]|rejectattr(\"key\", \"even\") }}\nmap-maps: {{ [-1, -2, 3, 4, -5]|map(\"abs\") }}\nmap-attr: {{ [dict(a=1), dict(a=2), {}]|map(attribute='a', default=None) }}\nmap-attr-undefined: {{ [dict(a=1), dict(a=2), {}]|map(attribute='a', default=definitely_undefined) }}\nmap-attr-deep: {{ [dict(a=[1]), dict(a=[2]), dict(a=[])]|map(attribute='a.0', default=None) }}\nmap-attr-int: {{ [[1], [1, 2]]|map(attribute=1, default=999) }}\nattr-filter: {{ map|attr(\"a\") }}\nunique-filter: {{ [1, 1, 1, 4, 3, 0, 0, 5]|unique }}\nunique-filter-ci: {{ [\"a\", \"A\", \"b\", \"c\", \"b\", \"D\", \"d\"]|unique }}\nunique-filter-cs: {{ [\"a\", \"A\", \"b\", \"c\", \"b\", \"D\", \"d\"]|unique(case_sensitive=true) }}\nunique-attr-filter: {{ [{'x': 1}, {'x': 1, 'y': 2}, {'x': 2}]|unique }}\npprint-filter: {{ objects|pprint }}\nint-filter: {{ true|int }}, {{ \"42\"|int }}, {{ \"-23\"|int }}, {{ 42.0|int }}, {{ 42.42|int }}, {{ \"42.42\"|int }}\nfloat-filter: {{ true|float }}, {{ \"42\"|float }}, {{ \"-23.5\"|float }}, {{ 42.5|float }}\nsplit: {{ three_words|split|list }}\nsplit-at-and: {{ three_words|split(\" and \")|list }}\nsplit-n-ws: {{ three_words|split(none, 1)|list }}\nsplit-n-d: {{ three_words|split(\"d\", 1)|list }}\nsplit-n-ws-filter-empty: {{ \"  foo    bar baz  \"|split(none, 1)|list }}\nsum: {{ range(10)|sum }}\nsum-empty: {{ []|sum }}\nsum-float: {{ [0.5, 1.0]|sum }}\nlines: {{ \"foo\\nbar\\r\\nbaz\"|lines }}\nstring: {{ [1|string, 2|string] }}"
+description: "lower: {{ word|lower }}\nupper: {{ word|upper }}\ntitle: {{ word|title }}\ntitle-sentence: {{ \"the bIrd, is The:word\"|title }}\ntitle-three-words: {{ three_words|title }}\ncapitalize: {{ word|capitalize }}\ncapitalize-three-words: {{ three_words|capitalize }}\nreplace: {{ word|replace(\"B\", \"th\") }}\nescape: {{ \"<\"|escape }}\ne: {{ \"<\"|e }}\ndouble-escape: {{ \"<\"|escape|escape }}\nsafe: {{ \"<\"|safe|escape }}\nlist-length: {{ list|length }}\nlist-from-list: {{ list|list }}\nlist-from-map: {{ map|list }}\nlist-from-word: {{ word|list }}\nlist-from-undefined: {{ undefined|list }}\nbool-empty-string: {{ \"\"|bool }}\nbool-non-empty-string: {{ \"hello\"|bool }}\nbool-empty-list: {{ []|bool }}\nbool-non-empty-list: {{ [42]|bool }}\nbool-undefined: {{ undefined|bool }}\nmap-length: {{ map|length }}\nstring-length: {{ word|length }}\nstring-count: {{ word|count }}\nreverse-list: {{ list|reverse }}\nreverse-string: {{ word|reverse }}\ntrim: |{{ word_with_spaces|trim }}|\ntrim-bird: {{ word|trim(\"Bd\") }}\njoin-default: {{ list|join }}\njoin-pipe: {{ list|join(\"|\") }}\njoin_string: {{ word|join('-') }}\njoin-empty-strings: {{ [undefined, \"\", \"blub\", \"\", undefined, 42]|join('|') }}\ndefault: {{ undefined|default == \"\" }}\ndefault-value: {{ undefined|default(42) }}\ndefault-value-falsy: {{ \"\"|default(\"b\", true) }}\nd: {{ undefined|d == \"\" }}\nd-value: {{ undefined|d(42) }}\nfirst-list: {{ list|first }}\nfirst-word: {{ word|first }}\nfirst-undefined: {{ []|first is undefined }}\nlast-list: {{ list|last }}\nlast-word: {{ word|last }}\nlast-undefined: {{ []|first is undefined }}\nmin: {{ other_list|min }}\nmax: {{ other_list|max }}\nsort: {{ other_list|sort }}\nsort-reverse: {{ other_list|sort(reverse=true) }}\nsort-case-insensitive: {{ [\"B\", \"a\", \"C\", \"z\"]|sort }}\nsort-case-sensitive: {{ [\"B\", \"a\", \"C\", \"z\"]|sort(case_sensitive=true) }}\nsort-case-insensitive-mixed: {{ [0, 1, \"true\", \"false\", \"True\", \"False\", true, false]|sort }}\nsort-case-sensitive-mixed: {{ [0, 1, \"true\", \"false\", \"True\", \"False\", true, false]|sort(case_sensitive=true) }}\nsort-attribute {{ objects|sort(attribute=\"name\") }}\nd: {{ undefined|d == \"\" }}\njson: {{ map|tojson }}\njson-pretty: {{ map|tojson(true) }}\njson-scary-html: {{ scary_html|tojson }}\nurlencode: {{ \"hello world/foo-bar_baz.txt\"|urlencode }}\nurlencode-kv: {{ dict(a=\"x y\", b=2, c=3, d=None)|urlencode }}\nbatch: {{ range(10)|batch(3) }}\nbatch-fill: {{ range(10)|batch(3, '-') }}\nslice: {{ range(10)|slice(3) }}\nslice-fill: {{ range(10)|slice(3, '-') }}\nitems: {{ dict(a=1)|items }}\nindent: {{ \"foo\\nbar\\nbaz\"|indent(2)|tojson }}\nindent-first-line: {{ \"foo\\nbar\\nbaz\"|indent(2, true)|tojson }}\nint-abs: {{ -42|abs }}\nfloat-abs: {{ -42.5|abs }}\nint-round: {{ 42|round }}\nfloat-round: {{ 42.5|round }}\nfloat-round-prec2: {{ 42.512345|round(2) }}\nselect-odd: {{ [1, 2, 3, 4, 5, 6]|select(\"odd\") }}\nselect-truthy: {{ [undefined, null, 0, 42, 23, \"\", \"aha\"]|select }}\nreject-truthy: {{ [undefined, null, 0, 42, 23, \"\", \"aha\"]|reject }}\nreject-odd: {{ [1, 2, 3, 4, 5, 6]|reject(\"odd\") }}\nselect-attr: {{ [dict(active=true, key=1), dict(active=false, key=2)]|selectattr(\"active\") }}\nreject-attr: {{ [dict(active=true, key=1), dict(active=false, key=2)]|rejectattr(\"active\") }}\nselect-attr: {{ [dict(active=true, key=1), dict(active=false, key=2)]|selectattr(\"key\", \"even\") }}\nreject-attr: {{ [dict(active=true, key=1), dict(active=false, key=2)]|rejectattr(\"key\", \"even\") }}\nmap-maps: {{ [-1, -2, 3, 4, -5]|map(\"abs\") }}\nmap-attr: {{ [dict(a=1), dict(a=2), {}]|map(attribute='a', default=None) }}\nmap-attr-undefined: {{ [dict(a=1), dict(a=2), {}]|map(attribute='a', default=definitely_undefined) }}\nmap-attr-deep: {{ [dict(a=[1]), dict(a=[2]), dict(a=[])]|map(attribute='a.0', default=None) }}\nmap-attr-int: {{ [[1], [1, 2]]|map(attribute=1, default=999) }}\nattr-filter: {{ map|attr(\"a\") }}\nunique-filter: {{ [1, 1, 1, 4, 3, 0, 0, 5]|unique }}\nunique-filter-ci: {{ [\"a\", \"A\", \"b\", \"c\", \"b\", \"D\", \"d\"]|unique }}\nunique-filter-cs: {{ [\"a\", \"A\", \"b\", \"c\", \"b\", \"D\", \"d\"]|unique(case_sensitive=true) }}\nunique-attr-filter: {{ [{'x': 1}, {'x': 1, 'y': 2}, {'x': 2}]|unique }}\npprint-filter: {{ objects|pprint }}\nint-filter: {{ true|int }}, {{ \"42\"|int }}, {{ \"-23\"|int }}, {{ 42.0|int }}, {{ 42.42|int }}, {{ \"42.42\"|int }}\nfloat-filter: {{ true|float }}, {{ \"42\"|float }}, {{ \"-23.5\"|float }}, {{ 42.5|float }}\nsplit: {{ three_words|split|list }}\nsplit-at-and: {{ three_words|split(\" and \")|list }}\nsplit-n-ws: {{ three_words|split(none, 1)|list }}\nsplit-n-d: {{ three_words|split(\"d\", 1)|list }}\nsplit-n-ws-filter-empty: {{ \"  foo    bar baz  \"|split(none, 1)|list }}\nsum: {{ range(10)|sum }}\nsum-empty: {{ []|sum }}\nsum-float: {{ [0.5, 1.0]|sum }}\nlines: {{ \"foo\\nbar\\r\\nbaz\"|lines }}\nstring: {{ [1|string, 2|string] }}"
 info:
   word: Bird
   word_with_spaces: " Spacebird\n"
@@ -56,6 +56,7 @@ trim-bird: ir
 join-default: 123
 join-pipe: 1|2|3
 join_string: B-i-r-d
+join-empty-strings: ||blub|||42
 default: true
 default-value: 42
 default-value-falsy: b