Rollup merge of #117256 - dtolnay:currentversion, r=compiler-errors

Parse rustc version at compile time

This PR eliminates a couple awkward codepaths where it was not clear how the compiler should proceed if its own version number is incomprehensible.

dab715641e/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs (L385)

dab715641e/compiler/rustc_attr/src/builtin.rs (L630)

We can guarantee that every compiled rustc comes with a working version number, so the ICE codepaths above shouldn't need to be written.

compiler/rustc_session/src/lib.rs

@@ -43,6 +43,9 @@ pub mod output;
 
 pub use getopts;
 
+mod version;
+pub use version::RustcVersion;
+
 fluent_messages! { "../messages.ftl" }
 
 /// Requirements for a `StableHashingContext` to be used in this crate.

compiler/rustc_session/src/version.rs

@@ -0,0 +1,19 @@
+use std::fmt::{self, Display};
+
+#[derive(Encodable, Decodable, Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(HashStable_Generic)]
+pub struct RustcVersion {
+    pub major: u16,
+    pub minor: u16,
+    pub patch: u16,
+}
+
+impl RustcVersion {
+    pub const CURRENT: Self = current_rustc_version!(env!("CFG_RELEASE"));
+}
+
+impl Display for RustcVersion {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(formatter, "{}.{}.{}", self.major, self.minor, self.patch)
+    }
+}