Auto merge of rust-lang#136932 - m-ou-se:fmt-width-precision-u16, r=<try>

Reduce formatting `width` and `precision` to 16 bits

This is reduces the `width` and `precision` fields in format strings to 16 bits. They are currently full `usize`s, but it's a bit nonsensical that we need to support the case where someone wants to pad their value to eighteen quintillion spaces and/or have eighteen quintillion digits of precision.

This is a breaking change, but probably affects virtually no code. Let's do a crater run to find out. Marking this as experiment for now.

---

There are three ways to set a width or precision today:

1. Directly a formatting string, e.g. `println!("{a:1234}")`
2. Indirectly in a formatting string, e.g. `println!("{a:width$}", width=1234)`
3. Through the unstable `FormattingOptions::width` method.

This PR:

- Adds a compiler error for 1. (`println!("{a:9999999}")` no longer compiles and gives a clear error.)
- Adds a runtime check for 2. (`println!("{a:width$}, width=9999999)` will panic.)
- Changes the signature of `FormattingOptions::width` to take a `u16` instead.

---

Additional context:

All the formatting flags and options are currently:

- The `+` flag (1 bit)
- The `-` flag (1 bit)
- The `#` flag (1 bit)
- The `0` flag (1 bit)
- The `x?` flag (1 bit)
- The `X?` flag (1 bit)
- The alignment (2 bits)
- The fill character (21 bits)
- Whether a width is specified (1 bit)
- Whether a precision is specified (1 bit)
- If used, the width (a full usize)
- If used, the precision (a full usize)

Everything except the last two can simply fit in a `u32` (those add up to 31 bits in total).

If we can accept a max width and precision of u16::MAX, we can make a `FormattingOptions` that is exactly 64 bits in size; the same size as a thin reference on most platforms.

If, additionally, we also limit the number of formatting arguments to u16::MAX, we can also reduce the size of `fmt::Arguments` (that is, of a `format_args!()` expression).

Author: bors

Diff for: compiler/rustc_ast/src/format.rs

@@ -266,7 +266,7 @@ pub enum FormatAlignment {
 #[derive(Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
 pub enum FormatCount {
     /// `{:5}` or `{:.5}`
-    Literal(usize),
+    Literal(u16),
     /// `{:.*}`, `{:.5$}`, or `{:a$}`, etc.
     Argument(FormatArgPosition),
 }

Diff for: compiler/rustc_ast_lowering/src/expr.rs

@@ -2151,6 +2151,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.expr(sp, hir::ExprKind::Lit(lit))
     }
 
+    pub(super) fn expr_u16(&mut self, sp: Span, value: u16) -> hir::Expr<'hir> {
+        let lit = self.arena.alloc(hir::Lit {
+            span: sp,
+            node: ast::LitKind::Int(
+                u128::from(value).into(),
+                ast::LitIntType::Unsigned(ast::UintTy::U16),
+            ),
+        });
+        self.expr(sp, hir::ExprKind::Lit(lit))
+    }
+
     pub(super) fn expr_char(&mut self, sp: Span, value: char) -> hir::Expr<'hir> {
         let lit = self.arena.alloc(hir::Lit { span: sp, node: ast::LitKind::Char(value) });
         self.expr(sp, hir::ExprKind::Lit(lit))

Diff for: compiler/rustc_ast_lowering/src/format.rs

@@ -292,7 +292,7 @@ fn make_count<'hir>(
                 hir::LangItem::FormatCount,
                 sym::Is,
             ));
-            let value = ctx.arena.alloc_from_iter([ctx.expr_usize(sp, *n)]);
+            let value = ctx.arena.alloc_from_iter([ctx.expr_u16(sp, *n)]);
             ctx.expr_call_mut(sp, count_is, value)
         }
         Some(FormatCount::Argument(arg)) => {

Diff for: compiler/rustc_parse_format/src/lib.rs

@@ -190,7 +190,7 @@ pub enum DebugHex {
 #[derive(Copy, Clone, Debug, PartialEq)]
 pub enum Count<'a> {
     /// The count is specified explicitly.
-    CountIs(usize),
+    CountIs(u16),
     /// The count is specified by the argument with the given name.
     CountIsName(&'a str, InnerSpan),
     /// The count is specified by the argument at the given index.
@@ -565,7 +565,7 @@ impl<'a> Parser<'a> {
     /// consuming a macro argument, `None` if it's the case.
     fn position(&mut self) -> Option<Position<'a>> {
         if let Some(i) = self.integer() {
-            Some(ArgumentIs(i))
+            Some(ArgumentIs(i.into()))
         } else {
             match self.cur.peek() {
                 Some(&(lo, c)) if rustc_lexer::is_id_start(c) => {
@@ -771,7 +771,7 @@ impl<'a> Parser<'a> {
     /// width.
     fn count(&mut self, start: usize) -> Count<'a> {
         if let Some(i) = self.integer() {
-            if self.consume('$') { CountIsParam(i) } else { CountIs(i) }
+            if self.consume('$') { CountIsParam(i.into()) } else { CountIs(i) }
         } else {
             let tmp = self.cur.clone();
             let word = self.word();
@@ -822,15 +822,15 @@ impl<'a> Parser<'a> {
         word
     }
 
