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Crates.md

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+# Crates
+The smallest amount of code that the rust compiler will consider
+
+This includes single binary exe files compiled with ``rustc``
+
+``cargo build`` also counts and so does ``cargo run`` (implicit ``cargo build``)
+
+Crates can contain modules and modules may be defined in other files that get compiled with the crate
+
+It comes in two forms a library or a binary
+
+## Binary Crates
+are programs that can be compiled then run like a cli application or a server
+each must have main function that defines what happens when the executable runs
+
+## Library Crates
+are programs that don't have a main function and don't compile to a executable
+instead it defines functionality that is intended to be shared with multiple projects
+
+most of the time when the term "crate" is said it means a library crate
+and crate is used interchangeably with the term library with other languages
+
+
+## Root Crate
+this is the source file that the compiler starts from and makes up the root module of your crate
+
+this is normally src/main.rs in a binary crate
+scr/lib.rs is normally the root in a library crate with the same name as the package
+hence why it doesn't need to be mentioned in the toml file this tells you what the packages are
+
+cargo passes root crates to rustc to build the library or the binary
+
+by replacing files in the src/bin directory each file will be its own binary crate