Auto merge of #118368 - GuillaumeGomez:env-flag, r=Nilstrieb

Implement `--env` compiler flag (without `tracked_env` support)

Part of https://github.com/rust-lang/rust/issues/80792.
Implementation of https://github.com/rust-lang/compiler-team/issues/653.
Not an implementation of https://github.com/rust-lang/rfcs/pull/2794.

It adds the `--env` compiler flag option which allows to set environment values used by `env!` and `option_env!`.

Important to note: When trying to retrieve an environment variable value, it will first look into the ones defined with `--env`, and if there isn't one, then only it will look into the environment variables. So if you use `--env PATH=a`, then `env!("PATH")` will return `"a"` and not the actual `PATH` value.

As mentioned in the title, `tracked_env` support is not added here. I'll do it in a follow-up PR.

r? rust-lang/compiler

1 files changed, 12 insertions, 2 deletions

diff --git a/compiler/rustc_builtin_macros/src/env.rs b/compiler/rustc_builtin_macros/src/env.rs
index 8c2fa6ee95f..d772642b88b 100644
--- a/compiler/rustc_builtin_macros/src/env.rs
+++ b/compiler/rustc_builtin_macros/src/env.rs
@@ -13,6 +13,16 @@ use thin_vec::thin_vec;
 
 use crate::errors;
 
+fn lookup_env<'cx>(cx: &'cx ExtCtxt<'_>, var: Symbol) -> Option<Symbol> {
+    let var = var.as_str();
+    if let Some(value) = cx.sess.opts.logical_env.get(var) {
+        return Some(Symbol::intern(value));
+    }
+    // If the environment variable was not defined with the `--env` option, we try to retrieve it
+    // from rustc's environment.
+    env::var(var).ok().as_deref().map(Symbol::intern)
+}
+
 pub fn expand_option_env<'cx>(
     cx: &'cx mut ExtCtxt<'_>,
     sp: Span,
@@ -23,7 +33,7 @@ pub fn expand_option_env<'cx>(
     };
 
     let sp = cx.with_def_site_ctxt(sp);
-    let value = env::var(var.as_str()).ok().as_deref().map(Symbol::intern);
+    let value = lookup_env(cx, var);
     cx.sess.parse_sess.env_depinfo.borrow_mut().insert((var, value));
     let e = match value {
         None => {
@@ -77,7 +87,7 @@ pub fn expand_env<'cx>(
     };
 
     let span = cx.with_def_site_ctxt(sp);
-    let value = env::var(var.as_str()).ok().as_deref().map(Symbol::intern);
+    let value = lookup_env(cx, var);
     cx.sess.parse_sess.env_depinfo.borrow_mut().insert((var, value));
     let e = match value {
         None => {