Presence of macro definition causes incorrect width settings warnings

[zmill]

With the following src/lib.rs:

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

rustfmt seems to be getting incorrect width settings from somewhere:

$ rustfmt --config max_width=80,chain_width=72 --print-config current . | grep '\(\<max\|chain\)_width'
max_width = 80
chain_width = 72

$ rustfmt --config max_width=80,chain_width=72 --check src/lib.rs
(no output)

$ rustfmt --config max_width=80,chain_width=73 --print-config current . | grep '\(\<max\|chain\)_width'
max_width = 80
chain_width = 73

$ rustfmt --config max_width=80,chain_width=73 --check src/lib.rs
`chain_width` cannot have a value that exceeds `max_width`. `chain_width` will be set to the same value as `max_width`

If I don't also override max_width, the warning occurs with chain_width >= 93 instead of 101 as expected; I checked several other max_widths as well, and it seems the highest chain_width accepted without warning is always 8 less than max_width.

---

If src/lib.rs doesn't contain a macro, e.g.:

fn f() {}

Then I get the warning (albeit twice) starting with a chain_width of one more than max_width as expected:

$ rustfmt --config max_width=80,chain_width=80 --print-config current . | grep '\(\<max\|chain\)_width'
max_width = 80
chain_width = 80

$ rustfmt --config max_width=80,chain_width=80 --check src/lib.rs
(no output)

$ rustfmt --config max_width=80,chain_width=81 --print-config current . | grep '\(\<max\|chain\)_width'
`chain_width` cannot have a value that exceeds `max_width`. `chain_width` will be set to the same value as `max_width`
max_width = 80
chain_width = 80

$ rustfmt --config max_width=80,chain_width=81 --check src/lib.rs
`chain_width` cannot have a value that exceeds `max_width`. `chain_width` will be set to the same value as `max_width`
`chain_width` cannot have a value that exceeds `max_width`. `chain_width` will be set to the same value as `max_width`

---

Behavior is the same for array_width and struct_lit_width; I didn't check the rest of the width settings. Behavior is also the same on stable and nightly.

$ rustfmt -V
rustfmt 1.4.37-stable (2a3635d5 2021-05-04)

$ rustfmt +nightly -V
rustfmt 1.4.37-nightly (d9aa2876 2021-07-24)

Full config in case it's helpful (should all be defaults):

$ rustfmt --print-config current .
max_width = 100
hard_tabs = false
tab_spaces = 4
newline_style = "Auto"
indent_style = "Block"
use_small_heuristics = "Default"
fn_call_width = 60
attr_fn_like_width = 70
struct_lit_width = 18
struct_variant_width = 35
array_width = 60
chain_width = 60
single_line_if_else_max_width = 50
wrap_comments = false
format_code_in_doc_comments = false
comment_width = 80
normalize_comments = false
normalize_doc_attributes = false
license_template_path = ""
format_strings = false
format_macro_matchers = false
format_macro_bodies = true
empty_item_single_line = true
struct_lit_single_line = true
fn_single_line = false
where_single_line = false
imports_indent = "Block"
imports_layout = "Mixed"
imports_granularity = "Preserve"
group_imports = "Preserve"
reorder_imports = true
reorder_modules = true
reorder_impl_items = false
type_punctuation_density = "Wide"
space_before_colon = false
space_after_colon = true
spaces_around_ranges = false
binop_separator = "Front"
remove_nested_parens = true
combine_control_expr = true
overflow_delimited_expr = false
struct_field_align_threshold = 0
enum_discrim_align_threshold = 0
match_arm_blocks = true
match_arm_leading_pipes = "Never"
force_multiline_blocks = false
fn_args_layout = "Tall"
brace_style = "SameLineWhere"
control_brace_style = "AlwaysSameLine"
trailing_semicolon = true
trailing_comma = "Vertical"
match_block_trailing_comma = false
blank_lines_upper_bound = 1
blank_lines_lower_bound = 0
edition = "2015"
version = "One"
inline_attribute_width = 0
merge_derives = true
use_try_shorthand = false
use_field_init_shorthand = false
force_explicit_abi = true
condense_wildcard_suffixes = false
color = "Auto"
required_version = "1.4.37"
unstable_features = false
disable_all_formatting = false
skip_children = false
hide_parse_errors = false
error_on_line_overflow = false
error_on_unformatted = false
report_todo = "Never"
report_fixme = "Never"
ignore = []
emit_mode = "Files"
make_backup = false

[calebcartwright]

The correlation with the presence of a macro def is baffling since the evaluation of the config settings should be happening long before any parsing stages.

For anyone interested in taking a look at fixing this the comparison point that's emitting the warnings can be found here:

rustfmt/src/config/config_type.rs

Lines 300 to 320 in e81c393

	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
	};

[ytmimi]

going to close this in favor of #5404 as there's more discussion there