5 files changed, 20 insertions, 2 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 87cf1bf27..8059a8809 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -524,6 +524,7 @@ name = "hir_expand"
 version = "0.0.0"
 dependencies = [
  "base_db",
+ "cfg",
  "either",
  "la-arena",
  "log",
diff --git a/crates/hir_expand/Cargo.toml b/crates/hir_expand/Cargo.toml
index 76cb03126..7bb40ba9b 100644
--- a/crates/hir_expand/Cargo.toml
+++ b/crates/hir_expand/Cargo.toml
@@ -16,6 +16,7 @@ rustc-hash = "1.0.0"
 la-arena = { version = "0.2.0", path = "../../lib/arena" }
 
 base_db = { path = "../base_db", version = "0.0.0" }
+cfg = { path = "../cfg", version = "0.0.0" }
 syntax = { path = "../syntax", version = "0.0.0" }
 parser = { path = "../parser", version = "0.0.0" }
 profile = { path = "../profile", version = "0.0.0" }
diff --git a/crates/hir_expand/src/builtin_macro.rs b/crates/hir_expand/src/builtin_macro.rs
index eb57ea7d6..b34f1a4f2 100644
--- a/crates/hir_expand/src/builtin_macro.rs
+++ b/crates/hir_expand/src/builtin_macro.rs
@@ -5,6 +5,7 @@ use crate::{
 };
 
 use base_db::{AnchoredPath, FileId};
+use cfg::CfgExpr;
 use either::Either;
 use mbe::{parse_exprs_with_sep, parse_to_token_tree, ExpandResult};
 use parser::FragmentKind;
@@ -97,6 +98,7 @@ register_builtin! {
     (format_args_nl, FormatArgsNl) => format_args_expand,
     (llvm_asm, LlvmAsm) => asm_expand,
     (asm, Asm) => asm_expand,
+    (cfg, Cfg) => cfg_expand,
 
     EAGER:
     (compile_error, CompileError) => compile_error_expand,
@@ -258,6 +260,18 @@ fn asm_expand(
     ExpandResult::ok(expanded)
 }
 
+fn cfg_expand(
+    db: &dyn AstDatabase,
+    id: LazyMacroId,
+    tt: &tt::Subtree,
+) -> ExpandResult<tt::Subtree> {
+    let loc = db.lookup_intern_macro(id);
+    let expr = CfgExpr::parse(tt);
+    let enabled = db.crate_graph()[loc.krate].cfg_options.check(&expr) != Some(false);
+    let expanded = if enabled { quote!(true) } else { quote!(false) };
+    ExpandResult::ok(expanded)
+}
+
 fn unquote_str(lit: &tt::Literal) -> Option<String> {
     let lit = ast::make::tokens::literal(&lit.to_string());
     let token = ast::String::cast(lit)?;
diff --git a/crates/hir_expand/src/name.rs b/crates/hir_expand/src/name.rs
index c94fb580a..e833e032c 100644
--- a/crates/hir_expand/src/name.rs
+++ b/crates/hir_expand/src/name.rs
@@ -154,6 +154,7 @@ pub mod known {
         macro_rules,
         derive,
         doc,
+        cfg,
         cfg_attr,
         // Components of known path (value or mod name)
         std,
diff --git a/crates/hir_expand/src/quote.rs b/crates/hir_expand/src/quote.rs
index 219bc2097..08bc5aa49 100644
--- a/crates/hir_expand/src/quote.rs
+++ b/crates/hir_expand/src/quote.rs
@@ -188,8 +188,9 @@ macro_rules! impl_to_to_tokentrees {
 
 impl_to_to_tokentrees! {
     u32 => self { tt::Literal{text: self.to_string().into(), id: tt::TokenId::unspecified()} };
-    usize => self { tt::Literal{text: self.to_string().into(), id: tt::TokenId::unspecified()}};
-    i32 => self { tt::Literal{text: self.to_string().into(), id: tt::TokenId::unspecified()}};
+    usize => self { tt::Literal{text: self.to_string().into(), id: tt::TokenId::unspecified()} };
+    i32 => self { tt::Literal{text: self.to_string().into(), id: tt::TokenId::unspecified()} };
+    bool => self { tt::Ident{text: self.to_string().into(), id: tt::TokenId::unspecified()} };
     tt::Leaf => self { self };
     tt::Literal => self { self };
     tt::Ident => self { self };