Try to support pre and post-change metavars

21 files changed, 255 insertions, 110 deletions

diff --git a/Cargo.lock b/Cargo.lock
index d1892b76899..2cb26d0c597 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -74,6 +74,7 @@ dependencies = [
  "profile",
  "rust-analyzer-salsa",
  "rustc-hash",
+ "semver",
  "span",
  "stdx",
  "syntax",
diff --git a/Cargo.toml b/Cargo.toml
index fdca4f6d160..80ec679db3d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -110,6 +110,7 @@ nohash-hasher = "0.2.0"
 rayon = "1.8.0"
 rust-analyzer-salsa = "0.17.0-pre.4"
 rustc-hash = "1.1.0"
+semver = "1.0.14"
 serde = { version = "1.0.192", features = ["derive"] }
 serde_json = "1.0.108"
 smallvec = { version = "1.10.0", features = [
diff --git a/crates/base-db/Cargo.toml b/crates/base-db/Cargo.toml
index 371d6ee74b9..13b26655581 100644
--- a/crates/base-db/Cargo.toml
+++ b/crates/base-db/Cargo.toml
@@ -16,6 +16,7 @@ la-arena.workspace = true
 rust-analyzer-salsa.workspace = true
 rustc-hash.workspace = true
 triomphe.workspace = true
+semver.workspace = true
 
 # local deps
 cfg.workspace = true
diff --git a/crates/base-db/src/input.rs b/crates/base-db/src/input.rs
index f0ce7e619d7..26f155adccc 100644
--- a/crates/base-db/src/input.rs
+++ b/crates/base-db/src/input.rs
@@ -11,6 +11,7 @@ use std::{fmt, mem, ops, str::FromStr};
 use cfg::CfgOptions;
 use la_arena::{Arena, Idx};
 use rustc_hash::{FxHashMap, FxHashSet};
+use semver::Version;
 use syntax::SmolStr;
 use triomphe::Arc;
 use vfs::{file_set::FileSet, AbsPathBuf, AnchoredPath, FileId, VfsPath};
@@ -258,7 +259,7 @@ impl ReleaseChannel {
 
     pub fn from_str(str: &str) -> Option<Self> {
         Some(match str {
-            "" => ReleaseChannel::Stable,
+            "" | "stable" => ReleaseChannel::Stable,
             "nightly" => ReleaseChannel::Nightly,
             _ if str.starts_with("beta") => ReleaseChannel::Beta,
             _ => return None,
@@ -289,7 +290,7 @@ pub struct CrateData {
     // things. This info does need to be somewhat present though as to prevent deduplication from
     // happening across different workspaces with different layouts.
     pub target_layout: TargetLayoutLoadResult,
-    pub channel: Option<ReleaseChannel>,
+    pub toolchain: Option<Version>,
 }
 
 impl CrateData {
@@ -346,6 +347,10 @@ impl CrateData {
 
         slf_deps.eq(other_deps)
     }
+
+    pub fn channel(&self) -> Option<ReleaseChannel> {
+        self.toolchain.as_ref().and_then(|v| ReleaseChannel::from_str(&v.pre))
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -427,7 +432,7 @@ impl CrateGraph {
         is_proc_macro: bool,
         origin: CrateOrigin,
         target_layout: Result<Arc<str>, Arc<str>>,
-        channel: Option<ReleaseChannel>,
+        toolchain: Option<Version>,
     ) -> CrateId {
         let data = CrateData {
             root_file_id,
@@ -441,7 +446,7 @@ impl CrateGraph {
             origin,
             target_layout,
             is_proc_macro,
-            channel,
+            toolchain,
         };
         self.arena.alloc(data)
     }
diff --git a/crates/base-db/src/lib.rs b/crates/base-db/src/lib.rs
index 918c1cd3e06..a0a55df5f99 100644
--- a/crates/base-db/src/lib.rs
+++ b/crates/base-db/src/lib.rs
@@ -23,6 +23,8 @@ pub use salsa::{self, Cancelled};
 pub use span::{FilePosition, FileRange};
 pub use vfs::{file_set::FileSet, AnchoredPath, AnchoredPathBuf, FileId, VfsPath};
 
+pub use semver::{BuildMetadata, Prerelease, Version, VersionReq};
+
 #[macro_export]
 macro_rules! impl_intern_key {
     ($name:ident) => {
diff --git a/crates/hir-expand/src/db.rs b/crates/hir-expand/src/db.rs
index dab9d5d5609..4585a532ab9 100644
--- a/crates/hir-expand/src/db.rs
+++ b/crates/hir-expand/src/db.rs
@@ -1,8 +1,10 @@
 //! Defines database & queries for macro expansion.
 
+use std::sync::OnceLock;
+
 use base_db::{
     salsa::{self, debug::DebugQueryTable},
-    CrateId, Edition, FileId, SourceDatabase,
+    CrateId, Edition, FileId, SourceDatabase, VersionReq,
 };
 use either::Either;
 use limit::Limit;
@@ -45,6 +47,9 @@ pub struct DeclarativeMacroExpander {
     pub transparency: Transparency,
 }
 
