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new lint
local_assigned_single_value
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,185 @@ | ||
| use clippy_utils::diagnostics::span_lint; | ||
| use clippy_utils::fn_has_unsatisfiable_preds; | ||
| use clippy_utils::source::snippet_opt; | ||
| use itertools::Itertools; | ||
| use rustc_const_eval::interpret::Scalar; | ||
| use rustc_data_structures::fx::FxHashMap; | ||
| use rustc_hir::def_id::LocalDefId; | ||
| use rustc_hir::{intravisit::FnKind, Body, FnDecl}; | ||
| use rustc_lint::{LateContext, LateLintPass}; | ||
| use rustc_middle::mir::{ | ||
| self, interpret::ConstValue, visit::Visitor, Constant, Location, Operand, Rvalue, Statement, StatementKind, | ||
| }; | ||
| use rustc_session::{declare_lint_pass, declare_tool_lint}; | ||
| use rustc_span::Span; | ||
|
|
||
| declare_clippy_lint! { | ||
| /// ### What it does | ||
| /// Checks for locals that are always assigned the same value. | ||
| /// | ||
| /// ### Why is this bad? | ||
| /// It's almost always a typo. If not, it can be made immutable, or turned into a constant. | ||
| /// | ||
| /// ### Example | ||
| /// ```rust | ||
| /// let mut x = 1; | ||
| /// x = 1; | ||
| /// ``` | ||
| /// Use instead: | ||
| /// ```rust | ||
| /// let x = 1; | ||
| /// ``` | ||
| #[clippy::version = "1.72.0"] | ||
| pub LOCAL_ASSIGNED_SINGLE_VALUE, | ||
| correctness, | ||
| "disallows assigning locals many times with the same value" | ||
| } | ||
| declare_lint_pass!(LocalAssignedSingleValue => [LOCAL_ASSIGNED_SINGLE_VALUE]); | ||
|
|
||
| impl LateLintPass<'_> for LocalAssignedSingleValue { | ||
| fn check_fn( | ||
| &mut self, | ||
| cx: &LateContext<'_>, | ||
| _: FnKind<'_>, | ||
| _: &FnDecl<'_>, | ||
| _: &Body<'_>, | ||
| _: Span, | ||
| def_id: LocalDefId, | ||
| ) { | ||
| // Building MIR for `fn`s with unsatisfiable preds results in ICE. | ||
| if fn_has_unsatisfiable_preds(cx, def_id.to_def_id()) { | ||
| return; | ||
| } | ||
|
|
||
| let mir = cx.tcx.optimized_mir(def_id.to_def_id()); | ||
| let mut v = V { | ||
| body: mir, | ||
| cx, | ||
| local_usage: mir | ||
| .local_decls | ||
| .iter_enumerated() | ||
| .map(|(local, _)| (local, LocalUsageValues::default())) | ||
| .collect(), | ||
| }; | ||
| v.visit_body(mir); | ||
|
|
||
| for (local, usage) in &v.local_usage { | ||
| if should_lint(&v.local_usage, *local, usage) { | ||
| let LocalUsageValues { | ||
| usage, | ||
| mut_ref_acquired: _, | ||
| assigned_non_const: _, | ||
| } = usage; | ||
|
|
||
| if let Some(local_decl) = mir.local_decls.get(*local) | ||
| && let [dbg_info] = &*mir | ||
| .var_debug_info | ||
| .iter() | ||
| .filter(|info| info.source_info.span == local_decl.source_info.span) | ||
| .collect_vec() | ||
| // Don't handle function arguments. | ||
| && dbg_info.argument_index.is_none() | ||
| // Ignore anything from a procedural macro, or locals we cannot prove aren't | ||
| // temporaries | ||
| && let Some(snippet) = snippet_opt(cx, dbg_info.source_info.span) | ||
| && snippet.ends_with(dbg_info.name.as_str()) | ||
| { | ||
| span_lint( | ||
| cx, | ||
| LOCAL_ASSIGNED_SINGLE_VALUE, | ||
| usage.iter().map(|(span, _)| *span).collect_vec(), | ||
