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rustdoc: reduce allocs in FnDecl::inner_full_print #109011

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Mar 13, 2023
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rustdoc: reduce allocs in FnDecl::inner_full_print #109011

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136 changes: 71 additions & 65 deletions src/librustdoc/html/format.rs
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Original file line number Diff line number Diff line change
@@ -1,13 +1,15 @@
//! HTML formatting module
//!
//! This module contains a large number of `fmt::Display` implementations for
//! various types in `rustdoc::clean`. These implementations all currently
//! assume that HTML output is desired, although it may be possible to redesign
//! them in the future to instead emit any format desired.
//! various types in `rustdoc::clean`.
//!
//! These implementations all emit HTML. As an internal implementation detail,
//! some of them support an alternate format that emits text, but that should
//! not be used external to this module.

use std::borrow::Cow;
use std::cell::Cell;
use std::fmt;
use std::fmt::{self, Write};
use std::iter::{self, once};

use rustc_ast as ast;
Expand Down Expand Up @@ -126,7 +128,6 @@ impl Buffer {
// the fmt::Result return type imposed by fmt::Write (and avoiding the trait
// import).
pub(crate) fn write_fmt(&mut self, v: fmt::Arguments<'_>) {
use fmt::Write;
self.buffer.write_fmt(v).unwrap();
}

Expand Down Expand Up @@ -279,8 +280,6 @@ pub(crate) fn print_where_clause<'a, 'tcx: 'a>(
indent: usize,
ending: Ending,
) -> impl fmt::Display + 'a + Captures<'tcx> {
use fmt::Write;

display_fn(move |f| {
let mut where_predicates = gens.where_predicates.iter().filter(|pred| {
!matches!(pred, clean::WherePredicate::BoundPredicate { bounds, .. } if bounds.is_empty())
Expand Down Expand Up @@ -1306,6 +1305,28 @@ impl clean::BareFunctionDecl {
}
}

// Implements Write but only counts the bytes "written".
struct WriteCounter(usize);

impl std::fmt::Write for WriteCounter {
fn write_str(&mut self, s: &str) -> fmt::Result {
self.0 += s.len();
Ok(())
}
}

// Implements Display by emitting the given number of spaces.
struct Indent(usize);

impl fmt::Display for Indent {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(0..self.0).for_each(|_| {
f.write_char(' ').unwrap();
});
Ok(())
}
}

impl clean::FnDecl {
pub(crate) fn print<'b, 'a: 'b, 'tcx: 'a>(
&'a self,
Expand Down Expand Up @@ -1345,95 +1366,80 @@ impl clean::FnDecl {
indent: usize,
cx: &'a Context<'tcx>,
) -> impl fmt::Display + 'a + Captures<'tcx> {
display_fn(move |f| self.inner_full_print(header_len, indent, f, cx))
display_fn(move |f| {
// First, generate the text form of the declaration, with no line wrapping, and count the bytes.
let mut counter = WriteCounter(0);
write!(&mut counter, "{:#}", display_fn(|f| { self.inner_full_print(None, f, cx) }))
.unwrap();
// If the text form was over 80 characters wide, we will line-wrap our output.
let line_wrapping_indent =
if header_len + counter.0 > 80 { Some(indent) } else { None };
// Generate the final output. This happens to accept `{:#}` formatting to get textual
// output but in practice it is only formatted with `{}` to get HTML output.
self.inner_full_print(line_wrapping_indent, f, cx)
})
}

fn inner_full_print(
&self,
header_len: usize,
indent: usize,
// For None, the declaration will not be line-wrapped. For Some(n),
// the declaration will be line-wrapped, with an indent of n spaces.
line_wrapping_indent: Option<usize>,
f: &mut fmt::Formatter<'_>,
cx: &Context<'_>,
) -> fmt::Result {
let amp = if f.alternate() { "&" } else { "&amp;" };
let mut args = Buffer::html();
let mut args_plain = Buffer::new();

write!(f, "(")?;
if let Some(n) = line_wrapping_indent {
write!(f, "\n{}", Indent(n + 4))?;
}
for (i, input) in self.inputs.values.iter().enumerate() {
if i > 0 {
match line_wrapping_indent {
None => write!(f, ", ")?,
Some(n) => write!(f, ",\n{}", Indent(n + 4))?,
};
}
if let Some(selfty) = input.to_self() {
match selfty {
clean::SelfValue => {
args.push_str("self");
args_plain.push_str("self");
write!(f, "self")?;
}
clean::SelfBorrowed(Some(ref lt), mtbl) => {
write!(args, "{}{} {}self", amp, lt.print(), mtbl.print_with_space());
write!(args_plain, "&{} {}self", lt.print(), mtbl.print_with_space());
write!(f, "{}{} {}self", amp, lt.print(), mtbl.print_with_space())?;
}
clean::SelfBorrowed(None, mtbl) => {
write!(args, "{}{}self", amp, mtbl.print_with_space());
write!(args_plain, "&{}self", mtbl.print_with_space());
write!(f, "{}{}self", amp, mtbl.print_with_space())?;
}
clean::SelfExplicit(ref typ) => {
if f.alternate() {
write!(args, "self: {:#}", typ.print(cx));
} else {
write!(args, "self: {}", typ.print(cx));
}
write!(args_plain, "self: {:#}", typ.print(cx));
write!(f, "self: ")?;
fmt::Display::fmt(&typ.print(cx), f)?;
}
}
} else {
if i > 0 {
args.push_str("\n");
}
if input.is_const {
args.push_str("const ");
args_plain.push_str("const ");
}
write!(args, "{}: ", input.name);
write!(args_plain, "{}: ", input.name);

if f.alternate() {
write!(args, "{:#}", input.type_.print(cx));
} else {
write!(args, "{}", input.type_.print(cx));
write!(f, "const ")?;
}
write!(args_plain, "{:#}", input.type_.print(cx));
}
if i + 1 < self.inputs.values.len() {
args.push_str(",");
args_plain.push_str(",");
write!(f, "{}: ", input.name)?;
fmt::Display::fmt(&input.type_.print(cx), f)?;
}
}

let mut args_plain = format!("({})", args_plain.into_inner());
let mut args = args.into_inner();

if self.c_variadic {
args.push_str(",\n ...");
args_plain.push_str(", ...");
match line_wrapping_indent {
None => write!(f, ", ...")?,
Some(n) => write!(f, "\n{}...", Indent(n + 4))?,
};
}

let arrow_plain = format!("{:#}", self.output.print(cx));
let arrow =
if f.alternate() { arrow_plain.clone() } else { format!("{}", self.output.print(cx)) };

let declaration_len = header_len + args_plain.len() + arrow_plain.len();
let output = if declaration_len > 80 {
let full_pad = format!("\n{}", " ".repeat(indent + 4));
let close_pad = format!("\n{}", " ".repeat(indent));
format!(
"({pad}{args}{close}){arrow}",
pad = if self.inputs.values.is_empty() { "" } else { &full_pad },
args = args.replace('\n', &full_pad),
close = close_pad,
arrow = arrow
)
} else {
format!("({args}){arrow}", args = args.replace('\n', " "), arrow = arrow)
match line_wrapping_indent {
None => write!(f, ")")?,
Some(n) => write!(f, "\n{})", Indent(n))?,
};

write!(f, "{}", output)
fmt::Display::fmt(&self.output.print(cx), f)?;
Ok(())
}
}

Expand Down