Why is the `panic` in the if branch that has a true condition not executed if no `else` branch is supplied in `build.rs`?

help
1 
// build.rs
fn main(){
    if std::env::var("DOCS_RS").is_ok(){
        panic!("{}",std::env::var("DOCS_RS").is_ok());
    }/*else{
        panic!("aaa");
    }*/
}

In PowerShell, $env:DOCS_RS="1"; cargo build can be successfully compiled, which is not expected. However, if you uncomment the comment

  --- stderr

  thread 'main' panicked at build.rs:3:9:
  true
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The expected error will be printed out. What's the reason here? It's confusing.

2

maybe the build script is not run at all. try add:

println!("cargo::rerun-if-env-changed=DOCS_RS");
3

However, if the else branch is supplied, the build.rs do work.

4

if you modified build.rs, cargo will automatically (rebuild and) re-run the build script. but if you only change the environment variable without changing the build script (or any ./src/* files), cargo build will not re-run the build script (unless it's a clean build).

5

Okay. I still found a confusion part. If I manually set $env:RUSTFLAGS="--cfg docsrs"; cargo doc --open, then I can still see the item in the generated documentation, where the item in the source code is written as:

// lib.rs

#[cfg(not(docsrs))]
pub fn add(x: i32, y: i32) -> i32 {
    x + y
}

However, if I used the build.rs, for example,

fn main(){
  println!("cargo:rustc-cfg=docsrs");
}

Then, the generated documentation won't include the add item? Shouldn't rustc-cfg equal RUSTFLAGS="--cfg docsrs"?

PS: If I want to manually set docsrs, I need to set RUSTDOCFLAGS="--cfg docsrs". Why does rustc-cfg in build.rs affect RUSTDOCFLAGS?

6

this, I don't really know, presumably cargo is hanlding the environment variables before invoking rustdoc.

I wonder does it behave differently between cargo doc and cargo rustdoc? I vaguely remember there's different hanlding for the flags between cargo build and cargo rustc, maybe it's similar for the case of documentation?