Rollup merge of #134661 - dtolnay:prefixattr, r=fmease

Reduce precedence of expressions that have an outer attr

Previously, `-Zunpretty=expanded` would expand this program as follows:

```rust
#![feature(stmt_expr_attributes)]

macro_rules! repro {
    ($e:expr) => {
        #[allow(deprecated)] $e
    };
}

#[derive(Default)]
struct Thing {
    #[deprecated]
    field: i32,
}

fn main() {
    let thing = Thing::default();
    let _ = repro!(thing).field;
}
```

```rs
#![feature(prelude_import)]
#![feature(stmt_expr_attributes)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;

struct Thing {
    #[deprecated]
    field: i32,
}

#[automatically_derived]
impl ::core::default::Default for Thing {
    #[inline]
    fn default() -> Thing {
        Thing { field: ::core::default::Default::default() }
    }
}

fn main() {
    let thing = Thing::default();
    let _ = #[allow(deprecated)] thing.field;
}
```

This is not the correct expansion. The correct output would have `(#[allow(deprecated)] thing).field` with the attribute applying only to `thing`, not to `thing.field`.

1 files changed, 15 insertions, 0 deletions

diff --git a/compiler/rustc_lint/src/context.rs b/compiler/rustc_lint/src/context.rs
index 5679d4566dc..b6bf45dfbcf 100644
--- a/compiler/rustc_lint/src/context.rs
+++ b/compiler/rustc_lint/src/context.rs
@@ -7,6 +7,7 @@ use std::cell::Cell;
 use std::slice;
 
 use rustc_ast::BindingMode;
+use rustc_ast::util::parser::ExprPrecedence;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sync;
 use rustc_data_structures::unord::UnordMap;
@@ -850,6 +851,20 @@ impl<'tcx> LateContext<'tcx> {
             })
     }
 
+    /// Returns the effective precedence of an expression for the purpose of
+    /// rendering diagnostic. This is not the same as the precedence that would
+    /// be used for pretty-printing HIR by rustc_hir_pretty.
+    pub fn precedence(&self, expr: &hir::Expr<'_>) -> ExprPrecedence {
+        let for_each_attr = |id: hir::HirId, callback: &mut dyn FnMut(&hir::Attribute)| {
+            for attr in self.tcx.hir_attrs(id) {
+                if attr.span().desugaring_kind().is_none() {
+                    callback(attr);
+                }
+            }
+        };
+        expr.precedence(&for_each_attr)
+    }
+
     /// If the given expression is a local binding, find the initializer expression.
     /// If that initializer expression is another local binding, find its initializer again.
     ///