Rollup merge of #145863 - EliasHolzmann:formatting_options_20250825, r=m-ou-se

formatting_options: Make all methods `const`

Related to #118117.

Having `const fn`s that take a `mut &` was unstable until Rust 1.83 (see #129195). Because of this, not all methods on `FormattingOptions` were implemented as `const`. As this has been stabilized now, there is no reason not to have all methods `const`.

Thanks to `@Ternvein` for bringing this to my attention (see [1]).

r? `@m-ou-se` (As you were the reviewer for the original implementation – feel free to reroll if you are busy or if you aren't interested)

[1]: #118117 (comment)

Author: GuillaumeGomez

library/core/src/fmt/mod.rs

@@ -359,7 +359,7 @@ impl FormattingOptions {
     ///   always be printed.
     /// - `-`: Currently not used
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn sign(&mut self, sign: Option<Sign>) -> &mut Self {
+    pub const fn sign(&mut self, sign: Option<Sign>) -> &mut Self {
         let sign = match sign {
             None => 0,
             Some(Sign::Plus) => flags::SIGN_PLUS_FLAG,
@@ -372,7 +372,7 @@ impl FormattingOptions {
     ///
     /// This is used to indicate for integer formats that the padding to width should both be done with a 0 character as well as be sign-aware
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn sign_aware_zero_pad(&mut self, sign_aware_zero_pad: bool) -> &mut Self {
+    pub const fn sign_aware_zero_pad(&mut self, sign_aware_zero_pad: bool) -> &mut Self {
         if sign_aware_zero_pad {
             self.flags |= flags::SIGN_AWARE_ZERO_PAD_FLAG;
         } else {
@@ -389,7 +389,7 @@ impl FormattingOptions {
     /// - [`Octal`] - precedes the argument with a `0b`
     /// - [`Binary`] - precedes the argument with a `0o`
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn alternate(&mut self, alternate: bool) -> &mut Self {
+    pub const fn alternate(&mut self, alternate: bool) -> &mut Self {
         if alternate {
             self.flags |= flags::ALTERNATE_FLAG;
         } else {
@@ -404,7 +404,7 @@ impl FormattingOptions {
     /// being formatted is smaller than width some extra characters will be
     /// printed around it.
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn fill(&mut self, fill: char) -> &mut Self {
+    pub const fn fill(&mut self, fill: char) -> &mut Self {
         self.flags = self.flags & (u32::MAX << 21) | fill as u32;
         self
     }
@@ -413,7 +413,7 @@ impl FormattingOptions {
     /// The alignment specifies how the value being formatted should be
     /// positioned if it is smaller than the width of the formatter.
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn align(&mut self, align: Option<Alignment>) -> &mut Self {
+    pub const fn align(&mut self, align: Option<Alignment>) -> &mut Self {
         let align: u32 = match align {
             Some(Alignment::Left) => flags::ALIGN_LEFT,
             Some(Alignment::Right) => flags::ALIGN_RIGHT,
@@ -430,7 +430,7 @@ 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<u16>) -> &mut Self {
+    pub const fn width(&mut self, width: Option<u16>) -> &mut Self {
         if let Some(width) = width {
             self.flags |= flags::WIDTH_FLAG;
             self.width = width;
@@ -450,7 +450,7 @@ 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<u16>) -> &mut Self {
+    pub const fn precision(&mut self, precision: Option<u16>) -> &mut Self {
         if let Some(precision) = precision {
             self.flags |= flags::PRECISION_FLAG;
             self.precision = precision;
@@ -463,7 +463,7 @@ impl FormattingOptions {
     /// Specifies whether the [`Debug`] trait should use lower-/upper-case
     /// hexadecimal or normal integers
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn debug_as_hex(&mut self, debug_as_hex: Option<DebugAsHex>) -> &mut Self {
+    pub const fn debug_as_hex(&mut self, debug_as_hex: Option<DebugAsHex>) -> &mut Self {
         let debug_as_hex = match debug_as_hex {
             None => 0,
             Some(DebugAsHex::Lower) => flags::DEBUG_LOWER_HEX_FLAG,
@@ -537,7 +537,7 @@ impl FormattingOptions {
     ///
     /// You may alternatively use [`Formatter::new()`].
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn create_formatter<'a>(self, write: &'a mut (dyn Write + 'a)) -> Formatter<'a> {
+    pub const fn create_formatter<'a>(self, write: &'a mut (dyn Write + 'a)) -> Formatter<'a> {
         Formatter { options: self, buf: write }
     }
 }
@@ -578,13 +578,13 @@ impl<'a> Formatter<'a> {
     ///
     /// You may alternatively use [`FormattingOptions::create_formatter()`].
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn new(write: &'a mut (dyn Write + 'a), options: FormattingOptions) -> Self {
+    pub const fn new(write: &'a mut (dyn Write + 'a), options: FormattingOptions) -> Self {
         Formatter { options, buf: write }
     }
 
     /// Creates a new formatter based on this one with given [`FormattingOptions`].
     #[unstable(feature = "formatting_options", issue = "118117")]
-    pub fn with_options<'b>(&'b mut self, options: FormattingOptions) -> Formatter<'b> {
+    pub const fn with_options<'b>(&'b mut self, options: FormattingOptions) -> Formatter<'b> {
         Formatter { options, buf: self.buf }
     }
 }