+// FIXME: Remove this once we drop support for 1.76
+static REQUIREMENT: OnceLock<VersionReq> = OnceLock::new();
+
 impl DeclarativeMacroExpander {
     pub fn expand(
         &self,
@@ -52,6 +57,18 @@ impl DeclarativeMacroExpander {
         tt: tt::Subtree,
         call_id: MacroCallId,
     ) -> ExpandResult<tt::Subtree> {
+        let toolchain = &db.crate_graph()[db.lookup_intern_macro_call(call_id).def.krate].toolchain;
+        let new_meta_vars = toolchain.as_ref().map_or(false, |version| {
+            REQUIREMENT.get_or_init(|| VersionReq::parse(">=1.76").unwrap()).matches(
+                &base_db::Version {
+                    pre: base_db::Prerelease::EMPTY,
+                    build: base_db::BuildMetadata::EMPTY,
+                    major: version.major,
+                    minor: version.minor,
+                    patch: version.patch,
+                },
+            )
+        });
         match self.mac.err() {
             Some(e) => ExpandResult::new(
                 tt::Subtree::empty(tt::DelimSpan::DUMMY),
@@ -59,18 +76,39 @@ impl DeclarativeMacroExpander {
             ),
             None => self
                 .mac
-                .expand(&tt, |s| s.ctx = apply_mark(db, s.ctx, call_id, self.transparency))
+                .expand(
+                    &tt,
+                    |s| s.ctx = apply_mark(db, s.ctx, call_id, self.transparency),
+                    new_meta_vars,
+                )
                 .map_err(Into::into),
         }
     }
 
-    pub fn expand_unhygienic(&self, tt: tt::Subtree) -> ExpandResult<tt::Subtree> {
+    pub fn expand_unhygienic(
+        &self,
+        db: &dyn ExpandDatabase,
+        tt: tt::Subtree,
+        krate: CrateId,
+    ) -> ExpandResult<tt::Subtree> {
+        let toolchain = &db.crate_graph()[krate].toolchain;
+        let new_meta_vars = toolchain.as_ref().map_or(false, |version| {
+            REQUIREMENT.get_or_init(|| VersionReq::parse(">=1.76").unwrap()).matches(
+                &base_db::Version {
+                    pre: base_db::Prerelease::EMPTY,
+                    build: base_db::BuildMetadata::EMPTY,
+                    major: version.major,
+                    minor: version.minor,
+                    patch: version.patch,
+                },
+            )
+        });
         match self.mac.err() {
             Some(e) => ExpandResult::new(
                 tt::Subtree::empty(tt::DelimSpan::DUMMY),
                 ExpandError::other(format!("invalid macro definition: {e}")),
             ),
-            None => self.mac.expand(&tt, |_| ()).map_err(Into::into),
+            None => self.mac.expand(&tt, |_| (), new_meta_vars).map_err(Into::into),
         }
     }
 }
@@ -278,7 +316,7 @@ pub fn expand_speculative(
             expander.expand(db, actual_macro_call, &adt, span_map)
         }
         MacroDefKind::Declarative(it) => {
-            db.decl_macro_expander(loc.krate, it).expand_unhygienic(tt)
+            db.decl_macro_expander(loc.krate, it).expand_unhygienic(db, tt, loc.def.krate)
         }
         MacroDefKind::BuiltIn(it, _) => it.expand(db, actual_macro_call, &tt).map_err(Into::into),
         MacroDefKind::BuiltInEager(it, _) => {
@@ -525,7 +563,8 @@ fn decl_macro_expander(
     def_crate: CrateId,
     id: AstId<ast::Macro>,
 ) -> Arc<DeclarativeMacroExpander> {
-    let is_2021 = db.crate_graph()[def_crate].edition >= Edition::Edition2021;
+    let crate_data = &db.crate_graph()[def_crate];
+    let is_2021 = crate_data.edition >= Edition::Edition2021;
     let (root, map) = parse_with_map(db, id.file_id);
     let root = root.syntax_node();
 
@@ -549,13 +588,25 @@ fn decl_macro_expander(
             _ => None,
         }
     };
+    let toolchain = crate_data.toolchain.as_ref();
+    let new_meta_vars = toolchain.as_ref().map_or(false, |version| {
+        REQUIREMENT.get_or_init(|| VersionReq::parse(">=1.76").unwrap()).matches(
+            &base_db::Version {
+                pre: base_db::Prerelease::EMPTY,
+                build: base_db::BuildMetadata::EMPTY,
+                major: version.major,
+                minor: version.minor,
+                patch: version.patch,
+            },
+        )
+    });
 
     let (mac, transparency) = match id.to_ptr(db).to_node(&root) {
         ast::Macro::MacroRules(macro_rules) => (
             match macro_rules.token_tree() {
                 Some(arg) => {
                     let tt = mbe::syntax_node_to_token_tree(arg.syntax(), map.as_ref());
-                    let mac = mbe::DeclarativeMacro::parse_macro_rules(&tt, is_2021);
+                    let mac = mbe::DeclarativeMacro::parse_macro_rules(&tt, is_2021, new_meta_vars);
                     mac
                 }
                 None => mbe::DeclarativeMacro::from_err(
@@ -569,7 +620,7 @@ fn decl_macro_expander(
             match macro_def.body() {
                 Some(arg) => {
                     let tt = mbe::syntax_node_to_token_tree(arg.syntax(), map.as_ref());
-                    let mac = mbe::DeclarativeMacro::parse_macro2(&tt, is_2021);
+                    let mac = mbe::DeclarativeMacro::parse_macro2(&tt, is_2021, new_meta_vars);
                     mac
                 }
                 None => mbe::DeclarativeMacro::from_err(
diff --git a/crates/ide-completion/src/context.rs b/crates/ide-completion/src/context.rs
index 0da7ba6d000..49acc3d7031 100644
--- a/crates/ide-completion/src/context.rs
+++ b/crates/ide-completion/src/context.rs
@@ -693,7 +693,7 @@ impl<'a> CompletionContext<'a> {
         let krate = scope.krate();
         let module = scope.module();
 
