Auto merge of #14878 - lowr:feat/metavar-expr-count, r=Veykril

Implement `${count()}` metavariable expression

This PR implements `${count()}` metavariable expression for MBE as described in [RFC 3086][rfc]. See [this section][sec-count] of the RFC for its semantics.

Additionally, this PR changes the type of depth parameter of `${index()}` from u32 to usize so as to match how [rustc parses it][index-usize].

Part of #11952
Fixes #14871

[rfc]: rust-lang/rfcs#3086
[sec-count]: https://github.com/rust-lang/rfcs/blob/master/text/3086-macro-metavar-expr.md#count
[index-usize]:https://github.com/rust-lang/rust/blob/ddad0576caf8d0515ed453e04b468977c7d3dfc1/compiler/rustc_expand/src/mbe/metavar_expr.rs#L22

Author: bors

crates/hir-def/src/macro_expansion_tests/mbe.rs

@@ -4,6 +4,7 @@
 mod tt_conversion;
 mod matching;
 mod meta_syntax;
+mod metavar_expr;
 mod regression;
 
 use expect_test::expect;
@@ -1614,92 +1615,6 @@ struct Foo;
     )
 }
 
-#[test]
-fn test_dollar_dollar() {
-    check(
-        r#"
-macro_rules! register_struct { ($Struct:ident) => {
-    macro_rules! register_methods { ($$($method:ident),*) => {
-        macro_rules! implement_methods { ($$$$($$val:expr),*) => {
-            struct $Struct;
-            impl $Struct { $$(fn $method() -> &'static [u32] { &[$$$$($$$$val),*] })*}
-        }}
-    }}
-}}
-
-register_struct!(Foo);
-register_methods!(alpha, beta);
-implement_methods!(1, 2, 3);
-"#,
-        expect![[r#"
-macro_rules! register_struct { ($Struct:ident) => {
-    macro_rules! register_methods { ($$($method:ident),*) => {
-        macro_rules! implement_methods { ($$$$($$val:expr),*) => {
-            struct $Struct;
-            impl $Struct { $$(fn $method() -> &'static [u32] { &[$$$$($$$$val),*] })*}
-        }}
-    }}
-}}
-
-macro_rules !register_methods {
-    ($($method: ident), *) = > {
-        macro_rules!implement_methods {
-            ($$($val: expr), *) = > {
-                struct Foo;
-                impl Foo {
-                    $(fn $method()-> &'static[u32] {
-                        &[$$($$val), *]
-                    }
-                    )*
-                }
-            }
-        }
-    }
-}
-macro_rules !implement_methods {
-    ($($val: expr), *) = > {
-        struct Foo;
-        impl Foo {
-            fn alpha()-> &'static[u32] {
-                &[$($val), *]
-            }
-            fn beta()-> &'static[u32] {
-                &[$($val), *]
-            }
-        }
-    }
-}
-struct Foo;
-impl Foo {
-    fn alpha() -> &'static[u32] {
-        &[1, 2, 3]
-    }
-    fn beta() -> &'static[u32] {
-        &[1, 2, 3]
-    }
-}
-"#]],
-    )
-}
-
-#[test]
-fn test_metavar_exprs() {
-    check(
-        r#"
-macro_rules! m {
-    ( $( $t:tt )* ) => ( $( ${ignore(t)} -${index()} )-* );
-}
-const _: i32 = m!(a b c);
-    "#,
-        expect![[r#"
-macro_rules! m {
-    ( $( $t:tt )* ) => ( $( ${ignore(t)} -${index()} )-* );
-}
-const _: i32 = -0--1--2;
-    "#]],
-    );
-}
-
 #[test]
 fn test_punct_without_space() {
     // Puncts are "glued" greedily.