Auto merge of rust-lang#13395 - GnomedDev:unnecessary-lit-bound, r=Manishearth

Add lint for unnecessary lifetime bounded &str return

Closes rust-lang#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 rust-lang#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.
```

Author: bors

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
+