Auto merge of #11859 - y21:issue11856, r=blyxyas

[`missing_asserts_for_indexing`]: work with bodies instead of blocks separately

Fixes #11856

Before this change, this lint would check blocks independently of each other, which means that it misses `assert!()`s from parent blocks.
```rs
// check_block
assert!(x.len() > 1);

{
  // check_block
  // no assert here
  let _ = x[0] + x[1];
}
```

This PR changes it to work with bodies rather than individual blocks. That means that a function will be checked in one go and we can remember if an `assert!` occurred anywhere.

Eventually it would be nice to have a more control flow-aware analysis, possibly by rewriting it as a MIR lint, but that's more complicated and I wanted this fixed first.

changelog: [`missing_asserts_for_indexing`]: accept `assert!`s from parent blocks

clippy_lints/src/missing_asserts_for_indexing.rs

@@ -9,7 +9,7 @@
 use rustc_ast::{LitKind, RangeLimits};
 use rustc_data_structures::unhash::UnhashMap;
 use rustc_errors::{Applicability, Diagnostic};
-use rustc_hir::{BinOp, Block, Expr, ExprKind, UnOp};
+use rustc_hir::{BinOp, Block, Body, Expr, ExprKind, UnOp};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::source_map::Spanned;
@@ -390,10 +390,10 @@ fn report_indexes(cx: &LateContext<'_>, map: &UnhashMap<u64, Vec<IndexEntry<'_>>
 }
 
 impl LateLintPass<'_> for MissingAssertsForIndexing {
-    fn check_block(&mut self, cx: &LateContext<'_>, block: &Block<'_>) {
+    fn check_body(&mut self, cx: &LateContext<'_>, body: &Body<'_>) {
         let mut map = UnhashMap::default();
 
-        for_each_expr(block, |expr| {
+        for_each_expr(body.value, |expr| {
             check_index(cx, expr, &mut map);
             check_assert(cx, expr, &mut map);
             ControlFlow::<!, ()>::Continue(())

tests/ui/missing_asserts_for_indexing_unfixable.rs

@@ -46,4 +46,26 @@ fn index_struct_different_fields(f: &Foo<'_>) {
     let _ = f.v[0] + f.v2[1];
 }
 
+fn shadowing() {
+    let x: &[i32] = &[1];
+    assert!(x.len() > 1);
+
+    let x: &[i32] = &[1];
+    let _ = x[0] + x[1];
+    //~^ ERROR: indexing into a slice multiple times without an `assert`
+}
+
+pub fn issue11856(values: &[i32]) -> usize {
+    let mut ascending = Vec::new();
+    for w in values.windows(2) {
+        assert!(w.len() > 1);
+        if w[0] < w[1] {
+            ascending.push((w[0], w[1]));
+        } else {
+            ascending.push((w[1], w[0]));
+        }
+    }
+    ascending.len()
+}
+
 fn main() {}

tests/ui/missing_asserts_for_indexing_unfixable.stderr

@@ -160,5 +160,24 @@ LL |     let _ = f.v[0] + f.v[1];
    |                      ^^^^^^
    = note: asserting the length before indexing will elide bounds checks
 
-error: aborting due to 7 previous errors
+error: indexing into a slice multiple times without an `assert`
+  --> $DIR/missing_asserts_for_indexing_unfixable.rs:54:13
+   |
+LL |     let _ = x[0] + x[1];
+   |             ^^^^^^^^^^^
+   |
+   = help: consider asserting the length before indexing: `assert!(x.len() > 1);`
+note: slice indexed here
+  --> $DIR/missing_asserts_for_indexing_unfixable.rs:54:13
+   |
+LL |     let _ = x[0] + x[1];
+   |             ^^^^
+note: slice indexed here
+  --> $DIR/missing_asserts_for_indexing_unfixable.rs:54:20
+   |
+LL |     let _ = x[0] + x[1];
+   |                    ^^^^
+   = note: asserting the length before indexing will elide bounds checks
+
+error: aborting due to 8 previous errors