| "local only ever assigned single value", | ||
| ); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
|
||
| type LocalUsage = FxHashMap<mir::Local, LocalUsageValues>; | ||
|
|
||
| /// Holds the data we have for the usage of a local. | ||
| #[derive(Default)] | ||
| struct LocalUsageValues { | ||
| /// Where and what this local is assigned. | ||
| usage: Vec<(Span, Scalar)>, | ||
| /// Whether it's mutably borrowed, ever. We should not lint this. | ||
| mut_ref_acquired: bool, | ||
| /// Whether it's assigned a value we cannot prove is constant, ever. We should not lint this. | ||
| assigned_non_const: bool, | ||
| } | ||
|
|
||
| struct V<'a, 'tcx> { | ||
| #[allow(dead_code)] | ||
| body: &'a mir::Body<'tcx>, | ||
| cx: &'a LateContext<'tcx>, | ||
| local_usage: LocalUsage, | ||
| } | ||
|
|
||
| impl<'a, 'tcx> Visitor<'tcx> for V<'a, 'tcx> { | ||
| fn visit_statement(&mut self, stmt: &Statement<'tcx>, _: Location) { | ||
| let Self { | ||
| body: _, | ||
| cx, | ||
| local_usage, | ||
| } = self; | ||
|
|
||
| if stmt.source_info.span.from_expansion() { | ||
| return; | ||
| } | ||
|
|
||
| if let StatementKind::Assign(assign) = &stmt.kind { | ||
| let (place, rvalue) = &**assign; | ||
| // Do not lint if there are any mutable borrows to a local | ||
| if let Rvalue::Ref(_, mir::BorrowKind::Unique | mir::BorrowKind::Mut { .. }, place) = rvalue | ||
| && let Some(other) = local_usage.get_mut(&place.local) | ||
| { | ||
| other.assigned_non_const = true; | ||
| return; | ||
| } | ||
| let Some(usage) = local_usage.get_mut(&place.local) else { | ||
| return; | ||
| }; | ||
|
|
||
| if let Rvalue::Use(Operand::Constant(constant)) = rvalue | ||
| && let Constant { literal, .. } = **constant | ||
| && let Some(ConstValue::Scalar(val)) = literal.try_to_value(cx.tcx) | ||
| { | ||
| usage.usage.push((stmt.source_info.span, val)); | ||
| } else if let Rvalue::Use(Operand::Copy(place)) = rvalue | ||
| && let [_base_proj, ..] = place.projection.as_slice() | ||
| { | ||
| // While this could be `let [x, y] = [1, 1]` or `let (x, y) = (1, 1)`, which should | ||
| // indeed be considered a constant, handling such a case would overcomplicate this | ||
| // lint. | ||
| // | ||
| // TODO(Centri3): Let's do the above as a follow-up, in the future! In particular, | ||
| // we need to handle `ProjectionElem::Field` and `ProjectionElem::Index`. | ||
| usage.assigned_non_const = true; | ||
| } else { | ||
| // We can also probably handle stuff like `x += 1` here, maybe. But this would be | ||
| // very very complex. Let's keep it simple enough. | ||
| usage.assigned_non_const = true; | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
|
||
| fn should_lint(_local_usage: &LocalUsage, _local: mir::Local, usage: &LocalUsageValues) -> bool { | ||
| let LocalUsageValues { | ||
| usage, | ||
| mut_ref_acquired, | ||
| assigned_non_const, | ||
| } = usage; | ||
|
|
||
| if usage.len() > 1 | ||
| && !mut_ref_acquired | ||
| && !assigned_non_const | ||
| && let [(_, head), tail @ ..] = &**usage | ||
| && tail.iter().all(|(_, i)| i == head) | ||
| { | ||
| return true; | ||
| } | ||
|
|
||
| false | ||
| } |
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