-        let toolchain = db.crate_graph()[krate.into()].channel;
+        let toolchain = db.crate_graph()[krate.into()].channel();
         // `toolchain == None` means we're in some detached files. Since we have no information on
         // the toolchain being used, let's just allow unstable items to be listed.
         let is_nightly = matches!(toolchain, Some(base_db::ReleaseChannel::Nightly) | None);
diff --git a/crates/ide/src/doc_links.rs b/crates/ide/src/doc_links.rs
index 9760f9daf0a..a36082bafcf 100644
--- a/crates/ide/src/doc_links.rs
+++ b/crates/ide/src/doc_links.rs
@@ -492,7 +492,7 @@ fn get_doc_base_urls(
     let Some(krate) = def.krate(db) else { return Default::default() };
     let Some(display_name) = krate.display_name(db) else { return Default::default() };
     let crate_data = &db.crate_graph()[krate.into()];
-    let channel = crate_data.channel.map_or("nightly", ReleaseChannel::as_str);
+    let channel = crate_data.channel().unwrap_or(ReleaseChannel::Nightly).as_str();
 
     let (web_base, local_base) = match &crate_data.origin {
         // std and co do not specify `html_root_url` any longer so we gotta handwrite this ourself.
diff --git a/crates/ide/src/shuffle_crate_graph.rs b/crates/ide/src/shuffle_crate_graph.rs
index af55b9cc987..bf6ad47a495 100644
--- a/crates/ide/src/shuffle_crate_graph.rs
+++ b/crates/ide/src/shuffle_crate_graph.rs
@@ -40,7 +40,7 @@ pub(crate) fn shuffle_crate_graph(db: &mut RootDatabase) {
             data.is_proc_macro,
             data.origin.clone(),
             data.target_layout.clone(),
-            data.channel,
+            data.toolchain.clone(),
         );
         new_proc_macros.insert(new_id, proc_macros[&old_id].clone());
         map.insert(old_id, new_id);
diff --git a/crates/mbe/src/benchmark.rs b/crates/mbe/src/benchmark.rs
index 1d8c94848f7..22e962a8147 100644
--- a/crates/mbe/src/benchmark.rs
+++ b/crates/mbe/src/benchmark.rs
@@ -20,7 +20,10 @@ fn benchmark_parse_macro_rules() {
     let rules = macro_rules_fixtures_tt();
     let hash: usize = {
         let _pt = bench("mbe parse macro rules");
-        rules.values().map(|it| DeclarativeMacro::parse_macro_rules(it, true).rules.len()).sum()
+        rules
+            .values()
+            .map(|it| DeclarativeMacro::parse_macro_rules(it, true, true).rules.len())
+            .sum()
     };
     assert_eq!(hash, 1144);
 }
@@ -38,7 +41,7 @@ fn benchmark_expand_macro_rules() {
         invocations
             .into_iter()
             .map(|(id, tt)| {
-                let res = rules[&id].expand(&tt, |_| ());
+                let res = rules[&id].expand(&tt, |_| (), true);
                 assert!(res.err.is_none());
                 res.value.token_trees.len()
             })
@@ -50,7 +53,7 @@ fn benchmark_expand_macro_rules() {
 fn macro_rules_fixtures() -> FxHashMap<String, DeclarativeMacro<DummyTestSpanData>> {
     macro_rules_fixtures_tt()
         .into_iter()
-        .map(|(id, tt)| (id, DeclarativeMacro::parse_macro_rules(&tt, true)))
+        .map(|(id, tt)| (id, DeclarativeMacro::parse_macro_rules(&tt, true, true)))
         .collect()
 }
 
@@ -105,7 +108,7 @@ fn invocation_fixtures(
                     for op in rule.lhs.iter() {
                         collect_from_op(op, &mut subtree, &mut seed);
                     }
-                    if it.expand(&subtree, |_| ()).err.is_none() {
+                    if it.expand(&subtree, |_| (), true).err.is_none() {
                         res.push((name.clone(), subtree));
                         break;
                     }
diff --git a/crates/mbe/src/expander.rs b/crates/mbe/src/expander.rs
index 0e755f69bf7..11c0ac25828 100644
--- a/crates/mbe/src/expander.rs
+++ b/crates/mbe/src/expander.rs
@@ -16,6 +16,7 @@ pub(crate) fn expand_rules<S: Span>(
     input: &tt::Subtree<S>,
     marker: impl Fn(&mut S) + Copy,
     is_2021: bool,
+    new_meta_vars: bool,
 ) -> ExpandResult<tt::Subtree<S>> {
     let mut match_: Option<(matcher::Match<S>, &crate::Rule<S>)> = None;
     for rule in rules {
@@ -26,7 +27,7 @@ pub(crate) fn expand_rules<S: Span>(
             // Unconditionally returning the transcription here makes the
             // `test_repeat_bad_var` test fail.
             let ExpandResult { value, err: transcribe_err } =
-                transcriber::transcribe(&rule.rhs, &new_match.bindings, marker);
+                transcriber::transcribe(&rule.rhs, &new_match.bindings, marker, new_meta_vars);
             if transcribe_err.is_none() {
                 return ExpandResult::ok(value);
             }
@@ -45,7 +46,7 @@ pub(crate) fn expand_rules<S: Span>(
     if let Some((match_, rule)) = match_ {
         // if we got here, there was no match without errors
         let ExpandResult { value, err: transcribe_err } =
-            transcriber::transcribe(&rule.rhs, &match_.bindings, marker);
+            transcriber::transcribe(&rule.rhs, &match_.bindings, marker, new_meta_vars);
         ExpandResult { value, err: match_.err.or(transcribe_err) }
     } else {
         ExpandResult::new(
diff --git a/crates/mbe/src/expander/transcriber.rs b/crates/mbe/src/expander/transcriber.rs
index a4dbd907a37..d1bcf3dbccc 100644
--- a/crates/mbe/src/expander/transcriber.rs
+++ b/crates/mbe/src/expander/transcriber.rs
@@ -131,8 +131,9 @@ pub(super) fn transcribe<S: Span>(
     template: &MetaTemplate<S>,
     bindings: &Bindings<S>,
     marker: impl Fn(&mut S) + Copy,
+    new_meta_vars: bool,
 ) -> ExpandResult<tt::Subtree<S>> {
-    let mut ctx = ExpandCtx { bindings, nesting: Vec::new() };
+    let mut ctx = ExpandCtx { bindings, nesting: Vec::new(), new_meta_vars };
     let mut arena: Vec<tt::TokenTree<S>> = Vec::new();
     expand_subtree(&mut ctx, template, None, &mut arena, marker)
 }
@@ -152,6 +153,7 @@ struct NestingState {
 struct ExpandCtx<'a, S> {
     bindings: &'a Bindings<S>,
     nesting: Vec<NestingState>,
+    new_meta_vars: bool,
 }
 
 fn expand_subtree<S: Span>(
@@ -284,7 +286,13 @@ fn expand_subtree<S: Span>(
                     }
                 }
 
