`if_same_then_else` triggers when bodies only differ in attributes
Author: bugadaniCreated Jan 8, 2026Updated Sep 18, 2026
LabelsC-bugI-false-positive
Summary
Clippy does not understand the defmt::write! macro and emits a lint warning when bodies differ in the interned string's value.
The macro currently expands to code like this:
{
let _typecheck_formatter: defmt::Formatter<'_> = fmt;
match () {
() => {
defmt::export::istr(&{
defmt::export::make_istr({
#[cfg_attr(target_os = "macos", link_section = ".defmt,119c41d6e2719ed6")]
#[cfg_attr(
not(target_os = "macos"),
link_section = ".defmt.{\"package\":\"esp-alloc\",\"tag\":\"defmt_write\",\"data\":\"Internal\",\"disambiguator\":\"13789615010068757779\",\"crate_name\":\"esp_alloc\"}"
)]
#[export_name = "{\"package\":\"esp-alloc\",\"tag\":\"defmt_write\",\"data\":\"Internal\",\"disambiguator\":\"13789615010068757779\",\"crate_name\":\"esp_alloc\"}"]
static S: u8 = 0;
&S as *const u8 as u16
})
});
}
}
}The macro expansions only differ in the attributes on static S - this is how defmt interns strings into the ELF without including them in the program binary that is downloaded to embedded devices.
Lint Name
if_same_then_else
Reproducer
I tried this code (the revision already contains the allow for the lint, which we had to add to work around this issue):
I saw this happen:
error: this `if` has identical blocks
Error: --> src/lib.rs:238:67
|
238 | if self.capabilities.contains(MemoryCapability::Internal) {
| ___________________________________________________________________^
239 | | defmt::write!(fmt, "Internal");
240 | | } else if self.capabilities.contains(MemoryCapability::External) {
| |_________^
|
note: same as this
--> src/lib.rs:240:74
|
240 | } else if self.capabilities.contains(MemoryCapability::External) {
| __________________________________________________________________________^
241 | | defmt::write!(fmt, "External");
242 | | } else {
| |_________^
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#if_same_then_else
= note: `-D clippy::if-same-then-else` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(clippy::if_same_then_else)]`
error: this `if` has identical blocks
Error: --> src/lib.rs:240:74
|
240 | } else if self.capabilities.contains(MemoryCapability::External) {
| __________________________________________________________________________^
241 | | defmt::write!(fmt, "External");
242 | | } else {
| |_________^
|
note: same as this
--> src/lib.rs:242:16
|
242 | } else {
| ________________^
243 | | defmt::write!(fmt, "Unknown");
244 | | }
| |_________^
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#if_same_then_elseI expected to see this happen:
Clippy should not emit a lint warning.
Version
clippy 0.1.94 (fecb335cba 2026-01-07)Additional Labels
No response
Source: rust-lang/rust-clippy