Auto merge of #125055 - nnethercote:Comment-FIXME, r=compiler-errors

Avoid clone in `Comments::next`

`Comments::next`, in `rustc_ast_pretty`, has this comment:
```
// FIXME: This shouldn't probably clone lmao
```
The obvious thing to try is to return `Option<&Comment>` instead of `Option<Comment>`. But that leads to multiple borrows all over the place, because `Comments` must be borrowed from `PrintState` and then processed by `&mut self` methods within `PrintState`.

This PR instead rearranges things so that comments are consumed as they are used, preserving the `Option<Comment>` return type without requiring any cloning.

r? `@compiler-errors`

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -55,8 +55,8 @@ impl PpAnn for NoAnn {}
 
 pub struct Comments<'a> {
     sm: &'a SourceMap,
-    comments: Vec<Comment>,
+    // Stored in reverse order so we can consume them by popping.
-    current: usize,
+    reversed_comments: Vec<Comment>,
 }
 
 /// Returns `None` if the first `col` chars of `s` contain a non-whitespace char.
@@ -182,21 +182,25 @@ fn gather_comments(sm: &SourceMap, path: FileName, src: String) -> Vec<Comment>
 
 impl<'a> Comments<'a> {
     pub fn new(sm: &'a SourceMap, filename: FileName, input: String) -> Comments<'a> {
-        let comments = gather_comments(sm, filename, input);
+        let mut comments = gather_comments(sm, filename, input);
-        Comments { sm, comments, current: 0 }
+        comments.reverse();
+        Comments { sm, reversed_comments: comments }
     }
 
-    // FIXME: This shouldn't probably clone lmao
+    fn peek(&self) -> Option<&Comment> {
-    fn next(&self) -> Option<Comment> {
+        self.reversed_comments.last()
-        self.comments.get(self.current).cloned()
+    }
+
+    fn next(&mut self) -> Option<Comment> {
+        self.reversed_comments.pop()
     }
 
     fn trailing_comment(
-        &self,
+        &mut self,
         span: rustc_span::Span,
         next_pos: Option<BytePos>,
     ) -> Option<Comment> {
-        if let Some(cmnt) = self.next() {
+        if let Some(cmnt) = self.peek() {
             if cmnt.style != CommentStyle::Trailing {
                 return None;
             }
@@ -204,7 +208,7 @@ fn trailing_comment(
             let comment_line = self.sm.lookup_char_pos(cmnt.pos);
             let next = next_pos.unwrap_or_else(|| cmnt.pos + BytePos(1));
             if span.hi() < cmnt.pos && cmnt.pos < next && span_line.line == comment_line.line {
-                return Some(cmnt);
+                return Some(self.next().unwrap());
             }
         }
 
@@ -400,7 +404,8 @@ fn deref_mut(&mut self) -> &mut Self::Target {
 
 /// This trait is used for both AST and HIR pretty-printing.
 pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::DerefMut {
-    fn comments(&mut self) -> &mut Option<Comments<'a>>;
+    fn comments(&self) -> Option<&Comments<'a>>;
+    fn comments_mut(&mut self) -> Option<&mut Comments<'a>>;
     fn ann_post(&mut self, ident: Ident);
     fn print_generic_args(&mut self, args: &ast::GenericArgs, colons_before_params: bool);
 
@@ -442,18 +447,18 @@ fn commasep<T, F>(&mut self, b: Breaks, elts: &[T], op: F)
 
     fn maybe_print_comment(&mut self, pos: BytePos) -> bool {
         let mut has_comment = false;
-        while let Some(cmnt) = self.next_comment() {
+        while let Some(cmnt) = self.peek_comment() {
-            if cmnt.pos < pos {
+            if cmnt.pos >= pos {
-                has_comment = true;
-                self.print_comment(&cmnt);
-            } else {
                 break;
             }
+            has_comment = true;
+            let cmnt = self.next_comment().unwrap();
+            self.print_comment(cmnt);
         }
         has_comment
     }
 
-    fn print_comment(&mut self, cmnt: &Comment) {
+    fn print_comment(&mut self, cmnt: Comment) {
         match cmnt.style {
             CommentStyle::Mixed => {
                 if !self.is_beginning_of_line() {
@@ -517,19 +522,20 @@ fn print_comment(&mut self, cmnt: &Comment) {
                 self.hardbreak();
             }
         }
-        if let Some(cmnts) = self.comments() {
-            cmnts.current += 1;
     }
+
+    fn peek_comment<'b>(&'b self) -> Option<&'b Comment> where 'a: 'b {
+        self.comments().and_then(|c| c.peek())
     }
 
     fn next_comment(&mut self) -> Option<Comment> {
-        self.comments().as_mut().and_then(|c| c.next())
+        self.comments_mut().and_then(|c| c.next())
     }
 
     fn maybe_print_trailing_comment(&mut self, span: rustc_span::Span, next_pos: Option<BytePos>) {
-        if let Some(cmnts) = self.comments() {
+        if let Some(cmnts) = self.comments_mut() {
             if let Some(cmnt) = cmnts.trailing_comment(span, next_pos) {
-                self.print_comment(&cmnt);
+                self.print_comment(cmnt);
             }
         }
     }
@@ -537,11 +543,11 @@ fn maybe_print_trailing_comment(&mut self, span: rustc_span::Span, next_pos: Opt
     fn print_remaining_comments(&mut self) {
         // If there aren't any remaining comments, then we need to manually
         // make sure there is a line break at the end.
-        if self.next_comment().is_none() {
+        if self.peek_comment().is_none() {
             self.hardbreak();
         }
         while let Some(cmnt) = self.next_comment() {
-            self.print_comment(&cmnt)
+            self.print_comment(cmnt)
         }
     }
 
@@ -994,8 +1000,12 @@ fn to_string(f: impl FnOnce(&mut State<'_>)) -> String {
 }
 
 impl<'a> PrintState<'a> for State<'a> {
-    fn comments(&mut self) -> &mut Option<Comments<'a>> {
+    fn comments(&self) -> Option<&Comments<'a>> {
-        &mut self.comments
+        self.comments.as_ref()
+    }
+
+    fn comments_mut(&mut self) -> Option<&mut Comments<'a>> {
+        self.comments.as_mut()
     }
 
     fn ann_post(&mut self, ident: Ident) {

compiler/rustc_hir_pretty/src/lib.rs

@@ -139,8 +139,12 @@ fn deref_mut(&mut self) -> &mut Self::Target {
 }
 
 impl<'a> PrintState<'a> for State<'a> {
-    fn comments(&mut self) -> &mut Option<Comments<'a>> {
+    fn comments(&self) -> Option<&Comments<'a>> {
-        &mut self.comments
+        self.comments.as_ref()
+    }
+
+    fn comments_mut(&mut self) -> Option<&mut Comments<'a>> {
+        self.comments.as_mut()
     }
 
     fn ann_post(&mut self, ident: Ident) {