Make #[cfg(version)] respect RUSTC_OVERRIDE_VERSION_STRING

compiler/rustc_attr_data_structures/src/version.rs

@@ -1,4 +1,5 @@
 use std::fmt::{self, Display};
+use std::sync::OnceLock;
 
 use rustc_macros::{
     Decodable, Encodable, HashStable_Generic, PrintAttribute, current_rustc_version,
@@ -16,8 +17,29 @@ pub struct RustcVersion {
 
 impl RustcVersion {
     pub const CURRENT: Self = current_rustc_version!();
+    pub fn current_overridable() -> Self {
+        *CURRENT_OVERRIDABLE.get_or_init(|| {
+            if let Ok(override_var) = std::env::var("RUSTC_OVERRIDE_VERSION_STRING")
+                && let Some(override_) = Self::parse_str(&override_var)
+            {
+                override_
+            } else {
+                Self::CURRENT
+            }
+        })
+    }
+    fn parse_str(value: &str) -> Option<Self> {
+        // Ignore any suffixes such as "-dev" or "-nightly".
+        let mut components = value.split('-').next().unwrap().splitn(3, '.');
+        let major = components.next()?.parse().ok()?;
+        let minor = components.next()?.parse().ok()?;
+        let patch = components.next().unwrap_or("0").parse().ok()?;
+        Some(RustcVersion { major, minor, patch })
+    }
 }
 
+static CURRENT_OVERRIDABLE: OnceLock<RustcVersion> = OnceLock::new();
+
 impl Display for RustcVersion {
     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(formatter, "{}.{}.{}", self.major, self.minor, self.patch)

compiler/rustc_attr_parsing/src/attributes/cfg.rs

@@ -129,9 +129,9 @@ pub fn eval_condition(
 
             // See https://github.com/rust-lang/rust/issues/64796#issuecomment-640851454 for details
             if sess.psess.assume_incomplete_release {
-                RustcVersion::CURRENT > min_version
+                RustcVersion::current_overridable() > min_version
             } else {
-                RustcVersion::CURRENT >= min_version
+                RustcVersion::current_overridable() >= min_version
             }
         }
         MetaItemKind::List(mis) => {

tests/ui/feature-gates/cfg-version-expand.rs

@@ -0,0 +1,30 @@
+//@ run-pass
+//@ rustc-env:RUSTC_OVERRIDE_VERSION_STRING=1.50.3
+
+#![feature(cfg_version)]
+
+#[cfg(version("1.49.0"))]
+const ON_1_49_0: bool = true;
+#[cfg(version("1.50"))]
+const ON_1_50_0: bool = true;
+#[cfg(not(version("1.51")))]
+const ON_1_51_0: bool = false;
+
+// This one uses the wrong syntax, so doesn't eval to true
+#[warn(unexpected_cfgs)]
+#[cfg(not(version = "1.48.0"))] //~ WARN unexpected `cfg` condition name: `version`
+const ON_1_48_0: bool = false;
+
+fn main() {
+    assert!(!ON_1_48_0);
+    assert!(ON_1_49_0);
+    assert!(ON_1_50_0);
+    assert!(!ON_1_51_0);
+    assert!(cfg!(version("1.1")));
+    assert!(cfg!(version("1.49")));
+    assert!(cfg!(version("1.50.0")));
+    assert!(cfg!(version("1.50.3")));
+    assert!(!cfg!(version("1.50.4")));
+    assert!(!cfg!(version("1.51")));
+    assert!(!cfg!(version("1.100")));
+}

tests/ui/feature-gates/cfg-version-expand.stderr

@@ -0,0 +1,13 @@
+warning: unexpected `cfg` condition name: `version`
+  --> $DIR/cfg-version-expand.rs:15:11
+   |
+LL | #[cfg(not(version = "1.48.0"))]
+   |           ^^^^^^^^^^^^^^^^^^
+   |
+   = help: expected names are: `FALSE` and `test` and 31 more
+   = help: to expect this configuration use `--check-cfg=cfg(version, values("1.48.0"))`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+   = note: `#[warn(unexpected_cfgs)]` on by default
+
+warning: 1 warning emitted
+