-                let c = match count(ctx, binding, 0, *depth) {
+                let res = if ctx.new_meta_vars {
+                    count(ctx, binding, 0, depth.unwrap_or(0))
+                } else {
+                    count_old(ctx, binding, 0, *depth)
+                };
+
+                let c = match res {
                     Ok(c) => c,
                     Err(e) => {
                         // XXX: It *might* make sense to emit a dummy integer value like `0` here.
@@ -548,3 +556,32 @@ fn count<S>(
         Binding::Fragment(_) | Binding::Missing(_) => Ok(1),
     }
 }
+
+fn count_old<S>(
+    ctx: &ExpandCtx<'_, S>,
+    binding: &Binding<S>,
+    our_depth: usize,
+    count_depth: Option<usize>,
+) -> Result<usize, CountError> {
+    match binding {
+        Binding::Nested(bs) => match count_depth {
+            None => bs.iter().map(|b| count_old(ctx, b, our_depth + 1, None)).sum(),
+            Some(0) => Ok(bs.len()),
+            Some(d) => bs.iter().map(|b| count_old(ctx, b, our_depth + 1, Some(d - 1))).sum(),
+        },
+        Binding::Empty => Ok(0),
+        Binding::Fragment(_) | Binding::Missing(_) => {
+            if our_depth == 0 {
+                // `${count(t)}` is placed inside the innermost repetition. This includes cases
+                // where `t` is not a repeated fragment.
+                Err(CountError::Misplaced)
+            } else if count_depth.is_none() {
+                Ok(1)
+            } else {
+                // We've reached at the innermost repeated fragment, but the user wants us to go
+                // further!
+                Err(CountError::OutOfBounds)
+            }
+        }
+    }
+}
diff --git a/crates/mbe/src/lib.rs b/crates/mbe/src/lib.rs
index ee9b2bf700c..d0b0da9724b 100644
--- a/crates/mbe/src/lib.rs
+++ b/crates/mbe/src/lib.rs
@@ -147,7 +147,12 @@ impl<S: Span> DeclarativeMacro<S> {
     }
 
     /// The old, `macro_rules! m {}` flavor.
-    pub fn parse_macro_rules(tt: &tt::Subtree<S>, is_2021: bool) -> DeclarativeMacro<S> {
+    pub fn parse_macro_rules(
+        tt: &tt::Subtree<S>,
+        is_2021: bool,
+        // FIXME: Remove this once we drop support for rust 1.76 (defaults to true then)
+        new_meta_vars: bool,
+    ) -> DeclarativeMacro<S> {
         // Note: this parsing can be implemented using mbe machinery itself, by
         // matching against `$($lhs:tt => $rhs:tt);*` pattern, but implementing
         // manually seems easier.
@@ -156,7 +161,7 @@ impl<S: Span> DeclarativeMacro<S> {
         let mut err = None;
 
         while src.len() > 0 {
-            let rule = match Rule::parse(&mut src, true) {
+            let rule = match Rule::parse(&mut src, true, new_meta_vars) {
                 Ok(it) => it,
                 Err(e) => {
                     err = Some(Box::new(e));
@@ -183,7 +188,12 @@ impl<S: Span> DeclarativeMacro<S> {
     }
 
     /// The new, unstable `macro m {}` flavor.
-    pub fn parse_macro2(tt: &tt::Subtree<S>, is_2021: bool) -> DeclarativeMacro<S> {
+    pub fn parse_macro2(
+        tt: &tt::Subtree<S>,
+        is_2021: bool,
+        // FIXME: Remove this once we drop support for rust 1.76 (defaults to true then)
+        new_meta_vars: bool,
+    ) -> DeclarativeMacro<S> {
         let mut src = TtIter::new(tt);
         let mut rules = Vec::new();
         let mut err = None;
@@ -191,7 +201,7 @@ impl<S: Span> DeclarativeMacro<S> {
         if tt::DelimiterKind::Brace == tt.delimiter.kind {
             cov_mark::hit!(parse_macro_def_rules);
             while src.len() > 0 {
-                let rule = match Rule::parse(&mut src, true) {
+                let rule = match Rule::parse(&mut src, true, new_meta_vars) {
                     Ok(it) => it,
                     Err(e) => {
                         err = Some(Box::new(e));
@@ -210,7 +220,7 @@ impl<S: Span> DeclarativeMacro<S> {
             }
         } else {
             cov_mark::hit!(parse_macro_def_simple);
-            match Rule::parse(&mut src, false) {
+            match Rule::parse(&mut src, false, new_meta_vars) {
                 Ok(rule) => {
                     if src.len() != 0 {
                         err = Some(Box::new(ParseError::expected("remaining tokens in macro def")));
@@ -241,13 +251,18 @@ impl<S: Span> DeclarativeMacro<S> {
         &self,
         tt: &tt::Subtree<S>,
         marker: impl Fn(&mut S) + Copy,
+        new_meta_vars: bool,
     ) -> ExpandResult<tt::Subtree<S>> {
-        expander::expand_rules(&self.rules, &tt, marker, self.is_2021)
+        expander::expand_rules(&self.rules, &tt, marker, self.is_2021, new_meta_vars)
     }
 }
 
