4 files changed, 46 insertions, 0 deletions
diff --git a/compiler/rustc_middle/src/query/erase.rs b/compiler/rustc_middle/src/query/erase.rs
index 7bbaa0496d5..6c6b9a5510c 100644
--- a/compiler/rustc_middle/src/query/erase.rs
+++ b/compiler/rustc_middle/src/query/erase.rs
@@ -1,3 +1,4 @@
+use std::ffi::OsStr;
 use std::intrinsics::transmute_unchecked;
 use std::mem::MaybeUninit;
 
@@ -67,6 +68,10 @@ impl<T> EraseType for &'_ [T] {
     type Result = [u8; size_of::<&'static [()]>()];
 }
 
+impl EraseType for &'_ OsStr {
+    type Result = [u8; size_of::<&'static OsStr>()];
+}
+
 impl<T> EraseType for &'_ ty::List<T> {
     type Result = [u8; size_of::<&'static ty::List<()>>()];
 }
@@ -174,6 +179,10 @@ impl<T> EraseType for Option<&'_ [T]> {
     type Result = [u8; size_of::<Option<&'static [()]>>()];
 }
 
+impl EraseType for Option<&'_ OsStr> {
+    type Result = [u8; size_of::<Option<&'static OsStr>>()];
+}
+
 impl EraseType for Option<mir::DestructuredConstant<'_>> {
     type Result = [u8; size_of::<Option<mir::DestructuredConstant<'static>>>()];
 }
diff --git a/compiler/rustc_middle/src/query/keys.rs b/compiler/rustc_middle/src/query/keys.rs
index 98314b5abfd..c382bcd726f 100644
--- a/compiler/rustc_middle/src/query/keys.rs
+++ b/compiler/rustc_middle/src/query/keys.rs
@@ -1,5 +1,7 @@
 //! Defines the set of legal keys that can be used in queries.
 
+use std::ffi::OsStr;
+
 use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE, LocalDefId, LocalModDefId, ModDefId};
 use rustc_hir::hir_id::{HirId, OwnerId};
 use rustc_query_system::dep_graph::DepNodeIndex;
@@ -498,6 +500,14 @@ impl Key for Option<Symbol> {
     }
 }
 
+impl<'tcx> Key for &'tcx OsStr {
+    type Cache<V> = DefaultCache<Self, V>;
+
+    fn default_span(&self, _tcx: TyCtxt<'_>) -> Span {
+        DUMMY_SP
+    }
+}
+
 /// Canonical query goals correspond to abstract trait operations that
 /// are not tied to any crate in particular.
 impl<'tcx, T: Clone> Key for CanonicalQueryInput<'tcx, T> {
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" }
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 834d1f2a4a8..08d4c1f9cf2 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -7,6 +7,8 @@ pub mod tls;
 use std::assert_matches::{assert_matches, debug_assert_matches};
 use std::borrow::Borrow;
 use std::cmp::Ordering;
+use std::env::VarError;
+use std::ffi::OsStr;
 use std::hash::{Hash, Hasher};
 use std::marker::PhantomData;
 use std::ops::{Bound, Deref};
@@ -1883,6 +1885,15 @@ impl<'tcx> TyCtxt<'tcx> {
         }
         None
     }
+
+    /// Helper to get a tracked environment variable via. [`TyCtxt::env_var_os`] and converting to
+    /// UTF-8 like [`std::env::var`].
+    pub fn env_var<K: ?Sized + AsRef<OsStr>>(self, key: &'tcx K) -> Result<&'tcx str, VarError> {
+        match self.env_var_os(key.as_ref()) {
+            Some(value) => value.to_str().ok_or_else(|| VarError::NotUnicode(value.to_os_string())),
+            None => Err(VarError::NotPresent),
+        }
+    }
 }
 
 impl<'tcx> TyCtxtAt<'tcx> {