What is the difference between Cargo's environment variables RUSTC and RUSTC_WRAPPER?

Question

Cargo's environment variables are explained here.

• RUSTC - Instead of running rustc, Cargo will execute this specified compiler instead.
• RUSTC_WRAPPER - Instead of simply running rustc, Cargo will execute this specified wrapper instead, passing as its commandline arguments the rustc invocation, with the first argument being rustc.

RUSTC and RUSTC_WRAPPER variables look like they are doing the same thing. How are they different?

Answer 1

As the documentation states:

• If RUSTC is defined, Cargo will not execute this line:

  rustc <parameters>
  

  but this one:

  $RUSTC <parameters>
  

  That is like the variable CC in Makefiles to set an alternative compiler.

• If RUSTC_WRAPPER is defined, Cargo will not call the compiler itself. It will instead call the following line:

  $RUSTC_WRAPPER rustc <parameters>
  

  You can add the options you want, or do whatever you want before the compilation, but you have to call the compiler yourself. Well, that's the purpose of a wrapper.