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.
```

CHANGELOG.md

@@ -6029,6 +6029,7 @@ Released 2018-09-13
 [`unnecessary_get_then_check`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_get_then_check
 [`unnecessary_join`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_join
 [`unnecessary_lazy_evaluations`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_lazy_evaluations
+[`unnecessary_literal_bound`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_literal_bound
 [`unnecessary_literal_unwrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_literal_unwrap
 [`unnecessary_map_on_constructor`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_map_on_constructor
 [`unnecessary_min_or_max`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_min_or_max

clippy_lints/src/declared_lints.rs

@@ -738,6 +738,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::unit_types::UNIT_CMP_INFO,
     crate::unnamed_address::FN_ADDRESS_COMPARISONS_INFO,
     crate::unnecessary_box_returns::UNNECESSARY_BOX_RETURNS_INFO,
+    crate::unnecessary_literal_bound::UNNECESSARY_LITERAL_BOUND_INFO,
     crate::unnecessary_map_on_constructor::UNNECESSARY_MAP_ON_CONSTRUCTOR_INFO,
     crate::unnecessary_owned_empty_strings::UNNECESSARY_OWNED_EMPTY_STRINGS_INFO,
     crate::unnecessary_self_imports::UNNECESSARY_SELF_IMPORTS_INFO,

clippy_lints/src/lib.rs

@@ -366,6 +366,7 @@ mod unit_return_expecting_ord;
 mod unit_types;
 mod unnamed_address;
 mod unnecessary_box_returns;
+mod unnecessary_literal_bound;
 mod unnecessary_map_on_constructor;
 mod unnecessary_owned_empty_strings;
 mod unnecessary_self_imports;
@@ -949,5 +950,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(non_zero_suggestions::NonZeroSuggestions));
     store.register_late_pass(move |_| Box::new(unused_trait_names::UnusedTraitNames::new(conf)));
     store.register_late_pass(|_| Box::new(manual_ignore_case_cmp::ManualIgnoreCaseCmp));
+    store.register_late_pass(|_| Box::new(unnecessary_literal_bound::UnnecessaryLiteralBound));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

clippy_lints/src/unnecessary_literal_bound.rs

@@ -0,0 +1,158 @@
+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;
+
+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"
+}
+
+declare_lint_pass!(UnnecessaryLiteralBound => [UNNECESSARY_LITERAL_BOUND]);
+
+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;
+    };
+
+    if !lifetime.is_anonymous() || !matches!(mutbl, Mutability::Not) {
+        return None;
+    }
+
+    Some(ty)
+}
+
+fn is_str_literal(expr: &Expr<'_>) -> bool {
+    matches!(
+        expr.kind,
+        ExprKind::Lit(Lit {
+            node: LitKind::Str(..),
+            ..
+        }),
+    )
+}
+
+struct FindNonLiteralReturn;
+
+impl<'hir> Visitor<'hir> for FindNonLiteralReturn {
+    type Result = std::ops::ControlFlow<()>;
+    type NestedFilter = intravisit::nested_filter::None;
+
+    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)
+        }
+    }
+}
+
+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);
+    }
+
+    false
+}
+
+fn check_explicit_returns_static_str(expr: &Expr<'_>) -> bool {
+    let mut visitor = FindNonLiteralReturn;
+    visitor.visit_expr(expr).is_continue()
+}
+
+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;
+        }
+
+        // Checking closures would be a little silly
+        if matches!(kind, FnKind::Closure) {
+            return;
+        }
+
+        // Check for `-> &str`
+        let FnRetTy::Return(ret_hir_ty) = decl.output else {
+            return;
+        };
+
+        let Some(inner_hir_ty) = extract_anonymous_ref(ret_hir_ty) else {
+            return;
+        };
+
+        if path_res(cx, inner_hir_ty) != Res::PrimTy(PrimTy::Str) {
+            return;
+        }
+
+        // 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,
+            );
+        }
+    }
+}

tests/ui/unnecessary_literal_bound.fixed

@@ -0,0 +1,65 @@
+#![warn(clippy::unnecessary_literal_bound)]
+
+struct Struct<'a> {
+    not_literal: &'a str,
+}
+
+impl Struct<'_> {
+    // Should warn
+    fn returns_lit(&self) -> &'static str {
+        "Hello"
+    }
+
+    // Should NOT warn
+    fn returns_non_lit(&self) -> &str {
+        self.not_literal
+    }
+
+    // Should warn, does not currently
+    fn conditionally_returns_lit(&self, cond: bool) -> &str {
+        if cond { "Literal" } else { "also a literal" }
+    }
+
+    // Should NOT warn
+    fn conditionally_returns_non_lit(&self, cond: bool) -> &str {
+        if cond { "Literal" } else { self.not_literal }
+    }
+
+    // Should warn
+    fn contionally_returns_literals_explicit(&self, cond: bool) -> &'static str {
+        if cond {
+            return "Literal";
+        }
+
+        "also a literal"
+    }
+
+    // Should NOT warn
+    fn conditionally_returns_non_lit_explicit(&self, cond: bool) -> &str {
+        if cond {
+            return self.not_literal;
+        }
+
+        "Literal"
+    }
+}
+
+trait ReturnsStr {
+    fn trait_method(&self) -> &str;
+}
+
+impl ReturnsStr for u8 {
+    // Should warn, even though not useful without trait refinement
+    fn trait_method(&self) -> &'static str {
+        "Literal"
+    }
+}
+
+impl ReturnsStr for Struct<'_> {
+    // Should NOT warn
+    fn trait_method(&self) -> &str {
+        self.not_literal
+    }
+}
+
+fn main() {}

tests/ui/unnecessary_literal_bound.rs

@@ -0,0 +1,65 @@
+#![warn(clippy::unnecessary_literal_bound)]
+
+struct Struct<'a> {
+    not_literal: &'a str,
+}
+
+impl Struct<'_> {
+    // Should warn
+    fn returns_lit(&self) -> &str {
+        "Hello"
+    }
+
+    // Should NOT warn
+    fn returns_non_lit(&self) -> &str {
+        self.not_literal
+    }
+
+    // Should warn, does not currently
+    fn conditionally_returns_lit(&self, cond: bool) -> &str {
+        if cond { "Literal" } else { "also a literal" }
+    }
+
+    // Should NOT warn
+    fn conditionally_returns_non_lit(&self, cond: bool) -> &str {
+        if cond { "Literal" } else { self.not_literal }
+    }
+
+    // Should warn
+    fn contionally_returns_literals_explicit(&self, cond: bool) -> &str {
+        if cond {
+            return "Literal";
+        }
+
+        "also a literal"
+    }
+
+    // Should NOT warn
+    fn conditionally_returns_non_lit_explicit(&self, cond: bool) -> &str {
+        if cond {
+            return self.not_literal;
+        }
+
+        "Literal"
+    }
+}
+
+trait ReturnsStr {
+    fn trait_method(&self) -> &str;
+}
+
+impl ReturnsStr for u8 {
+    // Should warn, even though not useful without trait refinement
+    fn trait_method(&self) -> &str {
+        "Literal"
+    }
+}
+
+impl ReturnsStr for Struct<'_> {
+    // Should NOT warn
+    fn trait_method(&self) -> &str {
+        self.not_literal
+    }
+}
+
+fn main() {}

tests/ui/unnecessary_literal_bound.stderr

@@ -0,0 +1,23 @@
+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)]`
+
+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`
+
+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`
+
+error: aborting due to 3 previous errors
+