-    fn integer(&mut self) -> Option<usize> {
-        let mut cur: usize = 0;
+    fn integer(&mut self) -> Option<u16> {
+        let mut cur: u16 = 0;
         let mut found = false;
         let mut overflow = false;
         let start = self.current_pos();
         while let Some(&(_, c)) = self.cur.peek() {
             if let Some(i) = c.to_digit(10) {
                 let (tmp, mul_overflow) = cur.overflowing_mul(10);
-                let (tmp, add_overflow) = tmp.overflowing_add(i as usize);
+                let (tmp, add_overflow) = tmp.overflowing_add(i as u16);
                 if mul_overflow || add_overflow {
                     overflow = true;
                 }
@@ -847,11 +847,11 @@ impl<'a> Parser<'a> {
             let overflowed_int = &self.input[start..end];
             self.err(
                 format!(
-                    "integer `{}` does not fit into the type `usize` whose range is `0..={}`",
+                    "integer `{}` does not fit into the type `u16` whose range is `0..={}`",
                     overflowed_int,
-                    usize::MAX
+                    u16::MAX
                 ),
-                "integer out of range for `usize`",
+                "integer out of range for `u16`",
                 self.span(start, end),
             );
         }

Diff for: compiler/rustc_span/src/symbol.rs

@@ -1000,6 +1000,7 @@ symbols! {
         from_residual,
         from_size_align_unchecked,
         from_str_method,
+        from_u16,
         from_usize,
         from_yeet,
         fs_create_dir,

Diff for: library/core/src/fmt/float.rs

@@ -29,7 +29,7 @@ fn float_to_decimal_common_exact<T>(
     fmt: &mut Formatter<'_>,
     num: &T,
     sign: flt2dec::Sign,
-    precision: usize,
+    precision: u16,
 ) -> Result
 where
     T: flt2dec::DecodableFloat,
@@ -40,7 +40,7 @@ where
         flt2dec::strategy::grisu::format_exact,
         *num,
         sign,
-        precision,
+        precision.into(),
         &mut buf,
         &mut parts,
     );
@@ -55,7 +55,7 @@ fn float_to_decimal_common_shortest<T>(
     fmt: &mut Formatter<'_>,
     num: &T,
     sign: flt2dec::Sign,
-    precision: usize,
+    precision: u16,
 ) -> Result
 where
     T: flt2dec::DecodableFloat,
@@ -68,7 +68,7 @@ where
         flt2dec::strategy::grisu::format_shortest,
         *num,
         sign,
-        precision,
+        precision.into(),
         &mut buf,
         &mut parts,
     );
@@ -101,7 +101,7 @@ fn float_to_exponential_common_exact<T>(
     fmt: &mut Formatter<'_>,
     num: &T,
     sign: flt2dec::Sign,
-    precision: usize,
+    precision: u16,
     upper: bool,
 ) -> Result
 where
@@ -113,7 +113,7 @@ where
         flt2dec::strategy::grisu::format_exact,
         *num,
         sign,
-        precision,
+        precision.into(),
         upper,
         &mut buf,
         &mut parts,

Diff for: library/core/src/fmt/mod.rs

@@ -294,8 +294,8 @@ pub struct FormattingOptions {
     flags: u32,
     fill: char,
     align: Option<Alignment>,
-    width: Option<usize>,
-    precision: Option<usize>,
+    width: Option<u16>,
+    precision: Option<u16>,
 }
 
