From 698f8c2f01ea549d77d7dc3338a12e04c11057b9 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Wed, 17 Apr 2024 14:02:58 +0200 Subject: Adding upstream version 1.64.0+dfsg1. Signed-off-by: Daniel Baumann --- compiler/rustc_ast_pretty/src/pp.rs | 451 ++++++++++++++++++++++++++++++++++++ 1 file changed, 451 insertions(+) create mode 100644 compiler/rustc_ast_pretty/src/pp.rs (limited to 'compiler/rustc_ast_pretty/src/pp.rs') diff --git a/compiler/rustc_ast_pretty/src/pp.rs b/compiler/rustc_ast_pretty/src/pp.rs new file mode 100644 index 000000000..c93022308 --- /dev/null +++ b/compiler/rustc_ast_pretty/src/pp.rs @@ -0,0 +1,451 @@ +//! This pretty-printer is a direct reimplementation of Philip Karlton's +//! Mesa pretty-printer, as described in the appendix to +//! Derek C. Oppen, "Pretty Printing" (1979), +//! Stanford Computer Science Department STAN-CS-79-770, +//! . +//! +//! The algorithm's aim is to break a stream into as few lines as possible +//! while respecting the indentation-consistency requirements of the enclosing +//! block, and avoiding breaking at silly places on block boundaries, for +//! example, between "x" and ")" in "x)". +//! +//! I am implementing this algorithm because it comes with 20 pages of +//! documentation explaining its theory, and because it addresses the set of +//! concerns I've seen other pretty-printers fall down on. Weirdly. Even though +//! it's 32 years old. What can I say? +//! +//! Despite some redundancies and quirks in the way it's implemented in that +//! paper, I've opted to keep the implementation here as similar as I can, +//! changing only what was blatantly wrong, a typo, or sufficiently +//! non-idiomatic rust that it really stuck out. +//! +//! In particular you'll see a certain amount of churn related to INTEGER vs. +//! CARDINAL in the Mesa implementation. Mesa apparently interconverts the two +//! somewhat readily? In any case, I've used usize for indices-in-buffers and +//! ints for character-sizes-and-indentation-offsets. This respects the need +//! for ints to "go negative" while carrying a pending-calculation balance, and +//! helps differentiate all the numbers flying around internally (slightly). +//! +//! I also inverted the indentation arithmetic used in the print stack, since +//! the Mesa implementation (somewhat randomly) stores the offset on the print +//! stack in terms of margin-col rather than col itself. I store col. +//! +//! I also implemented a small change in the String token, in that I store an +//! explicit length for the string. For most tokens this is just the length of +//! the accompanying string. But it's necessary to permit it to differ, for +//! encoding things that are supposed to "go on their own line" -- certain +//! classes of comment and blank-line -- where relying on adjacent +//! hardbreak-like Break tokens with long blankness indication doesn't actually +//! work. To see why, consider when there is a "thing that should be on its own +//! line" between two long blocks, say functions. If you put a hardbreak after +//! each function (or before each) and the breaking algorithm decides to break +//! there anyways (because the functions themselves are long) you wind up with +//! extra blank lines. If you don't put hardbreaks you can wind up with the +//! "thing which should be on its own line" not getting its own line in the +//! rare case of "really small functions" or such. This re-occurs with comments +//! and explicit blank lines. So in those cases we use a string with a payload +//! we want isolated to a line and an explicit length that's huge, surrounded +//! by two zero-length breaks. The algorithm will try its best to fit it on a +//! line (which it can't) and so naturally place the content on its own line to +//! avoid combining it with other lines and making matters even worse. +//! +//! # Explanation +//! +//! In case you do not have the paper, here is an explanation of what's going +//! on. +//! +//! There is a stream of input tokens flowing through this printer. +//! +//! The printer buffers up to 3N tokens inside itself, where N is linewidth. +//! Yes, linewidth is chars and tokens are multi-char, but in the worst +//! case every token worth buffering is 1 char long, so it's ok. +//! +//! Tokens are String, Break, and Begin/End to delimit blocks. +//! +//! Begin tokens can carry an offset, saying "how far to indent when you break +//! inside here", as well as a flag indicating "consistent" or "inconsistent" +//! breaking. Consistent breaking means that after the first break, no attempt +//! will be made to flow subsequent breaks together onto lines. Inconsistent +//! is the opposite. Inconsistent breaking example would be, say: +//! +//! ```ignore (illustrative) +//! foo(hello, there, good, friends) +//! ``` +//! +//! breaking inconsistently to become +//! +//! ```ignore (illustrative) +//! foo(hello, there, +//! good, friends); +//! ``` +//! +//! whereas a consistent breaking would yield: +//! +//! ```ignore (illustrative) +//! foo(hello, +//! there, +//! good, +//! friends); +//! ``` +//! +//! That is, in the consistent-break blocks we value vertical alignment +//! more than the ability to cram stuff onto a line. But in all cases if it +//! can make a block a one-liner, it'll do so. +//! +//! Carrying on with high-level logic: +//! +//! The buffered tokens go through a ring-buffer, 'tokens'. The 'left' and +//! 