Assume the effects of RUSTC_BOOTSTRAP in sysroot

compiler/rustc_feature/src/lib.rs

@@ -48,12 +48,20 @@ pub enum UnstableFeatures {
 }
 
 impl UnstableFeatures {
+    /// This takes into account `RUSTC_BOOTSTRAP` and whether the input is in the sysroot.
+    ///
+    /// If `krate` is [`Some`], then setting `RUSTC_BOOTSTRAP=krate` will enable the nightly
+    /// features. Otherwise, only `RUSTC_BOOTSTRAP=1` or `input_in_sysroot` will work.
+    pub fn from_environment_check_in_sysroot(krate: Option<&str>, input_in_sysroot: bool) -> Self {
+        Self::from_environment_value(krate, std::env::var("RUSTC_BOOTSTRAP"), input_in_sysroot)
+    }
+
     /// This takes into account `RUSTC_BOOTSTRAP`.
     ///
     /// If `krate` is [`Some`], then setting `RUSTC_BOOTSTRAP=krate` will enable the nightly
     /// features. Otherwise, only `RUSTC_BOOTSTRAP=1` will work.
     pub fn from_environment(krate: Option<&str>) -> Self {
-        Self::from_environment_value(krate, std::env::var("RUSTC_BOOTSTRAP"))
+        Self::from_environment_value(krate, std::env::var("RUSTC_BOOTSTRAP"), false)
     }
 
     /// Avoid unsafe `std::env::set_var()` by allowing tests to inject
@@ -62,10 +70,8 @@ impl UnstableFeatures {
     fn from_environment_value(
         krate: Option<&str>,
         env_var_rustc_bootstrap: Result<String, std::env::VarError>,
+        input_in_sysroot: bool,
     ) -> Self {
-        // `true` if this is a feature-staged build, i.e., on the beta or stable channel.
-        let disable_unstable_features =
-            option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some_and(|s| s != "0");
         // Returns whether `krate` should be counted as unstable
         let is_unstable_crate =
             |var: &str| krate.is_some_and(|name| var.split(',').any(|new_krate| new_krate == name));
@@ -81,6 +87,14 @@ impl UnstableFeatures {
             }
         }
 
+        if input_in_sysroot {
+            return UnstableFeatures::Cheat;
+        }
+
+        // `true` if this is a feature-staged build, i.e., on the beta or stable channel.
+        let disable_unstable_features =
+            option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some_and(|s| s != "0");
+
         if disable_unstable_features { UnstableFeatures::Disallow } else { UnstableFeatures::Allow }
     }
 

compiler/rustc_feature/src/tests.rs

@@ -4,7 +4,7 @@ use super::UnstableFeatures;
 fn rustc_bootstrap_parsing() {
     let is_bootstrap = |env: &str, krate: Option<&str>| {
         matches!(
-            UnstableFeatures::from_environment_value(krate, Ok(env.to_string())),
+            UnstableFeatures::from_environment_value(krate, Ok(env.to_string()), false),
             UnstableFeatures::Cheat
         )
     };
@@ -23,10 +23,22 @@ fn rustc_bootstrap_parsing() {
     // `RUSTC_BOOTSTRAP=0` is not recognized.
     assert!(!is_bootstrap("0", None));
 
+    let is_bootstrap = |env: &str, krate: Option<&str>, input_in_sysroot: bool| {
+        matches!(
+            UnstableFeatures::from_environment_value(krate, Ok(env.to_string()), input_in_sysroot),
+            UnstableFeatures::Cheat
+        )
+    };
+    // Whether RUSTC_BOOTSTRAP is set or unset, when input_in_sysroot is_bootstrap should be true
+    assert!(is_bootstrap("1", None, true));
+    assert!(is_bootstrap("0", None, true));
+    // Even when input_in_sysroot, disabling RUSTC_BOOTSTRAP is honoured.
+    assert!(!is_bootstrap("-1", None, true));
+
     // `RUSTC_BOOTSTRAP=-1` is force-stable, no unstable features allowed.
     let is_force_stable = |krate: Option<&str>| {
         matches!(
-            UnstableFeatures::from_environment_value(krate, Ok("-1".to_string())),
+            UnstableFeatures::from_environment_value(krate, Ok("-1".to_string()), false),
             UnstableFeatures::Disallow
         )
     };

compiler/rustc_session/src/config.rs

@@ -9,7 +9,7 @@ use std::collections::btree_map::{
 use std::collections::{BTreeMap, BTreeSet};
 use std::ffi::OsStr;
 use std::hash::Hash;
-use std::path::{Path, PathBuf};
+use std::path::{Path, PathBuf, absolute};
 use std::str::{self, FromStr};
 use std::sync::LazyLock;
 use std::{cmp, fmt, fs, iter};
@@ -2434,7 +2434,10 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
     let edition = parse_crate_edition(early_dcx, matches);
 
     let crate_name = matches.opt_str("crate-name");
-    let unstable_features = UnstableFeatures::from_environment(crate_name.as_deref());
+    let unstable_features = UnstableFeatures::from_environment_check_in_sysroot(
+        crate_name.as_deref(),
+        input_in_sysroot(matches),
+    );
     let JsonConfig {
         json_rendered,
         json_color,
@@ -2863,10 +2866,26 @@ pub fn parse_crate_types_from_list(list_list: Vec<String>) -> Result<Vec<CrateTy
     Ok(crate_types)
 }
 
+pub fn input_in_sysroot(matches: &getopts::Matches) -> bool {
+    let sysroot = Sysroot::new(matches.opt_str("sysroot").map(PathBuf::from));
+    // Input parsing derived from the make_input function. If there are multiple files, or stdin input, then
+    // input_in_sysroot is either irrelevant, or false.
+    let Some((Ok(input), Ok(sysroot))) = matches
+        .free
+        .as_slice()
+        .first()
+        .map(|path| (absolute(Path::new(path)), absolute(sysroot.path())))
+    else {
+        return false;
+    };
+
+    input.starts_with(sysroot)
+}
+
 pub mod nightly_options {
     use rustc_feature::UnstableFeatures;
 
-    use super::{OptionStability, RustcOptGroup};
+    use super::{OptionStability, RustcOptGroup, input_in_sysroot};
     use crate::EarlyDiagCtxt;
 
     pub fn is_unstable_enabled(matches: &getopts::Matches) -> bool {
@@ -2875,11 +2894,12 @@ pub mod nightly_options {
     }
 
     pub fn match_is_nightly_build(matches: &getopts::Matches) -> bool {
-        is_nightly_build(matches.opt_str("crate-name").as_deref())
+        is_nightly_build(matches.opt_str("crate-name").as_deref(), input_in_sysroot(matches))
     }
 
-    fn is_nightly_build(krate: Option<&str>) -> bool {
-        UnstableFeatures::from_environment(krate).is_nightly_build()
+    fn is_nightly_build(krate: Option<&str>, input_in_sysroot: bool) -> bool {
+        UnstableFeatures::from_environment_check_in_sysroot(krate, input_in_sysroot)
+            .is_nightly_build()
     }
 
     pub fn check_nightly_options(