 impl<S: Span> Rule<S> {
-    fn parse(src: &mut TtIter<'_, S>, expect_arrow: bool) -> Result<Self, ParseError> {
+    fn parse(
+        src: &mut TtIter<'_, S>,
+        expect_arrow: bool,
+        new_meta_vars: bool,
+    ) -> Result<Self, ParseError> {
         let lhs = src.expect_subtree().map_err(|()| ParseError::expected("expected subtree"))?;
         if expect_arrow {
             src.expect_char('=').map_err(|()| ParseError::expected("expected `=`"))?;
@@ -256,7 +271,7 @@ impl<S: Span> Rule<S> {
         let rhs = src.expect_subtree().map_err(|()| ParseError::expected("expected subtree"))?;
 
         let lhs = MetaTemplate::parse_pattern(lhs)?;
-        let rhs = MetaTemplate::parse_template(rhs)?;
+        let rhs = MetaTemplate::parse_template(rhs, new_meta_vars)?;
 
         Ok(crate::Rule { lhs, rhs })
     }
diff --git a/crates/mbe/src/parser.rs b/crates/mbe/src/parser.rs
index 5f1815304ed..afdbbef2314 100644
--- a/crates/mbe/src/parser.rs
+++ b/crates/mbe/src/parser.rs
@@ -25,23 +25,26 @@ pub(crate) struct MetaTemplate<S>(pub(crate) Box<[Op<S>]>);
 
 impl<S: Span> MetaTemplate<S> {
     pub(crate) fn parse_pattern(pattern: &tt::Subtree<S>) -> Result<Self, ParseError> {
-        MetaTemplate::parse(pattern, Mode::Pattern)
+        MetaTemplate::parse(pattern, Mode::Pattern, false)
     }
 
-    pub(crate) fn parse_template(template: &tt::Subtree<S>) -> Result<Self, ParseError> {
-        MetaTemplate::parse(template, Mode::Template)
+    pub(crate) fn parse_template(
+        template: &tt::Subtree<S>,
+        new_meta_vars: bool,
+    ) -> Result<Self, ParseError> {
+        MetaTemplate::parse(template, Mode::Template, new_meta_vars)
     }
 
     pub(crate) fn iter(&self) -> impl Iterator<Item = &Op<S>> {
         self.0.iter()
     }
 
-    fn parse(tt: &tt::Subtree<S>, mode: Mode) -> Result<Self, ParseError> {
+    fn parse(tt: &tt::Subtree<S>, mode: Mode, new_meta_vars: bool) -> Result<Self, ParseError> {
         let mut src = TtIter::new(tt);
 
         let mut res = Vec::new();
         while let Some(first) = src.peek_n(0) {
-            let op = next_op(first, &mut src, mode)?;
+            let op = next_op(first, &mut src, mode, new_meta_vars)?;
             res.push(op);
         }
 
@@ -51,13 +54,35 @@ impl<S: Span> MetaTemplate<S> {
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub(crate) enum Op<S> {
-    Var { name: SmolStr, kind: Option<MetaVarKind>, id: S },
-    Ignore { name: SmolStr, id: S },
-    Index { depth: usize },
-    Length { depth: usize },
-    Count { name: SmolStr, depth: usize },
-    Repeat { tokens: MetaTemplate<S>, kind: RepeatKind, separator: Option<Separator<S>> },
-    Subtree { tokens: MetaTemplate<S>, delimiter: tt::Delimiter<S> },
+    Var {
+        name: SmolStr,
+        kind: Option<MetaVarKind>,
+        id: S,
+    },
+    Ignore {
+        name: SmolStr,
+        id: S,
+    },
+    Index {
+        depth: usize,
+    },
+    Length {
+        depth: usize,
+    },
+    Count {
+        name: SmolStr,
+        // FIXME: `usize`` once we drop support for 1.76
+        depth: Option<usize>,
+    },
+    Repeat {
+        tokens: MetaTemplate<S>,
+        kind: RepeatKind,
+        separator: Option<Separator<S>>,
+    },
+    Subtree {
+        tokens: MetaTemplate<S>,
+        delimiter: tt::Delimiter<S>,
+    },
     Literal(tt::Literal<S>),
     Punct(SmallVec<[tt::Punct<S>; 3]>),
     Ident(tt::Ident<S>),
@@ -123,6 +148,7 @@ fn next_op<S: Span>(
     first_peeked: &tt::TokenTree<S>,
     src: &mut TtIter<'_, S>,
     mode: Mode,
+    new_meta_vars: bool,
 ) -> Result<Op<S>, ParseError> {
     let res = match first_peeked {
         tt::TokenTree::Leaf(tt::Leaf::Punct(p @ tt::Punct { char: '$', .. })) => {
@@ -136,14 +162,14 @@ fn next_op<S: Span>(
                 tt::TokenTree::Subtree(subtree) => match subtree.delimiter.kind {
                     tt::DelimiterKind::Parenthesis => {
                         let (separator, kind) = parse_repeat(src)?;
-                        let tokens = MetaTemplate::parse(subtree, mode)?;
+                        let tokens = MetaTemplate::parse(subtree, mode, new_meta_vars)?;
                         Op::Repeat { tokens, separator, kind }
                     }
                     tt::DelimiterKind::Brace => match mode {
                         Mode::Template => {
-                            parse_metavar_expr(&mut TtIter::new(subtree)).map_err(|()| {
-                                ParseError::unexpected("invalid metavariable expression")
-                            })?
+                            parse_metavar_expr(new_meta_vars, &mut TtIter::new(subtree)).map_err(
+                                |()| ParseError::unexpected("invalid metavariable expression"),
+                            )?
                         }
                         Mode::Pattern => {
                             return Err(ParseError::unexpected(
@@ -207,7 +233,7 @@ fn next_op<S: Span>(
 
         tt::TokenTree::Subtree(subtree) => {
             src.next().expect("first token already peeked");
-            let tokens = MetaTemplate::parse(subtree, mode)?;
+            let tokens = MetaTemplate::parse(subtree, mode, new_meta_vars)?;
             Op::Subtree { tokens, delimiter: subtree.delimiter }
         }
     };
@@ -288,7 +314,7 @@ fn parse_repeat<S: Span>(
     Err(ParseError::InvalidRepeat)
 }
 
