Add option to control trailing zero in floating-point literals

Configurations.md

@@ -1256,6 +1256,50 @@ Control the case of the letters in hexadecimal literal values
 - **Possible values**: `Preserve`, `Upper`, `Lower`
 - **Stable**: No (tracking issue: [#5081](https://github.com/rust-lang/rustfmt/issues/5081))
 
+## `float_literal_trailing_zero`
+
+Control the presence of trailing zero in floating-point literal values
+
+- **Default value**: `Preserve`
+- **Possible values**: `Preserve`, `Always`, `IfNoPostfix`, `Never`
+- **Stable**: No (tracking issue: [#3187](https://github.com/rust-lang/rustfmt/issues/3187))
+
+#### `Preserve` (default):
+
+Leave the literal as-is.
+
+#### `Always`:
+
+Add a trailing zero to the literal:
+
+```rust
+fn main() {
+    let values = [1.0, 2.0e10, 3.0f32];
+}
+```
+
+#### `IfNoPostfix`:
+
+Add a trailing zero by default. If the literal contains an exponent or a suffix, the zero
+and the preceding period are removed:
+
+```rust
+fn main() {
+    let values = [1.0, 2e10, 3f32];
+}
+```
+
+#### `Never`:
+
+Remove the trailing zero. If the literal contains an exponent or a suffix, the preceding
+period is also removed:
+
+```rust
+fn main() {
+    let values = [1., 2e10, 3f32];
+}
+```
+
 ## `hide_parse_errors`
 
 This option is deprecated and has been renamed to `show_parse_errors` to avoid confusion around the double negative default of `hide_parse_errors=false`.

src/config/mod.rs

@@ -79,6 +79,8 @@ create_config! {
         "Skip formatting the bodies of macros invoked with the following names.";
     hex_literal_case: HexLiteralCase, HexLiteralCase::Preserve, false,
         "Format hexadecimal integer literals";
+    float_literal_trailing_zero: FloatLiteralTrailingZero, FloatLiteralTrailingZero::Preserve,
+        false, "Add or remove trailing zero in floating-point literals";
 
     // Single line expressions and items
     empty_item_single_line: bool, true, false,
@@ -645,6 +647,7 @@ format_macro_matchers = false
 format_macro_bodies = true
 skip_macro_invocations = []
 hex_literal_case = "Preserve"
+float_literal_trailing_zero = "Preserve"
 empty_item_single_line = true
 struct_lit_single_line = true
 fn_single_line = false

src/config/options.rs

@@ -144,6 +144,21 @@ pub enum HexLiteralCase {
     Lower,
 }
 
+/// How to treat trailing zeros in floating-point literals.
+#[config_type]
+pub enum FloatLiteralTrailingZero {
+    /// Leave the literal as-is.
+    Preserve,
+    /// Add a trailing zero to the literal.
+    Always,
+    /// Add a trailing zero by default. If the literal contains an exponent or a suffix, the zero
+    /// and the preceding period are removed.
+    IfNoPostfix,
+    /// Remove the trailing zero. If the literal contains an exponent or a suffix, the preceding
+    /// period is also removed.
+    Never,
+}
+
 #[config_type]
 pub enum ReportTactic {
     Always,

src/expr.rs

@@ -2,6 +2,8 @@ use std::borrow::Cow;
 use std::cmp::min;
 
 use itertools::Itertools;
+use lazy_static::lazy_static;
+use regex::Regex;
 use rustc_ast::token::{Delimiter, Lit, LitKind};
 use rustc_ast::{ast, ptr, token, ForLoopKind};
 use rustc_span::{BytePos, Span};
@@ -13,7 +15,9 @@ use crate::comment::{
     rewrite_missing_comment, CharClasses, FindUncommented,
 };
 use crate::config::lists::*;
-use crate::config::{Config, ControlBraceStyle, HexLiteralCase, IndentStyle, Version};
+use crate::config::{
+    Config, ControlBraceStyle, FloatLiteralTrailingZero, HexLiteralCase, IndentStyle, Version,
+};
 use crate::lists::{
     definitive_tactic, itemize_list, shape_for_tactic, struct_lit_formatting, struct_lit_shape,
     struct_lit_tactic, write_list, ListFormatting, Separator,
@@ -37,6 +41,14 @@ use crate::utils::{
 use crate::vertical::rewrite_with_alignment;
 use crate::visitor::FmtVisitor;
 
