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rust-lang/style-team#189: rhs-should-use-indent-of-last-line-of-lhs #6305

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rust-lang/style-team#189: rhs-should-use-indent-of-last-line-of-lhs #6305

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rust-lang/style-team#189: rhs-should-use-indent-of-last-line-of-lhs
johnhuichen Aug 18, 2024
58ca556
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johnhuichen Sep 4, 2024
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18 changes: 18 additions & 0 deletions src/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2124,6 +2124,24 @@ pub(crate) fn rewrite_assign_rhs_expr<R: Rewrite>(
rhs_kind: &RhsAssignKind<'_>,
rhs_tactics: RhsTactics,
) -> RewriteResult {
let get_rhs_shape = || {
let last_line = lhs.lines().last().unwrap_or_default();
let tab_spaces = context.config.tab_spaces();
let new_shape = shape
.block_indent(tab_spaces)
.saturating_sub_width(tab_spaces);
Comment on lines +2130 to +2132
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Can you explain the call to .saturating_sub_width(tab_spaces). I'm not sure if that's necessary.

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@johnhuichen johnhuichen Sep 4, 2024 •
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Without saturating_sub_width,

impl SomeType {
    fn method(&mut self) {
        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info =
            long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;
    }
}

is formatted to

impl SomeType {
    fn method(&mut self) {
        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info = long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;
    }
}

and throws

line formatted, but exceeded maximum width (maximum: 100 (see `max_width` option), found: 101)

rather than

impl SomeType {
    fn method(&mut self) {
        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info =
                long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;
    }
}

I think the remaining width is not accounted for properly if I only use block_indent

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Could you explain why we're using tab_spaces? What happens if we don't use .saturating_sub_width(tab_spaces), but the rhs was even longer? Does it wrap correctly then?

Part of me wonders if the shape is off to begin with.

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@johnhuichen johnhuichen Sep 4, 2024 •
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the shape that was used before adding extra block_indent is

old shape = Shape { width: 91, indent: Indent { block_indent: 8, alignment: 0 }, offset: 0 }

and the new shape with block_indent but without saturating_sub_width is

new shape = Shape { width: 91, indent: Indent { block_indent: 12, alignment: 0 }, offset: 0

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@johnhuichen johnhuichen Sep 4, 2024 •
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the new shape is also the shape of the last line of lhs

            .extra_info = long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;

has 12 spaces before .extra_info and .extra_info = long_long_long_long_long_long_long_long_long_long_long_long_long_long_long; is 89 chars. So the remaining width shouldn't be 91, but 87

i.e. when indent was 8 spaces, width = 91, max width = 100, so
width = max - indent - 1

when indent is 12, width = 100 - 12 - 1 =87

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@ytmimi I think the above should answer your last question in this PR. Can you take a look?

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@ytmimi ytmimi Sep 9, 2024 •
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Thanks for the detailed explanation. I didn't have time to revisit this PR over the weekend, but I plan on giving this another look later this week

let extra_indent_string = new_shape.to_string(&context.config);
if last_line.starts_with(extra_indent_string.as_ref()) {
new_shape
} else {
shape
}
};
let shape = if context.config.style_edition() >= StyleEdition::Edition2024 {
get_rhs_shape()
} else {
shape
};
let last_line_width = last_line_width(lhs).saturating_sub(if lhs.contains('\n') {
shape.indent.width()
} else {
Expand Down
7 changes: 7 additions & 0 deletions src/shape.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -278,6 +278,13 @@ impl Shape {
offset_indent.to_string_inner(config, 0)
}

/// similar to to_string_with_newline, except the result does not start with a new line
pub(crate) fn to_string(&self, config: &Config) -> Cow<'static, str> {
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Probably worth adding a doc comment so that we know that this function gives us a single line of indentation.

let mut offset_indent = self.indent;
offset_indent.alignment = self.offset;
offset_indent.to_string_inner(config, 1)
}

/// Creates a `Shape` with a virtually infinite width.
pub(crate) fn infinite_width(&self) -> Shape {
Shape {
Expand Down
24 changes: 24 additions & 0 deletions tests/source/rust-lang/style-team#189.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,24 @@
// rustfmt-style_edition: 2024

impl SomeType {
fn method(&mut self) {
self.array[array_index as usize]
.as_mut()
.expect("thing must exist")
.extra_info = Some(ExtraInfo {
parent,
count: count as u16,
children: children.into_boxed_slice(),
});
}
}

impl SomeType {
fn method(&mut self) {
self.array[array_index as usize]
.as_mut()
.expect("thing must exist")
.extra_info =
long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;
}
}
24 changes: 24 additions & 0 deletions tests/target/rust-lang/style-team#189.rs
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There are a few other test cases that I think we should add:

impl SomeType {
    fn method(&mut self) {
        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info = Some(ExtraInfo {
                parent,
                count: count as u16,
                children: children.into_boxed_slice(),
            }) + Some(ExtraInfo {
                parent,
                count: count as u16,
                children: children.into_boxed_slice(),
            });
    }
}

impl SomeType {
    fn method(&mut self) {
        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info =
                long_long_long_long_long_long_long_long_long_long_long_long_long_long_long
                    .as_mut()
                    .another_call()
                    .get_extra_info();
    }
}

impl SomeType {
    fn method(&mut self) {
        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info =
                long_long_long_long_long_long_long_long_long_long_long_long_long_long_long
                    .as_mut()
                    .another_call()
                    .get_extra_info() + Some(ExtraInfo {
                        parent,
                        count: count as u16,
                        children: children.into_boxed_slice(),
                    });
    }
}

Original file line number Diff line number Diff line change
@@ -0,0 +1,24 @@
// rustfmt-style_edition: 2024

impl SomeType {
fn method(&mut self) {
self.array[array_index as usize]
.as_mut()
.expect("thing must exist")
.extra_info = Some(ExtraInfo {
parent,
count: count as u16,
children: children.into_boxed_slice(),
});
}
}

impl SomeType {
fn method(&mut self) {
self.array[array_index as usize]
.as_mut()
.expect("thing must exist")
.extra_info =
long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;
}
}
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