Introduce BoxMarker to improve pretty-printing correctness

compiler/rustc_ast_pretty/src/lib.rs

@@ -2,6 +2,7 @@
 #![allow(internal_features)]
 #![doc(rust_logo)]
 #![feature(box_patterns)]
+#![feature(negative_impls)]
 #![feature(rustdoc_internals)]
 // tidy-alphabetical-end
 

compiler/rustc_ast_pretty/src/pp.rs

@@ -234,6 +234,38 @@ struct BufEntry {
     size: isize,
 }
 
+// Boxes opened with methods like `Printer::{cbox,ibox}` must be closed with
+// `Printer::end`. Failure to do so can result in bad indenting, or in extreme
+// cases, cause no output to be produced at all.
+//
+// Box opening and closing used to be entirely implicit, which was hard to
+// understand and easy to get wrong. This marker type is now returned from the
+// box opening methods and forgotten by `Printer::end`. Any marker that isn't
+// forgotten will trigger a panic in `drop`. (Closing a box more than once
+// isn't possible because `BoxMarker` doesn't implement `Copy` or `Clone`.)
+//
+// FIXME(nnethercote): the panic in `drop` is currently disabled because a few
+// places fail to close their boxes. It can be enabled once they are fixed.
+//
+// Note: it would be better to make open/close mismatching impossible and avoid
+// the need for this marker type altogether by having functions like
+// `with_ibox` that open a box, call a closure, and then close the box. That
+// would work for simple cases, but box lifetimes sometimes interact with
+// complex control flow and across function boundaries in ways that are
+// difficult to handle with such a technique.
+#[must_use]
+pub struct BoxMarker;
+
+impl !Clone for BoxMarker {}
+impl !Copy for BoxMarker {}
+
+impl Drop for BoxMarker {
+    fn drop(&mut self) {
+        // FIXME(nnethercote): enable once the bad cases are fixed
+        //panic!("BoxMarker not ended with `Printer::end()`");
+    }
+}
+
 impl Printer {
     pub fn new() -> Self {
         Printer {
@@ -270,23 +302,27 @@ impl Printer {
         }
     }
 
-    fn scan_begin(&mut self, token: BeginToken) {
+    // This is is where `BoxMarker`s are produced.
+    fn scan_begin(&mut self, token: BeginToken) -> BoxMarker {
         if self.scan_stack.is_empty() {
             self.left_total = 1;
             self.right_total = 1;
             self.buf.clear();
         }
         let right = self.buf.push(BufEntry { token: Token::Begin(token), size: -self.right_total });
         self.scan_stack.push_back(right);
+        BoxMarker
     }
 
-    fn scan_end(&mut self) {
+    // This is is where `BoxMarker`s are consumed.
+    fn scan_end(&mut self, b: BoxMarker) {
         if self.scan_stack.is_empty() {
             self.print_end();
         } else {
             let right = self.buf.push(BufEntry { token: Token::End, size: -1 });
             self.scan_stack.push_back(right);
         }
+        std::mem::forget(b)
     }
 
     fn scan_break(&mut self, token: BreakToken) {

compiler/rustc_ast_pretty/src/pp/convenience.rs

@@ -1,25 +1,27 @@
 use std::borrow::Cow;
 
-use crate::pp::{BeginToken, BreakToken, Breaks, IndentStyle, Printer, SIZE_INFINITY, Token};
+use crate::pp::{
+    BeginToken, BoxMarker, BreakToken, Breaks, IndentStyle, Printer, SIZE_INFINITY, Token,
+};
 
 impl Printer {
     /// "raw box"
-    pub fn rbox(&mut self, indent: isize, breaks: Breaks) {
+    pub fn rbox(&mut self, indent: isize, breaks: Breaks) -> BoxMarker {
         self.scan_begin(BeginToken { indent: IndentStyle::Block { offset: indent }, breaks })
     }
 
     /// Inconsistent breaking box
-    pub fn ibox(&mut self, indent: isize) {
+    pub fn ibox(&mut self, indent: isize) -> BoxMarker {
         self.rbox(indent, Breaks::Inconsistent)
     }
 
     /// Consistent breaking box
-    pub fn cbox(&mut self, indent: isize) {
+    pub fn cbox(&mut self, indent: isize) -> BoxMarker {
         self.rbox(indent, Breaks::Consistent)
     }
 
-    pub fn visual_align(&mut self) {
-        self.scan_begin(BeginToken { indent: IndentStyle::Visual, breaks: Breaks::Consistent });
+    pub fn visual_align(&mut self) -> BoxMarker {
+        self.scan_begin(BeginToken { indent: IndentStyle::Visual, breaks: Breaks::Consistent })
     }
 
     pub fn break_offset(&mut self, n: usize, off: isize) {
@@ -30,8 +32,8 @@ impl Printer {
         });
     }
 
-    pub fn end(&mut self) {
-        self.scan_end()
+    pub fn end(&mut self, b: BoxMarker) {
+        self.scan_end(b)
     }
 
     pub fn eof(mut self) -> String {