+lazy_static! {
+    // This regex may accept invalid float literals (such as `1`, `_` or `2.e3`). That's ok.
+    // We only use it to parse literals whose validity has already been established.
+    static ref FLOAT_LITERAL: Regex =
+        Regex::new(r"^([0-9_]+)(?:\.([0-9_]+)?)?([eE][+-]?[0-9_]+)?$").unwrap();
+    static ref ZERO_LITERAL: Regex = Regex::new(r"^[0_]+$").unwrap();
+}
+
 impl Rewrite for ast::Expr {
     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
         format_expr(self, ExprType::SubExpression, context, shape)
@@ -1242,6 +1254,7 @@ pub(crate) fn rewrite_literal(
     match token_lit.kind {
         token::LitKind::Str => rewrite_string_lit(context, span, shape),
         token::LitKind::Integer => rewrite_int_lit(context, token_lit, span, shape),
+        token::LitKind::Float => rewrite_float_lit(context, token_lit, span, shape),
         _ => wrap_str(
             context.snippet(span).to_owned(),
             context.config.max_width(),
@@ -1283,6 +1296,11 @@ fn rewrite_int_lit(
     shape: Shape,
 ) -> Option<String> {
     let symbol = token_lit.symbol.as_str();
+    let suffix = token_lit.suffix.as_ref().map(|s| s.as_str());
+
+    if suffix == Some("f32") || suffix == Some("f64") {
+        return rewrite_float_lit(context, token_lit, span, shape);
+    }
 
     if let Some(symbol_stripped) = symbol.strip_prefix("0x") {
         let hex_lit = match context.config.hex_literal_case() {
@@ -1292,11 +1310,7 @@ fn rewrite_int_lit(
         };
         if let Some(hex_lit) = hex_lit {
             return wrap_str(
-                format!(
-                    "0x{}{}",
-                    hex_lit,
-                    token_lit.suffix.map_or(String::new(), |s| s.to_string())
-                ),
+                format!("0x{}{}", hex_lit, suffix.unwrap_or("")),
                 context.config.max_width(),
                 shape,
             );
@@ -1310,6 +1324,68 @@ fn rewrite_int_lit(
     )
 }
 
+fn rewrite_float_lit(
+    context: &RewriteContext<'_>,
+    token_lit: token::Lit,
+    span: Span,
+    shape: Shape,
+) -> Option<String> {
+    if matches!(
+        context.config.float_literal_trailing_zero(),
+        FloatLiteralTrailingZero::Preserve
+    ) {
+        return wrap_str(
+            context.snippet(span).to_owned(),
+            context.config.max_width(),
+            shape,
+        );
+    }
+
+    let symbol = token_lit.symbol.as_str();
+    let suffix = token_lit.suffix.as_ref().map(|s| s.as_str());
+
+    let caps = FLOAT_LITERAL.captures(symbol).unwrap();
+    let integer_part = caps.get(1).unwrap().as_str();
+    let fractional_part = caps.get(2).map(|s| s.as_str());
+    let exponent = caps.get(3).map(|s| s.as_str());
+
+    let has_postfix = exponent.is_some() || suffix.is_some();
+    let fractional_part_nonzero = fractional_part.map_or(false, |s| !ZERO_LITERAL.is_match(s));
+
+    let (include_period, include_fractional_part) =
+        match context.config.float_literal_trailing_zero() {
+            FloatLiteralTrailingZero::Preserve => unreachable!("handled above"),
+            FloatLiteralTrailingZero::Always => (true, true),
+            FloatLiteralTrailingZero::IfNoPostfix => (
+                fractional_part_nonzero || !has_postfix,
+                fractional_part_nonzero || !has_postfix,
+            ),
+            FloatLiteralTrailingZero::Never => (
+                fractional_part_nonzero || !has_postfix,
+                fractional_part_nonzero,
+            ),
+        };
+
+    let period = if include_period { "." } else { "" };
+    let fractional_part = if include_fractional_part {
+        fractional_part.unwrap_or("0")
+    } else {
+        ""
+    };
+    wrap_str(
+        format!(
+            "{}{}{}{}{}",
+            integer_part,
+            period,
+            fractional_part,
+            exponent.unwrap_or(""),
+            suffix.unwrap_or(""),
+        ),
+        context.config.max_width(),
+        shape,
+    )
+}
+
 fn choose_separator_tactic(context: &RewriteContext<'_>, span: Span) -> Option<SeparatorTactic> {
     if context.inside_macro() {
         if span_ends_with_comma(context, span) {

