Cannot set granular width configuration settings to 100% if max_width = 100

@ytmimi

Thanks for your work on rustfmt!

The granular width configuration settings are documented to be percentages of max_width (e.g. chain_width). However when max_width is set to 100 setting their values to 100 causes a warning

`fn_call_width` cannot have a value that exceeds `max_width`. `fn_call_width` will be set to the same value as `max_width`

The error message is both surprising and incorrect since fn_call_width = 100 is not a value that exceeds max_width.

This does not happen if max_width is 99 or 101.

The exact config options that trigger this bug are:

max_width = 100
use_small_heuristics = "Off"
fn_call_width = 100

$ rustfmt --version
rustfmt 1.4.38-stable (fe5b13d 2022-05-18)

Edit by @ytmimi (add a minimal reproducible code snippet):

macro_rules! foo {
    () => {};
}

Thanks for the report!

Just as a heads up, when filing an issue please try to include a minimum code example that can be used to reproduce the issue along with any configuration options needed to produce the issue. It was a little tricky to track down since I tried running rustfmt on a few files and I couldn't reproduce the error until I ran rustfmt against src/utils.rs. Doing so the error is emitted 4 times.

Running rustfmt with debug logging enabled (RUSTFMT_LOG=rustfmt=DEBUG rustfmt --config-path=issue_5404.toml --emit stdout src/utils.rs) reveals that this error is always emitted after calling replace_names:

[2022-06-28T17:05:55Z DEBUG rustfmt_nightly::macros] replace_names `zsp.source_callsite()` {"$sp": "zsp"}
`fn_call_width` cannot have a value that exceeds `max_width`. `fn_call_width` will be set to the same value as `max_width`

[2022-06-28T17:05:55Z DEBUG rustfmt_nightly::macros] replace_names `!zself.config.file_lines().is_all()
                && !zself
                    .config
                    .file_lines()
                    .intersects(&zself.parse_sess.lookup_line_range(zspan))` {"$span": "zspan", "$self": "zself"}
`fn_call_width` cannot have a value that exceeds `max_width`. `fn_call_width` will be set to the same value as `max_width`

[2022-06-28T17:05:55Z DEBUG rustfmt_nightly::macros] replace_names `if out_of_file_lines_range!(zself, zspan) {
                return None;
            }` {"$self": "zself", "$span": "zspan"}
`fn_call_width` cannot have a value that exceeds `max_width`. `fn_call_width` will be set to the same value as `max_width`

[2022-06-28T17:05:55Z DEBUG rustfmt_nightly::macros] replace_names `if out_of_file_lines_range!(zself, zspan) {
                zself.push_rewrite(zspan, None);
                return;
            }` {"$span": "zspan", "$self": "zself"}
`fn_call_width` cannot have a value that exceeds `max_width`. `fn_call_width` will be set to the same value as `max_width`

replace_names only gets called in MacroBranch::rewrite

rustfmt/src/macros.rs

Lines 483 to 486 in c4416f2

    
           // Replaces `$foo` with `zfoo`. We must check for name overlap to ensure we 
        
           // aren't causing problems. 
        
           // This should also work for escaped `$` variables, where we leave earlier `$`s. 
        
           fn replace_names(input: &str) -> Option<(String, HashMap<String, String>)> {

And on line 1230 we adjust the max_width, which is where the issue is occurring.

rustfmt/src/macros.rs

Lines 1216 to 1230 in c4416f2

    
           let (body_str, substs) = replace_names(old_body)?; 
        
           let has_block_body = old_body.starts_with('{'); 
        
           let mut config = context.config.clone(); 
        
           config.set().hide_parse_errors(true); 
        
           result += " {"; 
        
           let body_indent = if has_block_body { 
        
               shape.indent 
        
           } else { 
        
               shape.indent.block_indent(&config) 
        
           }; 
        
           let new_width = config.max_width() - body_indent.width(); 
        
           config.set().max_width(new_width);

Config::set returns a ConfigSetter, and trying to set any of the width settings results in calling Config::set_heuristics.

rustfmt/src/config/config_type.rs

Lines 89 to 103 in c4416f2

    
           impl<'a> ConfigSetter<'a> { 
        
               $( 
        
               #[allow(unreachable_pub)] 
        
               pub fn $i(&mut self, value: $ty) { 
        
                   (self.0).$i.2 = value; 
        
                   match stringify!($i) { 
        
                       "max_width" 
        
                       | "use_small_heuristics" 
        
                       | "fn_call_width" 
        
                       | "single_line_if_else_max_width" 
        
                       | "attr_fn_like_width" 
        
                       | "struct_lit_width" 
        
                       | "struct_variant_width" 
        
                       | "array_width" 
        
                       | "chain_width" => self.0.set_heuristics(),

Config::set_heuristics calls Config::set_width_heuristics.

rustfmt/src/config/config_type.rs

Lines 373 to 381 in c4416f2

    
           fn set_heuristics(&mut self) { 
        
               let max_width = self.max_width.2; 
        
               match self.use_small_heuristics.2 { 
        
                   Heuristics::Default => 
        
                       self.set_width_heuristics(WidthHeuristics::scaled(max_width)), 
        
                   Heuristics::Max => self.set_width_heuristics(WidthHeuristics::set(max_width)), 
        
                   Heuristics::Off => self.set_width_heuristics(WidthHeuristics::null()), 
        
               }; 
        
           }

Config::set_width_heuristics pretty much goes through all the width settings one by one and calls the get_width_value closure on each, leading to the error that you're seeing:

rustfmt/src/config/config_type.rs

Lines 294 to 314 in c4416f2

    
                       fn set_width_heuristics(&mut self, heuristics: WidthHeuristics) { 
        
                           let max_width = self.max_width.2; 
        
                           let get_width_value = | 
        
                               was_set: bool, 
        
                               override_value: usize, 
        
                               heuristic_value: usize, 
        
                               config_key: &str, 
        
                           | -> usize { 
        
                               if !was_set { 
        
                                   return heuristic_value; 
        
                               } 
        
                               if override_value > max_width { 
        
                                   eprintln!( 
        
                                       "`{0}` cannot have a value that exceeds `max_width`. \ 
        
                                       `{0}` will be set to the same value as `max_width`", 
        
                                       config_key, 
        
                                   ); 
        
                                   return max_width; 
        
                               } 
        
                               override_value 
        
                           };

fwiw I know this has been reported before, but can't find the prior issues atm

@ytmimi

Wow @ytmimi, thorough research/response, thank you. I'll add the minimal repro next time.

