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Auto merge of #147641 - cjgillot:liveness-cache-strings, r=estebank

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Liveness: Cache the set of string constants for suggestions.

Even on a slow diagnostic path, listing all the constants in MIR can be expensive, so cache it.
This commit is contained in:
bors 2025-11-07 09:50:49 +00:00
commit 96064126a0
1 changed files with 40 additions and 41 deletions
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81
compiler/rustc_mir_transform/src/liveness.rs
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@ -164,45 +164,6 @@ fn is_capture(place: PlaceRef<'_>) -> bool {
}
}
/// Give a diagnostic when any of the string constants look like a naked format string that would
/// interpolate our dead local.
fn maybe_suggest_literal_matching_name(
body: &Body<'_>,
name: Symbol,
) -> Vec<errors::UnusedVariableStringInterp> {
struct LiteralFinder<'body, 'tcx> {
body: &'body Body<'tcx>,
name: String,
name_colon: String,
found: Vec<errors::UnusedVariableStringInterp>,
}
impl<'tcx> Visitor<'tcx> for LiteralFinder<'_, 'tcx> {
fn visit_const_operand(&mut self, constant: &ConstOperand<'tcx>, loc: Location) {
if let ty::Ref(_, ref_ty, _) = constant.ty().kind()
&& ref_ty.kind() == &ty::Str
{
let rendered_constant = constant.const_.to_string();
if rendered_constant.contains(&self.name)
|| rendered_constant.contains(&self.name_colon)
{
let lit = self.body.source_info(loc).span;
self.found.push(errors::UnusedVariableStringInterp { lit });
}
}
}
}
let mut finder = LiteralFinder {
body,
name: format!("{{{name}}}"),
name_colon: format!("{{{name}:"),
found: vec![],
};
finder.visit_body(body);
finder.found
}
/// Give a diagnostic when an unused variable may be a typo of a unit variant or a struct.
fn maybe_suggest_unit_pattern_typo<'tcx>(
tcx: TyCtxt<'tcx>,
@ -879,6 +840,44 @@ impl<'a, 'tcx> AssignmentResult<'a, 'tcx> {
fn report_fully_unused(&mut self) {
let tcx = self.tcx;
// Give a diagnostic when any of the string constants look like a naked format string that
// would interpolate our dead local.
let mut string_constants_in_body = None;
let mut maybe_suggest_literal_matching_name = |name: Symbol| {
// Visiting MIR to enumerate string constants can be expensive, so cache the result.
let string_constants_in_body = string_constants_in_body.get_or_insert_with(|| {
struct LiteralFinder {
found: Vec<(Span, String)>,
}
impl<'tcx> Visitor<'tcx> for LiteralFinder {
fn visit_const_operand(&mut self, constant: &ConstOperand<'tcx>, _: Location) {
if let ty::Ref(_, ref_ty, _) = constant.ty().kind()
&& ref_ty.kind() == &ty::Str
{
let rendered_constant = constant.const_.to_string();
self.found.push((constant.span, rendered_constant));
}
}
}
let mut finder = LiteralFinder { found: vec![] };
finder.visit_body(self.body);
finder.found
});
let brace_name = format!("{{{name}");
string_constants_in_body
.iter()
.filter(|(_, rendered_constant)| {
rendered_constant
.split(&brace_name)
.any(|c| matches!(c.chars().next(), Some('}' | ':')))
})
.map(|&(lit, _)| errors::UnusedVariableStringInterp { lit })
.collect::<Vec<_>>()
};
// First, report fully unused locals.
for (index, place) in self.checked_places.iter() {
if self.ever_live.contains(index) {
@ -940,7 +939,7 @@ impl<'a, 'tcx> AssignmentResult<'a, 'tcx> {
def_span,
errors::UnusedVariable {
name,
string_interp: maybe_suggest_literal_matching_name(self.body, name),
string_interp: maybe_suggest_literal_matching_name(name),
sugg,
},
);
@ -1030,7 +1029,7 @@ impl<'a, 'tcx> AssignmentResult<'a, 'tcx> {
spans,
errors::UnusedVariable {
name,
string_interp: maybe_suggest_literal_matching_name(self.body, name),
string_interp: maybe_suggest_literal_matching_name(name),
sugg,
},
);