3 files changed, 84 insertions, 7 deletions

diff --git a/crates/ra_hir/src/ty/tests.rs b/crates/ra_hir/src/ty/tests.rs
index e56b9356e..896bf2924 100644
--- a/crates/ra_hir/src/ty/tests.rs
+++ b/crates/ra_hir/src/ty/tests.rs
@@ -4810,3 +4810,22 @@ fn no_such_field_diagnostics() {
     "###
     );
 }
+
+#[test]
+fn infer_builtin_macros_line() {
+    assert_snapshot!(
+        infer(r#"
+#[rustc_builtin_macro]
+macro_rules! line {() => {}}
+
+fn main() {
+    let x = line!();
+}
+"#),
+        @r###"
+        ![0; 1) '6': i32
+        [64; 88) '{     ...!(); }': ()
+        [74; 75) 'x': i32        
+    "###
+    );
+}
diff --git a/crates/ra_hir_expand/src/builtin_macro.rs b/crates/ra_hir_expand/src/builtin_macro.rs
index dca2f17ef..acb62da27 100644
--- a/crates/ra_hir_expand/src/builtin_macro.rs
+++ b/crates/ra_hir_expand/src/builtin_macro.rs
@@ -1,14 +1,28 @@
 //! Builtin macro
-use crate::{ast, name, AstId, BuiltinMacro, CrateId, MacroDefId};
+use crate::db::AstDatabase;
+use crate::{
+    ast::{self, AstNode},
+    name, AstId, BuiltinMacro, CrateId, HirFileId, MacroCallId, MacroDefId, MacroFileKind,
+    TextUnit,
+};
+
+use crate::quote;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum BuiltinExpander {
-    Line
+    Line,
 }
 
 impl BuiltinExpander {
-    pub fn expand(&self, _tt: &tt::Subtree) -> Result<tt::Subtree, mbe::ExpandError> {
-        Err(mbe::ExpandError::UnexpectedToken)
+    pub fn expand(
+        &self,
+        db: &dyn AstDatabase,
+        id: MacroCallId,
+        tt: &tt::Subtree,
+    ) -> Result<tt::Subtree, mbe::ExpandError> {
+        match self {
+            BuiltinExpander::Line => line_expand(db, id, tt),
+        }
     }
 }
 
@@ -18,9 +32,53 @@ pub fn find_builtin_macro(
     ast_id: AstId<ast::MacroCall>,
 ) -> Option<MacroDefId> {
     // FIXME: Better registering method
-    if ident == &name::LINE {
-        Some(MacroDefId::BuiltinMacro(BuiltinMacro { expander: BuiltinExpander::Line, krate, ast_id }))
+    if ident == &name::LINE_MACRO {
+        Some(MacroDefId::BuiltinMacro(BuiltinMacro {
+            expander: BuiltinExpander::Line,
+            krate,
+            ast_id,
+        }))
     } else {
         None
     }
 }
+
+fn to_line_number(db: &dyn AstDatabase, file: HirFileId, pos: TextUnit) -> usize {
+    // FIXME: Use expansion info
+    let file_id = file.original_file(db);
+    let text = db.file_text(file_id);
+    let mut line_num = 1;
+
+    // Count line end
+    for (i, c) in text.chars().enumerate() {
+        if i == pos.to_usize() {
+            break;
+        }
+        if c == '\n' {
+            line_num += 1;
+        }
+    }
+
+    line_num
+}
+
+fn line_expand(
+    db: &dyn AstDatabase,
+    id: MacroCallId,
+    _tt: &tt::Subtree,
+) -> Result<tt::Subtree, mbe::ExpandError> {
+    let loc = db.lookup_intern_macro(id);
+    let macro_call = loc.ast_id.to_node(db);
+
+    let arg = macro_call.token_tree().ok_or_else(|| mbe::ExpandError::UnexpectedToken)?;
+    let arg_start = arg.syntax().text_range().start();
+
+    let file = id.as_file(MacroFileKind::Expr);
+    let line_num = to_line_number(db, file, arg_start);
+
+    let expanded = quote! {
+        #line_num
+    };
+
+    Ok(expanded)
+}
diff --git a/crates/ra_hir_expand/src/db.rs b/crates/ra_hir_expand/src/db.rs
index 343c9e6bf..009ff5312 100644
--- a/crates/ra_hir_expand/src/db.rs
+++ b/crates/ra_hir_expand/src/db.rs
@@ -28,7 +28,7 @@ impl TokenExpander {
     ) -> Result<tt::Subtree, mbe::ExpandError> {
         match self {
             TokenExpander::MacroRules(it) => it.expand(tt),
-            TokenExpander::Builtin(it) => it.expand(tt),
+            TokenExpander::Builtin(it) => it.expand(db, id, tt),
         }
     }