Rollup merge of #130883 - madsmtm:env-var-query, r=petrochenkov

Add environment variable query

Generally, `rustc` prefers command-line arguments, but in some cases, an environment variable really is the most sensible option. We should make sure that this works properly with the compiler's change-tracking mechanisms, such that changing the relevant environment variable causes a rebuild.

This PR is a first step forwards in doing that.

Part of the work needed to do https://github.com/rust-lang/rust/issues/118204, see https://github.com/rust-lang/rust/pull/129342 for some discussion.

r? ``@petrochenkov``

1 files changed, 16 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 527c18addbe..850de3f0c2a 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -6,6 +6,7 @@
 
 #![allow(unused_parens)]
 
+use std::ffi::OsStr;
 use std::mem;
 use std::path::PathBuf;
 use std::sync::Arc;
@@ -119,6 +120,21 @@ rustc_queries! {
         desc { "perform lints prior to AST lowering" }
     }
 
+    /// Tracked access to environment variables.
+    ///
+    /// Useful for the implementation of `std::env!`, `proc-macro`s change
+    /// detection and other changes in the compiler's behaviour that is easier
+    /// to control with an environment variable than a flag.
+    ///
+    /// NOTE: This currently does not work with dependency info in the
+    /// analysis, codegen and linking passes, place extra code at the top of
+    /// `rustc_interface::passes::write_dep_info` to make that work.
+    query env_var_os(key: &'tcx OsStr) -> Option<&'tcx OsStr> {
+        // Environment variables are global state
+        eval_always
+        desc { "get the value of an environment variable" }
+    }
+
     query resolutions(_: ()) -> &'tcx ty::ResolverGlobalCtxt {
         no_hash
         desc { "getting the resolver outputs" }