Trailing space on doc comment

[Ciznia]

Summary

I tried to format this code:

use std::{borrow::Cow, hash::Hash};

/// A CSS-like string value.
///
/// Used for `font-family`, `content` (pseudo-elements), custom identifiers,
/// and any other property that takes a textual value.
///
/// # Zero-copy for static strings
///
/// The inner `Cow<'static, str>` means proc_macro-generated code like:
/// ```rust
/// let font = Str::from_static("JetBrains Mono");
/// ```
/// involves **zero heap allocation** — the slice lives in the binary's
/// read-only data segment. Runtime-computed strings fall back to
/// [`Str::from_string`] which heap-allocates via `Cow::Owned`.
///
/// Two `Str` values are equal if their **contents** are equal, regardless
/// of whether one is borrowed and the other owned.
///
/// # Examples
///
/// ```rust
/// use oxidui_style::str::Str;
/// let a: Str = "monospace".into(); // static borrow, no alloc
/// let b = Str::from_string(format!("Font-{}", 42)); // heap-allocated
/// ```
#[derive(Debug, Clone, PartialEq, Eq, Hash, Default)]
pub struct Str(pub Cow<'static, str>);

Expected behavior

I expect no change

Actual behavior

Add a trailing whitespace on the /// let font = Str::from_static("JetBrains Mono"); line
And then fail due to error_on_unformated=true

/// ```rust
/// let font = Str::from_static("JetBrains Mono"); 
/// ```

Configuration

rustfmt.toml

# Global config
edition = "2024"
style_edition = "2024"
unstable_features = true
newline_style = "Unix"

# Comment / Raw string formatting
error_on_unformatted = true
wrap_comments = true
format_strings = true
comment_width = 80
normalize_comments = true
format_code_in_doc_comments = true
doc_comment_code_block_width = 80
normalize_doc_attributes = true

# code formatting config
error_on_line_overflow = true
max_width = 80

# Function / Struct / Enum / Trait formatting
brace_style = "SameLineWhere"
empty_item_single_line = true
fn_single_line = true
fn_params_layout = "Tall"
reorder_impl_items = true
type_punctuation_density = "Wide"
use_field_init_shorthand = true
overflow_delimited_expr = true

# Macro formatting
merge_derives = false

# Imports / Modules formatting
imports_layout = "HorizontalVertical"
imports_granularity = "Crate"
group_imports = "StdExternalCrate"
reorder_imports = true
reorder_modules = true

# Control flow
control_brace_style = "AlwaysSameLine"
match_arm_blocks = true
match_arm_indent = true
match_block_trailing_comma = true
force_multiline_blocks = true

# Other
condense_wildcard_suffixes = true
hex_literal_case = "Upper"
tab_spaces = 4
trailing_comma = "Vertical"
trailing_semicolon = true
use_try_shorthand = true
float_literal_trailing_zero = "IfNoPostfix"

rustfmt cli options used (if applicable):

cargo +nightly fmt

Reproduction Steps

1. Copy this code, and that config, and try to format it

Meta

cargo +nighlty fmt -- --version:

rustfmt 1.9.0-nightly (2d76d9bc76 2026-03-09)

[ytmimi]

@Ciznia can you get the rustfmt.toml down to just the options that are needed to reproduce this issue?

[Ciznia]

yes i will do that when i'm back at home

[utakotoba]

I've narrowed it down to a minimal reproducible snippet using binary search. Here it is:

# error_on_unformatted = true
format_code_in_doc_comments = true
fn_single_line = true

The first option enables error logging for unformatted code. The second enables formatting inside doc comments. The issue is clearly triggered by fn_single_line = true inside documentation comments.

As a reference, here’s the error log produced on @Ciznia code snippet:

Diff in /workspace/src/main.rs:9:
 ///
 /// The inner `Cow<'static, str>` means proc_macro-generated code like:
 /// ```rust
-/// let font = Str::from_static("JetBrains Mono");
+/// let font = Str::from_static("JetBrains Mono"); 
 /// ```
 /// involves **zero heap allocation** — the slice lives in the binary's
 /// read-only data segment. Runtime-computed strings fall back to
error[internal]: left behind trailing whitespace
  --> /workspace/src/main.rs:12:12:51
   |
12 | /// let font = Str::from_static("JetBrains Mono"); 
   |                                                   ^
   |
   = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

warning: rustfmt has failed to format. See previous 1 errors.

[utakotoba]

@rustbot claim

I think this won't be complex work, as it's mostly caused by rustfmt wrapping code inside doc comments with fn main() { ... }, along with a trailing space added in the single-line representation.