-fn parse_metavar_expr<S: Span>(src: &mut TtIter<'_, S>) -> Result<Op<S>, ()> {
+fn parse_metavar_expr<S: Span>(new_meta_vars: bool, src: &mut TtIter<'_, S>) -> Result<Op<S>, ()> {
     let func = src.expect_ident()?;
     let args = src.expect_subtree()?;
 
@@ -300,16 +326,20 @@ fn parse_metavar_expr<S: Span>(src: &mut TtIter<'_, S>) -> Result<Op<S>, ()> {
 
     let op = match &*func.text {
         "ignore" => {
-            args.expect_dollar()?;
+            if new_meta_vars {
+                args.expect_dollar()?;
+            }
             let ident = args.expect_ident()?;
             Op::Ignore { name: ident.text.clone(), id: ident.span }
         }
         "index" => Op::Index { depth: parse_depth(&mut args)? },
         "length" => Op::Length { depth: parse_depth(&mut args)? },
         "count" => {
-            args.expect_dollar()?;
+            if new_meta_vars {
+                args.expect_dollar()?;
+            }
             let ident = args.expect_ident()?;
-            let depth = if try_eat_comma(&mut args) { parse_depth(&mut args)? } else { 0 };
+            let depth = if try_eat_comma(&mut args) { Some(parse_depth(&mut args)?) } else { None };
             Op::Count { name: ident.text.clone(), depth }
         }
         _ => return Err(()),
diff --git a/crates/project-model/Cargo.toml b/crates/project-model/Cargo.toml
index c1243b5932a..a36c9aca52d 100644
--- a/crates/project-model/Cargo.toml
+++ b/crates/project-model/Cargo.toml
@@ -15,7 +15,7 @@ doctest = false
 anyhow.workspace = true
 cargo_metadata.workspace = true
 rustc-hash.workspace = true
-semver = "1.0.14"
+semver.workspace = true
 serde_json.workspace = true
 serde.workspace = true
 tracing.workspace = true
diff --git a/crates/project-model/src/workspace.rs b/crates/project-model/src/workspace.rs
index 9333570354a..4057493fa3a 100644
--- a/crates/project-model/src/workspace.rs
+++ b/crates/project-model/src/workspace.rs
@@ -7,7 +7,7 @@ use std::{collections::VecDeque, fmt, fs, iter, process::Command, str::FromStr,
 use anyhow::{format_err, Context};
 use base_db::{
     CrateDisplayName, CrateGraph, CrateId, CrateName, CrateOrigin, Dependency, DependencyKind,
-    Edition, Env, FileId, LangCrateOrigin, ProcMacroPaths, ReleaseChannel, TargetLayoutLoadResult,
+    Edition, Env, FileId, LangCrateOrigin, ProcMacroPaths, TargetLayoutLoadResult,
 };
 use cfg::{CfgDiff, CfgOptions};
 use paths::{AbsPath, AbsPathBuf};
@@ -619,7 +619,7 @@ impl ProjectWorkspace {
                     sysroot.as_ref().ok(),
                     extra_env,
                     Err("rust-project.json projects have no target layout set".into()),
-                    toolchain.as_ref().and_then(|it| ReleaseChannel::from_str(it.pre.as_str())),
+                    toolchain.clone(),
                 )
             }
             ProjectWorkspace::Cargo {
@@ -644,7 +644,7 @@ impl ProjectWorkspace {
                     Ok(it) => Ok(Arc::from(it.as_str())),
                     Err(it) => Err(Arc::from(it.as_str())),
                 },
-                toolchain.as_ref().and_then(|it| ReleaseChannel::from_str(it.pre.as_str())),
+                toolchain.as_ref(),
             ),
             ProjectWorkspace::DetachedFiles { files, sysroot, rustc_cfg } => {
                 detached_files_to_crate_graph(
@@ -733,7 +733,7 @@ fn project_json_to_crate_graph(
     sysroot: Option<&Sysroot>,
     extra_env: &FxHashMap<String, String>,
     target_layout: TargetLayoutLoadResult,
-    channel: Option<ReleaseChannel>,
+    toolchain: Option<Version>,
 ) -> (CrateGraph, ProcMacroPaths) {
     let mut res = (CrateGraph::default(), ProcMacroPaths::default());
     let (crate_graph, proc_macros) = &mut res;
@@ -744,7 +744,7 @@ fn project_json_to_crate_graph(
             rustc_cfg.clone(),
             target_layout.clone(),
             load,
-            channel,
+            toolchain.as_ref(),
         )
     });
 
