Rollup merge of #138682 - Alexendoo:extra-symbols, r=fee1-dead

Allow drivers to supply a list of extra symbols to intern

Allows adding new symbols as `const`s in external drivers, desirable in Clippy so we can use them in patterns to replace code like 75530e9f72/src/tools/clippy/clippy_lints/src/casts/cast_ptr_alignment.rs (L66)

The Clippy change adds a couple symbols as a demo, the exact `clippy_utils` API and replacing other usages can be done on the Clippy side to minimise sync conflicts

---

try-job: aarch64-gnu

clippy_utils/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(f128)]
 #![feature(f16)]
 #![feature(if_let_guard)]
+#![feature(macro_metavar_expr)]
 #![feature(macro_metavar_expr_concat)]
 #![feature(let_chains)]
 #![feature(never_type)]
@@ -74,6 +75,7 @@ pub mod qualify_min_const_fn;
 pub mod source;
 pub mod str_utils;
 pub mod sugg;
+pub mod sym;
 pub mod ty;
 pub mod usage;
 pub mod visitors;
@@ -126,7 +128,7 @@ use rustc_middle::ty::{
 use rustc_span::hygiene::{ExpnKind, MacroKind};
 use rustc_span::source_map::SourceMap;
 use rustc_span::symbol::{Ident, Symbol, kw};
-use rustc_span::{InnerSpan, Span, sym};
+use rustc_span::{InnerSpan, Span};
 use visitors::{Visitable, for_each_unconsumed_temporary};
 
 use crate::consts::{ConstEvalCtxt, Constant, mir_to_const};

clippy_utils/src/sym.rs

@@ -0,0 +1,23 @@
+#![allow(non_upper_case_globals)]
+
+use rustc_span::symbol::{Symbol, PREDEFINED_SYMBOLS_COUNT};
+
+pub use rustc_span::sym::*;
+
+macro_rules! generate {
+    ($($sym:ident,)*) => {
+        /// To be supplied to `rustc_interface::Config`
+        pub const EXTRA_SYMBOLS: &[&str] = &[
+            $(stringify!($sym),)*
+        ];
+
+        $(
+            pub const $sym: Symbol = Symbol::new(PREDEFINED_SYMBOLS_COUNT + ${index()});
+        )*
+    };
+}
+
+generate! {
+    rustfmt_skip,
+    unused_extern_crates,
+}