muon/doc/differences.md

Differences between muon and Meson

This document describes functional differences between muon and Meson. None of these is a set-in-stone design decision, just a reflection of the current state of affairs. This document is also not exhaustive, but is a best-effort list.

Some other small differences may be found by searching the tests/project for "# different than meson" comments.

nested subproject promotion

Meson performs nested subproject promotion. This means that nested subprojects become top-level subprojects, and all subprojects share the same namespace.

For example, given the following project structure:

.
├── meson.build
└── subprojects
    ├── a
    │   ├── meson.build
    │   └── subprojects
    │       └── b
    │           └── meson.build
    └── b
        └── meson.build

The order of subproject calls determines which subprojects will be used:

project('main')

# This causes all subprojects under subprojects/a/subprojects/ to be "promoted"
subproject('a')

# This will now use subprojects/a/subprojects/b, instead of subprojects/b
subproject('b')

muon does not perform subproject promotion.

malformed escape sequences

Meson silently accepts malformed escape sequences and outputs them literally, removing the leading escape character. For example:

'\c'          # becomes 'c'
'\Uabcdefghi' # becomes 'Uabcdefghi'
'\xqr'        # becomes 'xqr'

In muon, malformed escape sequences are parse errors.

format strings

Format strings in various parts of Meson use @ as the delimiter. The behavior is inconsistent, configure_file() recognizes \ as an escape character, but format strings in string.format() and custom_target command arguments do not.

configure_file() will also warn you about invalid substitutions, and will remove them in the output, string.format() will error on invalid substitutions, and custom_target command arguments will be silently treated as literals if they are invalid substitutions (e.g. @BAZ@).

Because some projects rely on the above custom target command argument behaviour, muon merely adds a warning for invalid substitutions. In all other cases muon will raise error.

custom_target replaces backslashes with slashes in the command arguments

In Meson, all backslashes in custom_target command line arguments are blindly replaced to forward slashes. This behavior is not present in muon.

Reference: https://github.com/mesonbuild/meson/issues/1564

build_target() functions with empty sources

Meson allows you to create build targets (executable(), shared_library(), static_library(), etc.) without specifying any sources. In muon this is an error.

global compiler cache

Meson maintains a global compiler cache, which means that all languages added by subprojects are available to the main project and vice-versa. This can hide bugs that will surface if the subproject is built by itself, or subproject calls are rearranged.

run_command() cwd

Meson executes run_command() commands in the current subdirectory, while muon executes them in the project root. Neither behaviour should be relied upon however, since the docs say that it runs commands from an unspecified directory.