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Auto merge of #13395 - GnomedDev:unnecessary-lit-bound, r=Manishearth
Add lint for unnecessary lifetime bounded &str return Closes #305. Currently implemented with a pretty strong limitation that it can only see the most basic implicit return, but this should be fixable by something with more time and brain energy than me. Cavets from #13388 apply, as I have not had a review on my clippy lints yet so am pretty new to this. ``` changelog: [`unnecessary_literal_bound`]: Add lint for unnecessary lifetime bounded &str return. ```
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use clippy_utils::diagnostics::span_lint_and_sugg; | ||
use clippy_utils::path_res; | ||
use rustc_ast::ast::LitKind; | ||
use rustc_errors::Applicability; | ||
use rustc_hir::def::Res; | ||
use rustc_hir::intravisit::{FnKind, Visitor}; | ||
use rustc_hir::{Body, Expr, ExprKind, FnDecl, FnRetTy, Lit, MutTy, Mutability, PrimTy, Ty, TyKind, intravisit}; | ||
use rustc_lint::{LateContext, LateLintPass}; | ||
use rustc_session::declare_lint_pass; | ||
use rustc_span::Span; | ||
use rustc_span::def_id::LocalDefId; | ||
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declare_clippy_lint! { | ||
/// ### What it does | ||
/// | ||
/// Detects functions that are written to return `&str` that could return `&'static str` but instead return a `&'a str`. | ||
/// | ||
/// ### Why is this bad? | ||
/// | ||
/// This leaves the caller unable to use the `&str` as `&'static str`, causing unneccessary allocations or confusion. | ||
/// This is also most likely what you meant to write. | ||
/// | ||
/// ### Example | ||
/// ```no_run | ||
/// # struct MyType; | ||
/// impl MyType { | ||
/// fn returns_literal(&self) -> &str { | ||
/// "Literal" | ||
/// } | ||
/// } | ||
/// ``` | ||
/// Use instead: | ||
/// ```no_run | ||
/// # struct MyType; | ||
/// impl MyType { | ||
/// fn returns_literal(&self) -> &'static str { | ||
/// "Literal" | ||
/// } | ||
/// } | ||
/// ``` | ||
/// Or, in case you may return a non-literal `str` in future: | ||
/// ```no_run | ||
/// # struct MyType; | ||
/// impl MyType { | ||
/// fn returns_literal<'a>(&'a self) -> &'a str { | ||
/// "Literal" | ||
/// } | ||
/// } | ||
/// ``` | ||
#[clippy::version = "1.83.0"] | ||
pub UNNECESSARY_LITERAL_BOUND, | ||
pedantic, | ||
"detects &str that could be &'static str in function return types" | ||
} | ||
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declare_lint_pass!(UnnecessaryLiteralBound => [UNNECESSARY_LITERAL_BOUND]); | ||
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fn extract_anonymous_ref<'tcx>(hir_ty: &Ty<'tcx>) -> Option<&'tcx Ty<'tcx>> { | ||
let TyKind::Ref(lifetime, MutTy { ty, mutbl }) = hir_ty.kind else { | ||
return None; | ||
}; | ||
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if !lifetime.is_anonymous() || !matches!(mutbl, Mutability::Not) { | ||
return None; | ||
} | ||
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Some(ty) | ||
} | ||
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fn is_str_literal(expr: &Expr<'_>) -> bool { | ||
matches!( | ||
expr.kind, | ||
ExprKind::Lit(Lit { | ||
node: LitKind::Str(..), | ||
.. | ||
}), | ||
) | ||
} | ||
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struct FindNonLiteralReturn; | ||
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impl<'hir> Visitor<'hir> for FindNonLiteralReturn { | ||
type Result = std::ops::ControlFlow<()>; | ||
type NestedFilter = intravisit::nested_filter::None; | ||
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fn visit_expr(&mut self, expr: &'hir Expr<'hir>) -> Self::Result { | ||
if let ExprKind::Ret(Some(ret_val_expr)) = expr.kind | ||
&& !is_str_literal(ret_val_expr) | ||
{ | ||
Self::Result::Break(()) | ||
} else { | ||
intravisit::walk_expr(self, expr) | ||
} | ||
} | ||
} | ||
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fn check_implicit_returns_static_str(body: &Body<'_>) -> bool { | ||
// TODO: Improve this to the same complexity as the Visitor to catch more implicit return cases. | ||
if let ExprKind::Block(block, _) = body.value.kind | ||
&& let Some(implicit_ret) = block.expr | ||
{ | ||
return is_str_literal(implicit_ret); | ||
} | ||
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false | ||
} | ||
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fn check_explicit_returns_static_str(expr: &Expr<'_>) -> bool { | ||
let mut visitor = FindNonLiteralReturn; | ||
visitor.visit_expr(expr).is_continue() | ||
} | ||
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impl<'tcx> LateLintPass<'tcx> for UnnecessaryLiteralBound { | ||
fn check_fn( | ||
&mut self, | ||
cx: &LateContext<'tcx>, | ||
kind: FnKind<'tcx>, | ||
decl: &'tcx FnDecl<'_>, | ||
body: &'tcx Body<'_>, | ||
span: Span, | ||
_: LocalDefId, | ||
) { | ||
if span.from_expansion() { | ||
return; | ||
} | ||
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// Checking closures would be a little silly | ||
if matches!(kind, FnKind::Closure) { | ||
return; | ||
} | ||
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// Check for `-> &str` | ||
let FnRetTy::Return(ret_hir_ty) = decl.output else { | ||
return; | ||
}; | ||
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let Some(inner_hir_ty) = extract_anonymous_ref(ret_hir_ty) else { | ||
return; | ||
}; | ||
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if path_res(cx, inner_hir_ty) != Res::PrimTy(PrimTy::Str) { | ||
return; | ||
} | ||
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// Check for all return statements returning literals | ||
if check_explicit_returns_static_str(body.value) && check_implicit_returns_static_str(body) { | ||
span_lint_and_sugg( | ||
cx, | ||