@@ -807,7 +807,7 @@ fn project_json_to_crate_graph(
                         CrateOrigin::Local { repo: None, name: None }
                     },
                     target_layout.clone(),
-                    channel,
+                    toolchain.clone(),
                 );
                 if *is_proc_macro {
                     if let Some(path) = proc_macro_dylib_path.clone() {
@@ -853,7 +853,7 @@ fn cargo_to_crate_graph(
     forced_cfg: Option<CfgOptions>,
     build_scripts: &WorkspaceBuildScripts,
     target_layout: TargetLayoutLoadResult,
-    channel: Option<ReleaseChannel>,
+    toolchain: Option<&Version>,
 ) -> (CrateGraph, ProcMacroPaths) {
     let _p = profile::span("cargo_to_crate_graph");
     let mut res = (CrateGraph::default(), ProcMacroPaths::default());
@@ -866,7 +866,7 @@ fn cargo_to_crate_graph(
             rustc_cfg.clone(),
             target_layout.clone(),
             load,
-            channel,
+            toolchain,
         ),
         None => (SysrootPublicDeps::default(), None),
     };
@@ -950,7 +950,7 @@ fn cargo_to_crate_graph(
                 is_proc_macro,
                 target_layout.clone(),
                 false,
-                channel,
+                toolchain.cloned(),
             );
             if kind == TargetKind::Lib {
                 lib_tgt = Some((crate_id, name.clone()));
@@ -1038,7 +1038,7 @@ fn cargo_to_crate_graph(
                     rustc_build_scripts
                 },
                 target_layout,
-                channel,
+                toolchain,
             );
         }
     }
@@ -1117,7 +1117,7 @@ fn handle_rustc_crates(
     override_cfg: &CfgOverrides,
     build_scripts: &WorkspaceBuildScripts,
     target_layout: TargetLayoutLoadResult,
-    channel: Option<ReleaseChannel>,
+    toolchain: Option<&Version>,
 ) {
     let mut rustc_pkg_crates = FxHashMap::default();
     // The root package of the rustc-dev component is rustc_driver, so we match that
@@ -1172,7 +1172,7 @@ fn handle_rustc_crates(
                         rustc_workspace[tgt].is_proc_macro,
                         target_layout.clone(),
                         true,
-                        channel,
+                        toolchain.cloned(),
                     );
                     pkg_to_lib_crate.insert(pkg, crate_id);
                     // Add dependencies on core / std / alloc for this crate
@@ -1248,7 +1248,7 @@ fn add_target_crate_root(
     is_proc_macro: bool,
     target_layout: TargetLayoutLoadResult,
     rustc_crate: bool,
-    channel: Option<ReleaseChannel>,
+    toolchain: Option<Version>,
 ) -> CrateId {
     let edition = pkg.edition;
     let potential_cfg_options = if pkg.features.is_empty() {
@@ -1304,7 +1304,7 @@ fn add_target_crate_root(
             CrateOrigin::Library { repo: pkg.repository.clone(), name: pkg.name.clone() }
         },
         target_layout,
-        channel,
+        toolchain,
     );
     if is_proc_macro {
         let proc_macro = match build_data.as_ref().map(|it| it.proc_macro_dylib_path.as_ref()) {
@@ -1346,7 +1346,7 @@ fn sysroot_to_crate_graph(
     rustc_cfg: Vec<CfgFlag>,
     target_layout: TargetLayoutLoadResult,
     load: &mut dyn FnMut(&AbsPath) -> Option<FileId>,
-    channel: Option<ReleaseChannel>,
+    toolchain: Option<&Version>,
 ) -> (SysrootPublicDeps, Option<CrateId>) {
     let _p = profile::span("sysroot_to_crate_graph");
     let cfg_options = create_cfg_options(rustc_cfg.clone());
@@ -1357,7 +1357,7 @@ fn sysroot_to_crate_graph(
             rustc_cfg,
             cfg_options,
             target_layout,
-            channel,
+            toolchain,
             crate_graph,
             sysroot,
         ),
@@ -1380,7 +1380,7 @@ fn sysroot_to_crate_graph(
                     false,
                     CrateOrigin::Lang(LangCrateOrigin::from(&*sysroot[krate].name)),
                     target_layout.clone(),
-                    channel,
+                    toolchain.cloned(),
                 );
                 Some((krate, crate_id))
             })
@@ -1412,7 +1412,7 @@ fn handle_hack_cargo_workspace(
     rustc_cfg: Vec<CfgFlag>,
     cfg_options: CfgOptions,
     target_layout: Result<Arc<str>, Arc<str>>,
-    channel: Option<ReleaseChannel>,
+    toolchain: Option<&Version>,
     crate_graph: &mut CrateGraph,
     sysroot: &Sysroot,
 ) -> FxHashMap<SysrootCrate, CrateId> {
@@ -1426,7 +1426,7 @@ fn handle_hack_cargo_workspace(
         Some(cfg_options),
         &WorkspaceBuildScripts::default(),
         target_layout,
-        channel,
+        toolchain,
     );
     crate_graph.extend(cg, &mut pm);
     for crate_name in ["std", "alloc", "core"] {
diff --git a/crates/project-model/test_data/output/cargo_hello_world_project_model.txt b/crates/project-model/test_data/output/cargo_hello_world_project_model.txt
index e98f016ca7d..d8d9e559e5c 100644
--- a/crates/project-model/test_data/output/cargo_hello_world_project_model.txt
+++ b/crates/project-model/test_data/output/cargo_hello_world_project_model.txt
@@ -62,7 +62,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     1: CrateData {
         root_file_id: FileId(
@@ -135,7 +135,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     2: CrateData {
         root_file_id: FileId(
@@ -208,7 +208,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     3: CrateData {
         root_file_id: FileId(
@@ -281,7 +281,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     4: CrateData {
         root_file_id: FileId(
@@ -350,6 +350,6 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
 }
\ No newline at end of file
diff --git a/crates/project-model/test_data/output/cargo_hello_world_project_model_with_selective_overrides.txt b/crates/project-model/test_data/output/cargo_hello_world_project_model_with_selective_overrides.txt
index e98f016ca7d..d8d9e559e5c 100644
--- a/crates/project-model/test_data/output/cargo_hello_world_project_model_with_selective_overrides.txt
+++ b/crates/project-model/test_data/output/cargo_hello_world_project_model_with_selective_overrides.txt
@@ -62,7 +62,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     1: CrateData {
         root_file_id: FileId(
@@ -135,7 +135,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     2: CrateData {
         root_file_id: FileId(
@@ -208,7 +208,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     3: CrateData {
         root_file_id: FileId(
@@ -281,7 +281,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     4: CrateData {
         root_file_id: FileId(
@@ -350,6 +350,6 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
 }
\ No newline at end of file
diff --git a/crates/project-model/test_data/output/cargo_hello_world_project_model_with_wildcard_overrides.txt b/crates/project-model/test_data/output/cargo_hello_world_project_model_with_wildcard_overrides.txt
index 7ecd53572e2..e0ba5ed498f 100644
--- a/crates/project-model/test_data/output/cargo_hello_world_project_model_with_wildcard_overrides.txt
+++ b/crates/project-model/test_data/output/cargo_hello_world_project_model_with_wildcard_overrides.txt
@@ -61,7 +61,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     1: CrateData {
         root_file_id: FileId(
@@ -133,7 +133,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     2: CrateData {
         root_file_id: FileId(
@@ -205,7 +205,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     3: CrateData {
         root_file_id: FileId(
@@ -277,7 +277,7 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
     4: CrateData {
         root_file_id: FileId(
@@ -346,6 +346,6 @@
         target_layout: Err(
             "target_data_layout not loaded",
         ),
-        channel: None,
+        toolchain: None,
     },
 }
\ No newline at end of file
diff --git a/crates/project-model/test_data/output/rust_project_hello_world_project_model.txt b/crates/project-model/test_data/output/rust_project_hello_world_project_model.txt
index 581a6afc148..e35f0fc7327 100644
--- a/crates/project-model/test_data/output/rust_project_hello_world_project_model.txt
+++ b/crates/project-model/test_data/output/rust_project_hello_world_project_model.txt
@@ -39,7 +39,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     1: CrateData {
         root_file_id: FileId(
@@ -72,7 +72,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     2: CrateData {
         root_file_id: FileId(
@@ -105,7 +105,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     3: CrateData {
         root_file_id: FileId(
@@ -138,7 +138,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     4: CrateData {
         root_file_id: FileId(
@@ -188,7 +188,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     5: CrateData {
         root_file_id: FileId(
@@ -221,7 +221,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     6: CrateData {
         root_file_id: FileId(
@@ -319,7 +319,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     7: CrateData {
         root_file_id: FileId(
@@ -352,7 +352,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     8: CrateData {
         root_file_id: FileId(
@@ -385,7 +385,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     9: CrateData {
         root_file_id: FileId(
@@ -418,7 +418,7 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
     10: CrateData {
         root_file_id: FileId(
@@ -495,6 +495,6 @@
         target_layout: Err(
             "rust-project.json projects have no target layout set",
         ),
-        channel: None,
+        toolchain: None,
     },
 }
\ No newline at end of file
diff --git a/crates/test-fixture/src/lib.rs b/crates/test-fixture/src/lib.rs
index 8c5f9309072..1a042b2dea2 100644
--- a/crates/test-fixture/src/lib.rs
+++ b/crates/test-fixture/src/lib.rs
@@ -3,8 +3,8 @@ use std::{mem, ops::Not, str::FromStr, sync};
 