'right' indices denote the active portion of the ring buffer as well as +//! describing hypothetical points-in-the-infinite-stream at most 3N tokens +//! apart (i.e., "not wrapped to ring-buffer boundaries"). The paper will switch +//! between using 'left' and 'right' terms to denote the wrapped-to-ring-buffer +//! and point-in-infinite-stream senses freely. +//! +//! There is a parallel ring buffer, `size`, that holds the calculated size of +//! each token. Why calculated? Because for Begin/End pairs, the "size" +//! includes everything between the pair. That is, the "size" of Begin is +//! actually the sum of the sizes of everything between Begin and the paired +//! End that follows. Since that is arbitrarily far in the future, `size` is +//! being rewritten regularly while the printer runs; in fact most of the +//! machinery is here to work out `size` entries on the fly (and give up when +//! they're so obviously over-long that "infinity" is a good enough +//! approximation for purposes of line breaking). +//! +//! The "input side" of the printer is managed as an abstract process called +//! SCAN, which uses `scan_stack`, to manage calculating `size`. SCAN is, in +//! other words, the process of calculating 'size' entries. +//! +//! The "output side" of the printer is managed by an abstract process called +//! PRINT, which uses `print_stack`, `margin` and `space` to figure out what to +//! do with each token/size pair it consumes as it goes. It's trying to consume +//! the entire buffered window, but can't output anything until the size is >= +//! 0 (sizes are set to negative while they're pending calculation). +//! +//! So SCAN takes input and buffers tokens and pending calculations, while +//! PRINT gobbles up completed calculations and tokens from the buffer. The +//! theory is that the two can never get more than 3N tokens apart, because +//! once there's "obviously" too much data to fit on a line, in a size +//! calculation, SCAN will write "infinity" to the size and let PRINT consume +//! it. +//! +//! In this implementation (following the paper, again) the SCAN process is the +//! methods called `Printer::scan_*`, and the 'PRINT' process is the +//! method called `Printer::print`. + +mod convenience; +mod ring; + +use ring::RingBuffer; +use std::borrow::Cow; +use std::cmp; +use std::collections::VecDeque; +use std::iter; + +/// How to break. Described in more detail in the module docs. +#[derive(Clone, Copy, PartialEq)] +pub enum Breaks { + Consistent, + Inconsistent, +} + +#[derive(Clone, Copy, PartialEq)] +enum IndentStyle { + /// Vertically aligned under whatever column this block begins at. + /// + /// fn demo(arg1: usize, + /// arg2: usize) {} + Visual, + /// Indented relative to the indentation level of the previous line. + /// + /// fn demo( + /// arg1: usize, + /// arg2: usize, + /// ) {} + Block { offset: isize }, +} + +#[derive(Clone, Copy, Default, PartialEq)] +pub struct BreakToken { + offset: isize, + blank_space: isize, + pre_break: Option, +} + +#[derive(Clone, Copy, PartialEq)] +pub struct BeginToken { + indent: IndentStyle, + breaks: Breaks, +} + +#[derive(Clone, PartialEq)] +pub enum Token { + // In practice a string token contains either a `&'static str` or a + // `String`. `Cow` is overkill for this because we never modify the data, + // but it's more convenient than rolling our own more specialized type. + String(Cow<'static, str>), + Break(BreakToken), + Begin(BeginToken), + End, +} + +#[derive(Copy, Clone)] +enum PrintFrame { + Fits, + Broken { indent: usize, breaks: Breaks }, +} + +const SIZE_INFINITY: isize = 0xffff; + +/// Target line width. +const MARGIN: isize = 78; +/// Every line is allowed at least this much space, even if highly indented. +const MIN_SPACE: isize = 60; + +pub struct Printer { + out: String, + /// Number of spaces left on line + space: isize, + /// Ring-buffer of tokens and calculated sizes + buf: RingBuffer, + /// Running size of stream "...left" + left_total: isize, + /// Running size of stream "...right" + right_total: isize, + /// Pseudo-stack, really a ring too. Holds the + /// primary-ring-buffers index of the Begin that started the + /// current block, possibly with the most recent Break after that + /// Begin (if there is any) on top of it. Stuff is flushed off the + /// bottom as it becomes irrelevant due to the primary ring-buffer + /// advancing. + scan_stack: VecDeque, + /// Stack of blocks-in-progress being flushed by print + print_stack: Vec, + /// Level of indentation of current line + indent: usize, + /// Buffered indentation to avoid writing trailing whitespace + pending_indentation: isize, + /// The token most recently popped from the left boundary of the + /// ring-buffer for printing + last_printed: Option, +} + +#[derive(Clone)] +struct BufEntry { + token: Token, + size: isize, +} + +impl Printer { + pub fn new() -> Self { + Printer { + out: String::new(), + space: MARGIN, + buf: RingBuffer::new(), + left_total: 0, + right_total: 0, + scan_stack: VecDeque::new(), + print_stack: Vec::new(), + indent: 0, + pending_indentation: 0, + last_printed: None, + } + } + + pub fn last_token(&self) -> Option<&Token> { + self.last_token_still_buffered().or_else(|| self.last_printed.as_ref()) + } + + pub fn last_token_still_buffered(&self) -> Option<&Token> { + self.buf.last().map(|last| &last.token) + } + + /// Be very careful with this! + pub fn replace_last_token_still_buffered(&mut self, token: Token) { + self.buf.last_mut().unwrap().token = token; + } + + fn scan_eof(&mut self) { + if !self.scan_stack.is_empty() { + self.check_stack(0); + self.advance_left(); + } + } + + fn scan_begin(&mut self, token: BeginToken) { + 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); + } + + fn scan_end(&mut self) { + 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); + } + } + + fn scan_break(&mut self, token: BreakToken) { + if self.scan_stack.is_empty() { + self.left_total = 1; + self.right_total = 1; + self.buf.clear(); + } else { + self.check_stack(0); + } + let right = self.buf.push(BufEntry { token: Token::Break(token), size: -self.right_total }); + self.scan_stack.push_back(right); + self.right_total += token.blank_space; + } + + fn scan_string(&mut self, string: Cow<'static, str>) { + if self.scan_stack.is_empty() { + self.print_string(&string); + } else { + let len = string.len() as isize; + self.buf.push(BufEntry { token: Token::String(string), size: len }); + self.right_total += len; + self.check_stream(); + } + } + + pub fn offset(&mut self, offset: isize) { + if let Some(BufEntry { token: Token::Break(token), .. }) = &mut self.buf.last_mut() { + token.offset += offset; + } + } + + fn check_stream(&mut self) { + while self.right_total - self.left_total > self.space { + if *self.scan_stack.front().unwrap() == self.buf.index_of_first() { + self.scan_stack.pop_front().unwrap(); + self.buf.first_mut().unwrap().size = SIZE_INFINITY; + } + self.advance_left(); + if self.buf.is_empty() { + break; + } + } + } + + fn advance_left(&mut self) { + while self.buf.first().unwrap().size >= 0 { + let left = self.buf.pop_first().unwrap(); + + match &left.token { + Token::String(string) => { + self.left_total += string.len() as isize; + self.print_string(string); + } + Token::Break(token) => { + self.left_total += token.blank_space; + self.print_break(*token, left.size); + } + Token::Begin(token) => self.print_begin(*token, left.size), + Token::End => self.print_end(), + } + + self.last_printed = Some(left.token); + + if self.buf.is_empty() { + break; + } + } + } + + fn check_stack(&mut self, mut depth: usize) { + while let Some(&index) = self.scan_stack.back() { + let mut entry = &mut self.buf[index]; + match entry.token { + Token::Begin(_) => { + if depth == 0 { + break; + } + self.scan_stack.pop_back().unwrap(); + entry.size += self.right_total; + depth -= 1; + } + Token::End => { + // paper says + not =, but that makes no sense. + self.scan_stack.pop_back().unwrap(); + entry.size = 1; + depth += 1; + } + _ => { + self.scan_stack.pop_back().unwrap(); + entry.size += self.right_total; + if depth == 0 { + break; + } + } + } + } + } + + fn get_top(&self) -> PrintFrame { + *self + .print_stack + .last() + .unwrap_or(&PrintFrame::Broken { indent: 0, breaks: Breaks::Inconsistent }) + } + + fn print_begin(&mut self, token: BeginToken, size: isize) { + if size > self.space { + self.print_stack.push(PrintFrame::Broken { indent: self.indent, breaks: token.breaks }); + self.indent = match token.indent { + IndentStyle::Block { offset } => { + usize::try_from(self.indent as isize + offset).unwrap() + } + IndentStyle::Visual => (MARGIN - self.space) as usize, + }; + } else { + self.print_stack.push(PrintFrame::Fits); + } + } + + fn print_end(&mut self) { + if let PrintFrame::Broken { indent, .. } = self.print_stack.pop().unwrap() { + self.indent = indent; + } + } + + fn print_break(&mut self, token: BreakToken, size: isize) { + let fits = match self.get_top() { + PrintFrame::Fits => true, + PrintFrame::Broken { breaks: Breaks::Consistent, .. } => false, + PrintFrame::Broken { breaks: Breaks::Inconsistent, .. } => size <= self.space, + }; + if fits { + self.pending_indentation += token.blank_space; + self.space -= token.blank_space; + } else { + if let Some(pre_break) = token.pre_break { + self.out.push(pre_break); + } + self.out.push('\n'); + let indent = self.indent as isize + token.offset; + self.pending_indentation = indent; + self.space = cmp::max(MARGIN - indent, MIN_SPACE); + } + } + + fn print_string(&mut self, string: &str) { + // Write the pending indent. A more concise way of doing this would be: + // + // write!(self.out, "{: >n$}", "", n = self.pending_indentation as usize)?; + // + // But that is significantly slower. This code is sufficiently hot, and indents can get + // sufficiently large, that the difference is significant on some workloads. + self.out.reserve(self.pending_indentation as usize); + self.out.extend(iter::repeat(' ').take(self.pending_indentation as usize)); + self.pending_indentation = 0; + + self.out.push_str(string); + self.space -= string.len() as isize; + } +} -- cgit v1.2.3