UNNECESSARY_LITERAL_BOUND, | ||
ret_hir_ty.span, | ||
"returning a `str` unnecessarily tied to the lifetime of arguments", | ||
"try", | ||
"&'static str".into(), // how ironic, a lint about `&'static str` requiring a `String` alloc... | ||
Applicability::MachineApplicable, | ||
); | ||
} | ||
} | ||
} |
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#![warn(clippy::unnecessary_literal_bound)] | ||
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struct Struct<'a> { | ||
not_literal: &'a str, | ||
} | ||
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impl Struct<'_> { | ||
// Should warn | ||
fn returns_lit(&self) -> &'static str { | ||
"Hello" | ||
} | ||
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// Should NOT warn | ||
fn returns_non_lit(&self) -> &str { | ||
self.not_literal | ||
} | ||
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// Should warn, does not currently | ||
fn conditionally_returns_lit(&self, cond: bool) -> &str { | ||
if cond { "Literal" } else { "also a literal" } | ||
} | ||
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// Should NOT warn | ||
fn conditionally_returns_non_lit(&self, cond: bool) -> &str { | ||
if cond { "Literal" } else { self.not_literal } | ||
} | ||
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// Should warn | ||
fn contionally_returns_literals_explicit(&self, cond: bool) -> &'static str { | ||
if cond { | ||
return "Literal"; | ||
} | ||
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"also a literal" | ||
} | ||
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// Should NOT warn | ||
fn conditionally_returns_non_lit_explicit(&self, cond: bool) -> &str { | ||
if cond { | ||
return self.not_literal; | ||
} | ||
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"Literal" | ||
} | ||
} | ||
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trait ReturnsStr { | ||
fn trait_method(&self) -> &str; | ||
} | ||
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impl ReturnsStr for u8 { | ||
// Should warn, even though not useful without trait refinement | ||
fn trait_method(&self) -> &'static str { | ||
"Literal" | ||
} | ||
} | ||
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impl ReturnsStr for Struct<'_> { | ||
// Should NOT warn | ||
fn trait_method(&self) -> &str { | ||
self.not_literal | ||
} | ||
} | ||
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fn main() {} |
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#![warn(clippy::unnecessary_literal_bound)] | ||
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struct Struct<'a> { | ||
not_literal: &'a str, | ||
} | ||
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impl Struct<'_> { | ||
// Should warn | ||
fn returns_lit(&self) -> &str { | ||
"Hello" | ||
} | ||
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// Should NOT warn | ||
fn returns_non_lit(&self) -> &str { | ||
self.not_literal | ||
} | ||
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// Should warn, does not currently | ||
fn conditionally_returns_lit(&self, cond: bool) -> &str { | ||
if cond { "Literal" } else { "also a literal" } | ||
} | ||
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// Should NOT warn | ||
fn conditionally_returns_non_lit(&self, cond: bool) -> &str { | ||
if cond { "Literal" } else { self.not_literal } | ||
} | ||
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// Should warn | ||
fn contionally_returns_literals_explicit(&self, cond: bool) -> &str { | ||
if cond { | ||
return "Literal"; | ||
} | ||
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"also a literal" | ||
} | ||
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// Should NOT warn | ||
fn conditionally_returns_non_lit_explicit(&self, cond: bool) -> &str { | ||
if cond { | ||
return self.not_literal; | ||
} | ||
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"Literal" | ||
} | ||
} | ||
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trait ReturnsStr { | ||
fn trait_method(&self) -> &str; | ||
} | ||
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impl ReturnsStr for u8 { | ||
// Should warn, even though not useful without trait refinement | ||
fn trait_method(&self) -> &str { | ||
"Literal" | ||
} | ||
} | ||
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impl ReturnsStr for Struct<'_> { | ||
// Should NOT warn | ||
fn trait_method(&self) -> &str { | ||
self.not_literal | ||
} | ||
} | ||
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fn main() {} |
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Original file line number | Diff line number | Diff line change |
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error: returning a `str` unnecessarily tied to the lifetime of arguments | ||
--> tests/ui/unnecessary_literal_bound.rs:9:30 | ||
| | ||
LL | fn returns_lit(&self) -> &str { | ||
| ^^^^ help: try: `&'static str` | ||
| | ||
= note: `-D clippy::unnecessary-literal-bound` implied by `-D warnings` | ||
= help: to override `-D warnings` add `#[allow(clippy::unnecessary_literal_bound)]` | ||
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error: returning a `str` unnecessarily tied to the lifetime of arguments | ||
--> tests/ui/unnecessary_literal_bound.rs:29:68 | ||
| | ||
LL | fn contionally_returns_literals_explicit(&self, cond: bool) -> &str { | ||
| ^^^^ help: try: `&'static str` | ||
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error: returning a `str` unnecessarily tied to the lifetime of arguments | ||
--> tests/ui/unnecessary_literal_bound.rs:53:31 | ||
| | ||
LL | fn trait_method(&self) -> &str { | ||
| ^^^^ help: try: `&'static str` | ||
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error: aborting due to 3 previous errors | ||
|