Fix sliced_string_as_bytes FP with a RangeFull (#15873)

changelog: [`sliced_string_as_bytes`]: don't fire on
`str[..].as_bytes()`

So I ran into this in some codebase I was working on,
where the lint fired on this line:
```rust
let string: &str;
string[..].as_bytes()
```
So I was trying to understand the rationale behind this lint, and it
says:
> It involves doing an unnecessary UTF-8 alignment check which is less
efficient, and can cause a panic.

This is obviously not true in the case where a `RangeFull` slice is
being taken, since there is no UTF-8 boundary check, and no panic can be
caused. So I created an exemption for `RangeFull`s.

Two other notes:
1. I'm not sure the word "alignment" in the lint's description (quoted
above) is really correct, should probably say "char boundary" instead?
2. I might be missing something, but isn't there a lint for doing
superfluous slice indexing, and then calling a slice method? e.g.
`str[..].len()` or `slice[..].len()`, where `str` and `slice` are `&str`
and `&[T]`, respectively. If we had one, I would expect *it* to fire for
the example code I quoted above.

Author: llogiq

clippy_lints/src/methods/sliced_string_as_bytes.rs

@@ -1,15 +1,21 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::higher;
 use clippy_utils::res::MaybeDef;
 use clippy_utils::source::snippet_with_applicability;
 use rustc_errors::Applicability;
-use rustc_hir::{Expr, ExprKind, LangItem, is_range_literal};
+use rustc_hir::{Expr, ExprKind, LangItem};
 use rustc_lint::LateContext;
 
 use super::SLICED_STRING_AS_BYTES;
 
+/// Checks if `index` is any type of range except `RangeFull` (i.e. `..`)
+fn is_bounded_range_literal(cx: &LateContext<'_>, index: &Expr<'_>) -> bool {
+    higher::Range::hir(cx, index).is_some_and(|range| Option::or(range.start, range.end).is_some())
+}
+
 pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>) {
     if let ExprKind::Index(indexed, index, _) = recv.kind
-        && is_range_literal(index)
+        && is_bounded_range_literal(cx, index)
         && let ty = cx.typeck_results().expr_ty(indexed).peel_refs()
         && (ty.is_str() || ty.is_lang_item(cx, LangItem::String))
     {

tests/ui/sliced_string_as_bytes.fixed

@@ -32,6 +32,12 @@ fn main() {
     let bytes = &"consectetur adipiscing".as_bytes()[..=5];
     //~^ sliced_string_as_bytes
 
+    // this lint is a perf lint meant to catch utf-8 alignment checks.
+    // while the slicing here *is* redundant, it's more like a needless borrow, and shouldn't affect
+    // perf
+    let bytes = s[..].as_bytes();
+    let bytes = string[..].as_bytes();
+
     let f = Foo;
     let bytes = f[0..4].as_bytes();
 }

tests/ui/sliced_string_as_bytes.rs

@@ -32,6 +32,12 @@ fn main() {
     let bytes = "consectetur adipiscing"[..=5].as_bytes();
     //~^ sliced_string_as_bytes
 
+    // this lint is a perf lint meant to catch utf-8 alignment checks.
+    // while the slicing here *is* redundant, it's more like a needless borrow, and shouldn't affect
+    // perf
+    let bytes = s[..].as_bytes();
+    let bytes = string[..].as_bytes();
+
     let f = Foo;
     let bytes = f[0..4].as_bytes();
 }