 impl FormattingOptions {
@@ -389,7 +389,7 @@ impl FormattingOptions {
     /// the padding specified by [`FormattingOptions::fill`]/[`FormattingOptions::align`]
     /// will be used to take up the required space.
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn width(&mut self, width: Option<usize>) -> &mut Self {
+    pub fn width(&mut self, width: Option<u16>) -> &mut Self {
         self.width = width;
         self
     }
@@ -403,7 +403,7 @@ impl FormattingOptions {
     /// - For floating-point types, this indicates how many digits after the
     /// decimal point should be printed.
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn precision(&mut self, precision: Option<usize>) -> &mut Self {
+    pub fn precision(&mut self, precision: Option<u16>) -> &mut Self {
         self.precision = precision;
         self
     }
@@ -455,12 +455,12 @@ impl FormattingOptions {
     }
     /// Returns the current width.
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub const fn get_width(&self) -> Option<usize> {
+    pub const fn get_width(&self) -> Option<u16> {
         self.width
     }
     /// Returns the current precision.
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub const fn get_precision(&self) -> Option<usize> {
+    pub const fn get_precision(&self) -> Option<u16> {
         self.precision
     }
     /// Returns the current precision.
@@ -1499,15 +1499,18 @@ unsafe fn run(fmt: &mut Formatter<'_>, arg: &rt::Placeholder, args: &[rt::Argume
     unsafe { value.fmt(fmt) }
 }
 
-unsafe fn getcount(args: &[rt::Argument<'_>], cnt: &rt::Count) -> Option<usize> {
+unsafe fn getcount(args: &[rt::Argument<'_>], cnt: &rt::Count) -> Option<u16> {
     match *cnt {
+        #[cfg(bootstrap)]
+        rt::Count::Is(n) => Some(n as u16),
+        #[cfg(not(bootstrap))]
         rt::Count::Is(n) => Some(n),
         rt::Count::Implied => None,
         rt::Count::Param(i) => {
             debug_assert!(i < args.len());
             // SAFETY: cnt and args come from the same Arguments,
             // which guarantees this index is always within bounds.
-            unsafe { args.get_unchecked(i).as_usize() }
+            unsafe { args.get_unchecked(i).as_u16() }
         }
     }
 }
@@ -1516,11 +1519,11 @@ unsafe fn getcount(args: &[rt::Argument<'_>], cnt: &rt::Count) -> Option<usize>
 #[must_use = "don't forget to write the post padding"]
 pub(crate) struct PostPadding {
     fill: char,
-    padding: usize,
+    padding: u16,
 }
 
 impl PostPadding {
-    fn new(fill: char, padding: usize) -> PostPadding {
+    fn new(fill: char, padding: u16) -> PostPadding {
         PostPadding { fill, padding }
     }
 
@@ -1634,7 +1637,7 @@ impl<'a> Formatter<'a> {
             }
             // Check if we're over the minimum width, if so then we can also
             // just write the bytes.
-            Some(min) if width >= min => {
+            Some(min) if width >= usize::from(min) => {
                 write_prefix(self, sign, prefix)?;
                 self.buf.write_str(buf)
             }
@@ -1645,7 +1648,7 @@ impl<'a> Formatter<'a> {
                 let old_align =
                     crate::mem::replace(&mut self.options.align, Some(Alignment::Right));
                 write_prefix(self, sign, prefix)?;
-                let post_padding = self.padding(min - width, Alignment::Right)?;
+                let post_padding = self.padding(min - width as u16, Alignment::Right)?;
                 self.buf.write_str(buf)?;
                 post_padding.write(self)?;
                 self.options.fill = old_fill;
@@ -1654,7 +1657,7 @@ impl<'a> Formatter<'a> {
             }
             // Otherwise, the sign and prefix goes after the padding
             Some(min) => {
-                let post_padding = self.padding(min - width, Alignment::Right)?;
+                let post_padding = self.padding(min - width as u16, Alignment::Right)?;
                 write_prefix(self, sign, prefix)?;
                 self.buf.write_str(buf)?;
                 post_padding.write(self)
@@ -1703,7 +1706,7 @@ impl<'a> Formatter<'a> {
             // If our string is longer that the precision, then we must have
             // truncation. However other flags like `fill`, `width` and `align`
             // must act as always.
-            if let Some((i, _)) = s.char_indices().nth(max) {
+            if let Some((i, _)) = s.char_indices().nth(usize::from(max)) {
                 // LLVM here can't prove that `..i` won't panic `&s[..i]`, but
                 // we know that it can't panic. Use `get` + `unwrap_or` to avoid
                 // `unsafe` and otherwise don't emit any panic-related code
@@ -1724,14 +1727,14 @@ impl<'a> Formatter<'a> {
                 let chars_count = s.chars().count();
                 // If we're under the maximum width, check if we're over the minimum
                 // width, if so it's as easy as just emitting the string.
-                if chars_count >= width {
+                if chars_count >= usize::from(width) {
                     self.buf.write_str(s)
                 }
                 // If we're under both the maximum and the minimum width, then fill
                 // up the minimum width with the specified string + some alignment.
                 else {
                     let align = Alignment::Left;
-                    let post_padding = self.padding(width - chars_count, align)?;
+                    let post_padding = self.padding(width - chars_count as u16, align)?;
                     self.buf.write_str(s)?;
                     post_padding.write(self)
                 }
@@ -1745,7 +1748,7 @@ impl<'a> Formatter<'a> {
     /// thing that is being padded.
     pub(crate) fn padding(
         &mut self,
-        padding: usize,
+        padding: u16,
         default: Alignment,
     ) -> result::Result<PostPadding, Error> {
         let align = self.align().unwrap_or(default);
@@ -1785,19 +1788,19 @@ impl<'a> Formatter<'a> {
 
