Macro config 

config!() { /* proc-macro */ }

Declares static variables containing config data.

A config block may contain any number of config items, in the form illustrated below:

config! {
    /// Optional documents
    #[optional_attributes]
    <VIS> static <IDENT> = <SRC>;

    /// Optional documents
    #[optional_attributes]
    <VIS> static <IDENT> = <SRC> + <SRC> + <SRC>;
}

Each declaration is simply a typical static item, except that the type is omitted.

The expression part looks like a sum of sources. This is where overwriting takes place. All variants are completely overwritten except for tables, which got merged recursively. Every <SRC> takes one of the following forms:

• #[toml] r#"name = "Tom""# - an inline literal config. We require an attribute to specify the format of config.
• include_config!("example_config.toml") - a file inclusion. The path is resolved relative to the current file (similar to include_str!()). The format specifier attribute may be omitted if it’s clear from the file extension.
• include_config_env!("$CARGO_MANIFEST_DIR/examples/example_config.toml") - also a file inclusion, but environment variables of form $ENV_VAR are interpolated. Escape $ with $$.

The support of environment variable interpolation is to aid any code analyzer to locate files, as environment variables like $CARGO_MANIFEST_DIR and $OUT_DIR resolve to absolute paths. This is mostly inspired by include_dir crate.