 use base_db::{
     CrateDisplayName, CrateGraph, CrateId, CrateName, CrateOrigin, Dependency, DependencyKind,
-    Edition, Env, FileChange, FileSet, LangCrateOrigin, ReleaseChannel, SourceDatabaseExt,
-    SourceRoot, VfsPath,
+    Edition, Env, FileChange, FileSet, LangCrateOrigin, SourceDatabaseExt, SourceRoot, Version,
+    VfsPath,
 };
 use cfg::CfgOptions;
 use hir_expand::{
@@ -120,12 +120,10 @@ impl ChangeFixture {
     ) -> ChangeFixture {
         let FixtureWithProjectMeta { fixture, mini_core, proc_macro_names, toolchain } =
             FixtureWithProjectMeta::parse(ra_fixture);
-        let toolchain = toolchain
-            .map(|it| {
-                ReleaseChannel::from_str(&it)
-                    .unwrap_or_else(|| panic!("unknown release channel found: {it}"))
-            })
-            .unwrap_or(ReleaseChannel::Stable);
+        let toolchain = Some({
+            let channel = toolchain.as_deref().unwrap_or("stable");
+            Version::parse(&format!("1.76.0-{channel}")).unwrap()
+        });
         let mut source_change = FileChange::new();
 
         let mut files = Vec::new();
@@ -193,7 +191,7 @@ impl ChangeFixture {
                         .as_deref()
                         .map(From::from)
                         .ok_or_else(|| "target_data_layout unset".into()),
-                    Some(toolchain),
+                    toolchain.clone(),
                 );
                 let prev = crates.insert(crate_name.clone(), crate_id);
                 assert!(prev.is_none(), "multiple crates with same name: {}", crate_name);
@@ -233,7 +231,7 @@ impl ChangeFixture {
                 default_target_data_layout
                     .map(|it| it.into())
                     .ok_or_else(|| "target_data_layout unset".into()),
-                Some(toolchain),
+                toolchain.clone(),
             );
         } else {
             for (from, to, prelude) in crate_deps {
@@ -280,7 +278,7 @@ impl ChangeFixture {
                 false,
                 CrateOrigin::Lang(LangCrateOrigin::Core),
                 target_layout.clone(),
-                Some(toolchain),
+                toolchain.clone(),
             );
 
             for krate in all_crates {
@@ -325,7 +323,7 @@ impl ChangeFixture {
                 true,
                 CrateOrigin::Local { repo: None, name: None },
                 target_layout,
-                Some(toolchain),
+                toolchain,
             );
             proc_macros.insert(proc_macros_crate, Ok(proc_macro));