Hi! I can also reproduce with the following:

.rustfmt.toml

max_width = 100 # Not strictly necessary to reproduce.
single_line_if_else_max_width = 100 # Any value above 92 produces the incorrect warning.

src/main.rs

macro_rules! anything { // Necessary to reproduce.
    () => {};
}

fn main() {
    let v = if a == b { String::new() } else { format!("anything") }; // Formatting does happen.
}

The following incorrect warning is then displayed:

$ cargo fmt
`single_line_if_else_max_width` cannot have a value that exceeds `max_width`. `single_line_if_else_max_width` will be set to the same value as `max_width`

Although the project seems correctly formatted. Was this minimal reproductible example the only blocking point?

I just experienced the same problem and put a few print statements into the rustfmt source code, which revealed the following behaviour:

// max_width = 100
// fn_call_width = 100

macro_rules! foo {
    () => {
        // max_width = 92
        // fn_call_width = 100

        // WARNING: `fn_call_width` cannot have a value that exceeds `max_width`. `fn_call_width` will be set to the same value as `max_width`
        // Which results in:
        // fn_call_width = 92
    };
}

Notice that rustfmt decreases max_width as it enters deeper scopes/nesting, but fn_call_width remains the same. I guess this saturation is actually desirable behaviour, but getting the context-less warning message causes more confusion than anything.

Interestingly, if you add an extra pair of brackets, max_width in the nested scope will be 96 instead of 92:

// max_width = 100
// fn_call_width = 100

macro_rules! foo {
    () => {{
        // max_width = 96
        // fn_call_width = 100

        // WARNING: `fn_call_width` cannot have a value that exceeds `max_width`. `fn_call_width` will be set to the same value as `max_width`
        // Which results in:
        // fn_call_width = 96
    }};
}

I have addressed this in #6651.

Reopening to track revert #7222 (#7223); we'll probably have to reland #6651 with a style edition gate.

Navigation Menu

Repository navigation

Cannot set granular width configuration settings to 100% if `max_width = 100` #5404

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

	// Replaces `$foo` with `zfoo`. We must check for name overlap to ensure we
	// aren't causing problems.
	// This should also work for escaped `$` variables, where we leave earlier `$`s.
	fn replace_names(input: &str) -> Option<(String, HashMap<String, String>)> {

	let (body_str, substs) = replace_names(old_body)?;
	let has_block_body = old_body.starts_with('{');

	let mut config = context.config.clone();
	config.set().hide_parse_errors(true);

	result += " {";

	let body_indent = if has_block_body {
	shape.indent
	} else {
	shape.indent.block_indent(&config)
	};
	let new_width = config.max_width() - body_indent.width();
	config.set().max_width(new_width);

	impl<'a> ConfigSetter<'a> {
	$(
	#[allow(unreachable_pub)]
	pub fn $i(&mut self, value: $ty) {
	(self.0).$i.2 = value;
	match stringify!($i) {
	"max_width"
	\| "use_small_heuristics"
	\| "fn_call_width"
	\| "single_line_if_else_max_width"
	\| "attr_fn_like_width"
	\| "struct_lit_width"
	\| "struct_variant_width"
	\| "array_width"
	\| "chain_width" => self.0.set_heuristics(),

	fn set_heuristics(&mut self) {
	let max_width = self.max_width.2;
	match self.use_small_heuristics.2 {
	Heuristics::Default =>
	self.set_width_heuristics(WidthHeuristics::scaled(max_width)),
	Heuristics::Max => self.set_width_heuristics(WidthHeuristics::set(max_width)),
	Heuristics::Off => self.set_width_heuristics(WidthHeuristics::null()),
	};
	}

	fn set_width_heuristics(&mut self, heuristics: WidthHeuristics) {
	let max_width = self.max_width.2;
	let get_width_value = \|
	was_set: bool,
	override_value: usize,
	heuristic_value: usize,
	config_key: &str,
	\| -> usize {
	if !was_set {
	return heuristic_value;
	}
	if override_value > max_width {
	eprintln!(
	"`{0}` cannot have a value that exceeds `max_width`. \
	`{0}` will be set to the same value as `max_width`",
	config_key,
	);
	return max_width;
	}
	override_value
	};

Cannot set granular width configuration settings to 100% if max_width = 100 #5404

Description

Activity

ytmimi commented on Jun 28, 2022

calebcartwright commented on Jun 28, 2022

tcharding commented on Jun 28, 2022

iago-lito commented on Apr 25, 2023

ferdonline commented on Oct 24, 2023

ytmimi commented on Oct 24, 2023

ferdonline commented on Oct 26, 2023

ytmimi commented on Oct 26, 2023

bergkvist commented on Feb 16, 2024

csmulhern commented on Sep 16, 2025

jieyouxu commented on Nov 2, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

Cannot set granular width configuration settings to 100% if `max_width = 100` #5404