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Fix empty lists/sets with comments idempotency #363

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Merged
MattSturgeon merged 1 commit into from
Jan 6, 2026
Merged

Fix empty lists/sets with comments idempotency #363

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18 changes: 15 additions & 3 deletions src/Nixfmt/Pretty.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -124,17 +124,24 @@ renderItems separator (Items items) = go items
go (item : rest) =
pretty item <> if null rest then mempty else separator <> go rest

hasOnlyComments :: Items a -> Bool
hasOnlyComments (Items xs) = not (null xs) && all isComment xs
where
isComment (Comments _) = True
isComment _ = False

-- Render a list given the separator between items
renderList :: Doc -> Ann Token -> Items Term -> Ann Token -> Doc
renderList itemSep paropen@Ann{trailComment = post} items parclose =
pretty (paropen{trailComment = Nothing})
<> surroundWith sur (nest $ pretty post <> renderItems itemSep items)
<> pretty parclose
where
-- If the brackets are on different lines, keep them like that
sur
| sourceLine paropen /= sourceLine parclose = hardline
| null $ unItems items = hardspace
| sourceLine paropen /= sourceLine parclose = hardline -- If the brackets are on different lines, keep them like that
| hasOnlyComments items = hardline -- If the list has only comments, use hardline to ensure idempotency. https://github.com/NixOS/nixfmt/issues/362
| hasTrivia paropen && null items = hardline -- even if the comment got associated with the opening bracket, keep the hardline
| null items = hardspace -- making sure we're not potentially adding extra newlines for empty lists
| otherwise = line

instance Pretty Trivia where
Expand Down Expand Up @@ -597,6 +604,11 @@ isAbsorbable (Set _ (Ann{sourceLine = line1}) (Items []) (Ann{sourceLine = line2
| line1 /= line2 = True
isAbsorbable (List (Ann{sourceLine = line1}) (Items []) (Ann{sourceLine = line2}))
| line1 /= line2 = True
-- Lists/sets with only comments are absorbable (https://github.com/NixOS/nixfmt/issues/362)
isAbsorbable (List paropen items _)
| hasTrivia paropen || hasOnlyComments items = True
isAbsorbable (Set _ paropen items _)
| hasTrivia paropen || hasOnlyComments items = True
isAbsorbable (Parenthesized (LoneAnn _) (Term t) _) = isAbsorbable t
isAbsorbable _ = False

Expand Down
55 changes: 55 additions & 0 deletions test/diff/attr_set/in.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -327,6 +327,61 @@
++ baz # newline!
++ meow;

# https://github.com/NixOS/nixfmt/issues/362
foobar = [ /* foobar */ ];

foobar =
[
# foobar
];

foobar =
[ # foobar
];

foobar =
[ # foobar
];

foobar = [
# foobar
];

foobar2 = [ /* foobar */

];

foobar3 = [

/* foobar */ ];

foobar = { /* foobar */ };

foobar =
{
# foobar
};

foobar =
{ # foobar
};

foobar =
{ # foobar
};

foobar = {
# foobar
};

foobar2 = { /* foobar */

};

foobar3 = {

/* foobar */ };

environment.systemPackages =
# Include the PAM modules in the system path mostly for the manpages.
[ package ]
Expand Down
61 changes: 61 additions & 0 deletions test/diff/attr_set/out-pure.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -432,6 +432,67 @@
++ baz # newline!
++ meow;

# https://github.com/NixOS/nixfmt/issues/362
foobar = [
# foobar
];

foobar = [
# foobar
];

foobar = [
# foobar
];

foobar = [
# foobar
];

foobar = [
# foobar
];

foobar2 = [
# foobar

];

foobar3 = [

# foobar
];

foobar = {
# foobar
};

foobar = {
# foobar
};

foobar = {
# foobar
};

foobar = {
# foobar
};

foobar = {
# foobar
};

foobar2 = {
# foobar

};

foobar3 = {

# foobar
};

environment.systemPackages =
# Include the PAM modules in the system path mostly for the manpages.
[ package ] ++ lib.optional config.users.ldap.enable pam_ldap;
Expand Down
61 changes: 61 additions & 0 deletions test/diff/attr_set/out.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -449,6 +449,67 @@
++ baz # newline!
++ meow;

# https://github.com/NixOS/nixfmt/issues/362
foobar = [
# foobar
];

foobar = [
# foobar
];

foobar = [
# foobar
];

foobar = [
# foobar
];

foobar = [
# foobar
];

foobar2 = [
# foobar

];

foobar3 = [

# foobar
];

foobar = {
# foobar
};

foobar = {
# foobar
};

foobar = {
# foobar
};

foobar = {
# foobar
};

foobar = {
# foobar
};

foobar2 = {
# foobar

};

foobar3 = {

# foobar
};

environment.systemPackages =
# Include the PAM modules in the system path mostly for the manpages.
[ package ] ++ lib.optional config.users.ldap.enable pam_ldap;
Expand Down
9 changes: 9 additions & 0 deletions test/diff/lists/in.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -82,6 +82,15 @@

]

[ /* foobar */ ]

[ # foobar
]

[
# foobar
]


[ [ multi line ] ]
[ [ [ singleton ] ] ]
Expand Down
12 changes: 12 additions & 0 deletions test/diff/lists/out-pure.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -108,6 +108,18 @@

]

[
# foobar
]

[
# foobar
]

[
# foobar
]

[
[
multi
Expand Down
12 changes: 12 additions & 0 deletions test/diff/lists/out.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -111,6 +111,18 @@

]

[
# foobar
]

[
# foobar
]

[
# foobar
]

[
[
multi
Expand Down