                 // remove the sign from the formatted parts
                 formatted.sign = "";
-                width = width.saturating_sub(sign.len());
+                width = width.saturating_sub(sign.len() as u16);
                 self.options.fill = '0';
                 self.options.align = Some(Alignment::Right);
             }
 
             // remaining parts go through the ordinary padding process.
             let len = formatted.len();
-            let ret = if width <= len {
+            let ret = if usize::from(width) <= len {
                 // no padding
                 // SAFETY: Per the precondition.
                 unsafe { self.write_formatted_parts(&formatted) }
             } else {
-                let post_padding = self.padding(width - len, Alignment::Right)?;
+                let post_padding = self.padding(width - len as u16, Alignment::Right)?;
                 // SAFETY: Per the precondition.
                 unsafe {
                     self.write_formatted_parts(&formatted)?;
@@ -2029,7 +2032,7 @@ impl<'a> Formatter<'a> {
     #[must_use]
     #[stable(feature = "fmt_flags", since = "1.5.0")]
     pub fn width(&self) -> Option<usize> {
-        self.options.width
+        self.options.width.map(|x| x as usize)
     }
 
     /// Returns the optionally specified precision for numeric types.
@@ -2060,7 +2063,7 @@ impl<'a> Formatter<'a> {
     #[must_use]
     #[stable(feature = "fmt_flags", since = "1.5.0")]
     pub fn precision(&self) -> Option<usize> {
-        self.options.precision
+        self.options.precision.map(|x| x as usize)
     }
 
     /// Determines if the `+` flag was specified.
@@ -2800,7 +2803,7 @@ pub(crate) fn pointer_fmt_inner(ptr_addr: usize, f: &mut Formatter<'_>) -> Resul
         f.options.flags |= 1 << (rt::Flag::SignAwareZeroPad as u32);
 
         if f.options.width.is_none() {
-            f.options.width = Some((usize::BITS / 4) as usize + 2);
+            f.options.width = Some((usize::BITS / 4) as u16 + 2);
         }
     }
     f.options.flags |= 1 << (rt::Flag::Alternate as u32);

Diff for: library/core/src/fmt/rt.rs

@@ -47,7 +47,11 @@ pub enum Alignment {
 #[derive(Copy, Clone)]
 pub enum Count {
     /// Specified with a literal number, stores the value
+    #[cfg(bootstrap)]
     Is(usize),
+    /// Specified with a literal number, stores the value
+    #[cfg(not(bootstrap))]
+    Is(u16),
     /// Specified using `$` and `*` syntaxes, stores the index into `args`
     Param(usize),
     /// Not specified
@@ -74,7 +78,7 @@ enum ArgumentType<'a> {
         formatter: unsafe fn(NonNull<()>, &mut Formatter<'_>) -> Result,
         _lifetime: PhantomData<&'a ()>,
     },
-    Count(usize),
+    Count(u16),
 }
 
 /// This struct represents a generic "argument" which is taken by format_args!().
@@ -151,7 +155,10 @@ impl Argument<'_> {
     }
     #[inline]
     pub const fn from_usize(x: &usize) -> Argument<'_> {
-        Argument { ty: ArgumentType::Count(*x) }
+        if *x > u16::MAX as usize {
+            panic!("Formatting argument out of range");
+        };
+        Argument { ty: ArgumentType::Count(*x as u16) }
     }
 
     /// Format this placeholder argument.
@@ -181,7 +188,7 @@ impl Argument<'_> {
     }
 
     #[inline]
-    pub(super) const fn as_usize(&self) -> Option<usize> {
+    pub(super) const fn as_u16(&self) -> Option<u16> {
         match self.ty {
             ArgumentType::Count(count) => Some(count),
             ArgumentType::Placeholder { .. } => None,