Fix inference with conditionally compiled tails

crates/hir_def/src/body/lower.rs

@@ -203,7 +203,7 @@ impl ExprCollector<'_> {
         self.maybe_collect_expr(expr).unwrap_or_else(|| self.missing_expr())
     }
 
-    /// Returns `None` if the expression is `#[cfg]`d out.
+    /// Returns `None` if and only if the expression is `#[cfg]`d out.
     fn maybe_collect_expr(&mut self, expr: ast::Expr) -> Option<ExprId> {
         let syntax_ptr = AstPtr::new(&expr);
         self.check_cfg(&expr)?;
@@ -665,7 +665,7 @@ impl ExprCollector<'_> {
                 if self.check_cfg(&stmt).is_none() {
                     return;
                 }
-
+                let has_semi = stmt.semicolon_token().is_some();
                 // Note that macro could be expended to multiple statements
                 if let Some(ast::Expr::MacroCall(m)) = stmt.expr() {
                     let macro_ptr = AstPtr::new(&m);
@@ -682,18 +682,19 @@ impl ExprCollector<'_> {
                                 statements.statements().for_each(|stmt| this.collect_stmt(stmt));
                                 if let Some(expr) = statements.expr() {
                                     let expr = this.collect_expr(expr);
-                                    this.statements_in_scope.push(Statement::Expr(expr));
+                                    this.statements_in_scope
+                                        .push(Statement::Expr { expr, has_semi });
                                 }
                             }
                             None => {
                                 let expr = this.alloc_expr(Expr::Missing, syntax_ptr.clone());
-                                this.statements_in_scope.push(Statement::Expr(expr));
+                                this.statements_in_scope.push(Statement::Expr { expr, has_semi });
                             }
                         },
                     );
                 } else {
                     let expr = self.collect_expr_opt(stmt.expr());
-                    self.statements_in_scope.push(Statement::Expr(expr));
+                    self.statements_in_scope.push(Statement::Expr { expr, has_semi });
                 }
             }
             ast::Stmt::Item(item) => {
@@ -722,8 +723,17 @@ impl ExprCollector<'_> {
         let prev_statements = std::mem::take(&mut self.statements_in_scope);
 
         block.statements().for_each(|s| self.collect_stmt(s));
-
-        let tail = block.tail_expr().map(|e| self.collect_expr(e));
+        block.tail_expr().and_then(|e| {
+            let expr = self.maybe_collect_expr(e)?;
+            Some(self.statements_in_scope.push(Statement::Expr { expr, has_semi: false }))
+        });
+
+        let mut tail = None;
+        if let Some(Statement::Expr { expr, has_semi: false }) = self.statements_in_scope.last() {
+            tail = Some(*expr);
+            self.statements_in_scope.pop();
+        }
+        let tail = tail;
         let statements = std::mem::replace(&mut self.statements_in_scope, prev_statements);
         let syntax_node_ptr = AstPtr::new(&block.into());
         let expr_id = self.alloc_expr(

crates/hir_def/src/body/scope.rs

@@ -157,7 +157,7 @@ fn compute_block_scopes(
                 scope = scopes.new_scope(scope);
                 scopes.add_bindings(body, scope, *pat);
             }
-            Statement::Expr(expr) => {
+            Statement::Expr { expr, .. } => {
                 scopes.set_scope(*expr, scope);
                 compute_expr_scopes(*expr, body, scopes, scope);
             }

crates/hir_def/src/expr.rs

@@ -242,7 +242,7 @@ pub struct RecordLitField {
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub enum Statement {
     Let { pat: PatId, type_ref: Option<Interned<TypeRef>>, initializer: Option<ExprId> },
-    Expr(ExprId),
+    Expr { expr: ExprId, has_semi: bool },
 }
 
 impl Expr {
@@ -265,7 +265,7 @@ impl Expr {
                                 f(*expr);
                             }
                         }
-                        Statement::Expr(e) => f(*e),
+                        Statement::Expr { expr: expression, .. } => f(*expression),
                     }
                 }
                 if let Some(expr) = tail {

crates/hir_ty/src/diagnostics/expr.rs

@@ -83,7 +83,7 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
         if let Expr::Block { statements, tail, .. } = body_expr {
             if let Some(t) = tail {
                 self.validate_results_in_tail_expr(body.body_expr, *t, db);
-            } else if let Some(Statement::Expr(id)) = statements.last() {
+            } else if let Some(Statement::Expr { expr: id, .. }) = statements.last() {
                 self.validate_missing_tail_expr(body.body_expr, *id, db);
             }
         }

crates/hir_ty/src/infer/expr.rs

@@ -809,7 +809,7 @@ impl<'a> InferenceContext<'a> {
                     let ty = self.resolve_ty_as_possible(ty);
                     self.infer_pat(*pat, &ty, BindingMode::default());
                 }
-                Statement::Expr(expr) => {
+                Statement::Expr { expr, .. } => {
                     self.infer_expr(*expr, &Expectation::none());
                 }
             }

crates/hir_ty/src/tests/regression.rs

@@ -1050,3 +1050,52 @@ fn test() {
         "#]],
     );
 }
+
+#[test]
+fn cfg_tail() {
+    // https://github.com/rust-analyzer/rust-analyzer/issues/8378
+    check_infer(
+        r#"
+        fn fake_tail(){
+            { "first" }
+            #[cfg(never)] 9
+        }
+        fn multiple_fake(){
+            { "fake" }
+            { "fake" }
+            { "second" }
+            #[cfg(never)] { 11 }
+            #[cfg(never)] 12;
+            #[cfg(never)] 13
+        }
+        fn no_normal_tail(){
+            { "third" }
+            #[cfg(never)] 14;
+            #[cfg(never)] 15;
+        }
+        fn no_actual_tail(){
+            { "fourth" };
+            #[cfg(never)] 14;
+            #[cfg(never)] 15
+        }
+        "#,
+        expect![[r#"
+            14..53 '{     ...)] 9 }': &str
+            20..31 '{ "first" }': &str
+            22..29 '"first"': &str
+            72..190 '{     ...] 13 }': &str
+            78..88 '{ "fake" }': &str
+            80..86 '"fake"': &str
+            93..103 '{ "fake" }': &str
+            95..101 '"fake"': &str
+            108..120 '{ "second" }': &str
+            110..118 '"second"': &str
+            210..273 '{     ... 15; }': &str
+            216..227 '{ "third" }': &str
+            218..225 '"third"': &str
+            293..357 '{     ...] 15 }': ()
+            299..311 '{ "fourth" }': &str
+            301..309 '"fourth"': &str
+        "#]],
+    )
+}