[Bug]: Empty blocks within @layer rule are not deleted

[Yoskutik]

Describe the bug

If my CSS code contains several instances of using @layer rule with the same layer name, after the minification CSS nano will keep each usage of it.

This code

@layer components {
  .O82 { display: flex; }
}
@layer components {
  .tK5 { border: 0; }
}
@layer components {
  .tK1 { font-size: inherit; }
}

Is compiled into this (I added whitespaces)

@layer components {
  .O82 { display: flex }

  .tK5 { border: 0 }

  .tK1 { font-size: inherit }
}

@layer components {}

@layer components {}

Expected behaviour

The following @layer components {} blocks do not affect the styles, therefore they can be removed

Steps to reproduce

The steps are described above

Version

6.1.2

Preset

default

Environment

System:
    OS: macOS 14.3.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 113.86 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh

Package details

├─┬ css-loader@6.10.0 -> ./node_modules/.pnpm/css-loader@6.10.0_webpack@5.91.0/node_modules/css-loader
│ ├─┬ icss-utils@5.1.0 -> ./node_modules/.pnpm/icss-utils@5.1.0_postcss@8.4.38/node_modules/icss-utils
│ │ └── postcss@8.4.38 -> ./node_modules/.pnpm/postcss@8.4.38/node_modules/postcss
│ ├─┬ postcss-modules-extract-imports@3.0.0 -> ./node_modules/.pnpm/postcss-modules-extract-imports@3.0.0_postcss@8.4.38/node_modules/postcss-modules-extract-imports
│ │ └── postcss@8.4.38 -> ./node_modules/.pnpm/postcss@8.4.38/node_modules/postcss
│ ├─┬ postcss-modules-local-by-default@4.0.4 -> ./node_modules/.pnpm/postcss-modules-local-by-default@4.0.4_postcss@8.4.38/node_modules/postcss-modules-local-by-default
│ │ └── postcss@8.4.38 -> ./node_modules/.pnpm/postcss@8.4.38/node_modules/postcss
│ ├─┬ postcss-modules-scope@3.1.1 -> ./node_modules/.pnpm/postcss-modules-scope@3.1.1_postcss@8.4.38/node_modules/postcss-modules-scope
│ │ └── postcss@8.4.38 -> ./node_modules/.pnpm/postcss@8.4.38/node_modules/postcss
│ ├─┬ postcss-modules-values@4.0.0 -> ./node_modules/.pnpm/postcss-modules-values@4.0.0_postcss@8.4.38/node_modules/postcss-modules-values
│ │ └── postcss@8.4.38 -> ./node_modules/.pnpm/postcss@8.4.38/node_modules/postcss
│ └── postcss@8.4.38 -> ./node_modules/.pnpm/postcss@8.4.38/node_modules/postcss
└─┬ css-minimizer-webpack-plugin@6.0.0 -> ./node_modules/.pnpm/css-minimizer-webpack-plugin@6.0.0_webpack@5.91.0/node_modules/css-minimizer-webpack-plugin
  ├─┬ cssnano@6.1.2 -> ./node_modules/.pnpm/cssnano@6.1.2_postcss@8.4.38/node_modules/cssnano
  │ └── postcss@8.4.38 -> ./node_modules/.pnpm/postcss@8.4.38/node_modules/postcss
  └── postcss@8.4.38 -> ./node_modules/.pnpm/postcss@8.4.38/node_modules/postcss

Additional context

No response

[ludofischer]

This happens because deleting an empty layer might change the layer priority (see closed issue #1544). To prevent keeping useless empty layers, cssnano should remember all the layer names it has seen before, but currently it does not do it.

[Yoskutik]

When why do you merge the styled into a single layer declaration? If you have to keep the layers, isn't better not to merge them?

[ludofischer]

When why do you merge the styled into a single layer declaration? If you have to keep the layers, isn't better not to merge them?

As far as I remember, the code that merges is the same for all at-rules, and predates even existence of @layer declarations. When layers became more widespread, a fix was applied to preserve empty at-rules only for the @layer case.