tests/source/float-lit-trailing-zero-always.rs

@@ -0,0 +1,17 @@
+// rustfmt-float_literal_trailing_zero: Always
+
+fn float_literals() {
+    let a = 0.;
+    let b = 0.0;
+    let c = 100.;
+    let d = 100.0;
+    let e = 5e3;
+    let f = 5.0e3;
+    let g = 7f32;
+    let h = 7.0f32;
+    let i = 9e3f32;
+    let j = 9.0e3f32;
+    let k = 1000.00;
+    let l = 1_000_.;
+    let m = 1_000_.000_000;
+}

tests/source/float-lit-trailing-zero-if-no-postfix.rs

@@ -0,0 +1,17 @@
+// rustfmt-float_literal_trailing_zero: IfNoPostfix
+
+fn float_literals() {
+    let a = 0.;
+    let b = 0.0;
+    let c = 100.;
+    let d = 100.0;
+    let e = 5e3;
+    let f = 5.0e3;
+    let g = 7f32;
+    let h = 7.0f32;
+    let i = 9e3f32;
+    let j = 9.0e3f32;
+    let k = 1000.00;
+    let l = 1_000_.;
+    let m = 1_000_.000_000;
+}

tests/source/float-lit-trailing-zero-never.rs

@@ -0,0 +1,17 @@
+// rustfmt-float_literal_trailing_zero: Never
+
+fn float_literals() {
+    let a = 0.;
+    let b = 0.0;
+    let c = 100.;
+    let d = 100.0;
+    let e = 5e3;
+    let f = 5.0e3;
+    let g = 7f32;
+    let h = 7.0f32;
+    let i = 9e3f32;
+    let j = 9.0e3f32;
+    let k = 1000.00;
+    let l = 1_000_.;
+    let m = 1_000_.000_000;
+}

tests/target/float-lit-trailing-zero-always.rs

@@ -0,0 +1,17 @@
+// rustfmt-float_literal_trailing_zero: Always
+
+fn float_literals() {
+    let a = 0.0;
+    let b = 0.0;
+    let c = 100.0;
+    let d = 100.0;
+    let e = 5.0e3;
+    let f = 5.0e3;
+    let g = 7.0f32;
+    let h = 7.0f32;
+    let i = 9.0e3f32;
+    let j = 9.0e3f32;
+    let k = 1000.00;
+    let l = 1_000_.0;
+    let m = 1_000_.000_000;
+}

tests/target/float-lit-trailing-zero-if-no-postfix.rs

@@ -0,0 +1,17 @@
+// rustfmt-float_literal_trailing_zero: IfNoPostfix
+
+fn float_literals() {
+    let a = 0.0;
+    let b = 0.0;
+    let c = 100.0;
+    let d = 100.0;
+    let e = 5e3;
+    let f = 5e3;
+    let g = 7f32;
+    let h = 7f32;
+    let i = 9e3f32;
+    let j = 9e3f32;
+    let k = 1000.00;
+    let l = 1_000_.0;
+    let m = 1_000_.000_000;
+}

tests/target/float-lit-trailing-zero-never.rs

@@ -0,0 +1,17 @@
+// rustfmt-float_literal_trailing_zero: Never
+
+fn float_literals() {
+    let a = 0.;
+    let b = 0.;
+    let c = 100.;
+    let d = 100.;
+    let e = 5e3;
+    let f = 5e3;
+    let g = 7f32;
+    let h = 7f32;
+    let i = 9e3f32;
+    let j = 9e3f32;
+    let k = 1000.;
+    let l = 1_000_.;
+    let m = 1_000_.;
+}