Trait ast_grep_core::replacer::IndentSensitive

source ·

pub trait IndentSensitive: Content {
    const NEW_LINE: Self::Underlying;
    const SPACE: Self::Underlying;
    const TAB: Self::Underlying;
}

Expand description

This module is for indentation-sensitive replacement.

Ideally, structural search and replacement should all be based on AST. But this means our changed AST need to be pretty-printed by structural rules, which we don’t have enough resource to support. An indentation solution is used.

The algorithm is quite complicated, uncomprehensive, sluggish and buggy. But let’s walk through it by example.

consider this code

if (true) {
  a(
    1
      + 2
      + 3
  )
}

and this pattern and replacement

// pattern
a($B)
// replacement
c(
  $B
)

We need to compute the relative indentation of the captured meta-var. When we insert the meta-var into replacement, keep the relative indent intact, while also respecting the replacement indent. Finally, the whole replacement should replace the matched node in a manner that maintains the indentation of the source.

We need to consider multiple indentations. Key concepts here:

meta-var node: in this case $B in pattern/replacement, or 1+2+3 in source.
matched node: in this case a($B) in pattern, a(1 + 2 + 3)` in source
meta-var source indentation: $B matches 1+2+3, the first line’s indentation in source code is 4.
meta-var replacement indentation: in this case 2
matched node source indentation: in this case 2

Extract Meta-var with de-indent

Initial meta-var node B text: The meta-var source indentation for $B is 4. However, meta-var node does not have the first line indentation.

1
      + 2
      + 3

Deindent meta-var node B, except first line: De-indenting all lines following the first line by 4 spaces gives us this relative code layout.

1
  + 2
  + 3

Insert meta-var into replacement with re-indent

Re-indent by meta-var replacement indentation. meta-var node $B occurs in replace with first line indentation of 2. We need to re-indent the meta-var code before replacement, except the first line

1
    + 2
    + 3

Insert meta-var code in to replacement

c(
  1
    + 2
    + 3
)

Insert replacement into source with re-indent

Re-indent the replaced template code except first line The whole matched node first line indentation is 2. We need to reindent the replacement code by 2, except the first line.

Inserted replacement code to original tree

if (true) {
  c(
    1
      + 2
      + 3
  )
}

The steps 3,4 and steps 5,6 are similar. We can define a replace_with_indent to it. Following the same path, we can define a extract_with_deindent for steps 1,2.