Start with the idea
Parameter types and the return type are always written out, never inferred across a function boundary. The last expression is the return value when you leave the semicolon off, which is why an accidental semicolon turns a function into one that returns the unit type and produces a type error.
The area function declares its contract before its body: two u32 parameters go in and one u32 comes out. That boundary lets rustc check a call without reading every line of the implementation. Inside the body, width * height is the tail expression. Its value becomes the return value because there is no semicolon after it. Adding one would change that final statement to unit, and rustc would point out the mismatch with -> u32.
The shout function shows that a body can have several steps before its tail value. text is borrowed as &str, to_uppercase creates an owned String, and push modifies that owned buffer. The final out expression moves the completed String to the caller.
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