Introduce function for comparing expression values

Introduce `eq_expr_value(cx, a, b)` as a shortcut for
`SpanlessEq::new(cx).ignore_fn().eq_expr(cx, a, b)`.

No functional changes intended.

clippy_lints/src/utils/hir_utils.rs

@@ -340,6 +340,11 @@ pub fn over<X>(left: &[X], right: &[X], mut eq_fn: impl FnMut(&X, &X) -> bool) -
     left.len() == right.len() && left.iter().zip(right).all(|(x, y)| eq_fn(x, y))
 }
 
+/// Checks if two expressions evaluate to the same value, and don't contain any side effects.
+pub fn eq_expr_value(cx: &LateContext<'_>, left: &Expr<'_>, right: &Expr<'_>) -> bool {
+    SpanlessEq::new(cx).ignore_fn().eq_expr(left, right)
+}
+
 /// Type used to hash an ast element. This is different from the `Hash` trait
 /// on ast types as this
 /// trait would consider IDs and spans.

clippy_lints/src/utils/internal_lints.rs

@@ -1,7 +1,6 @@
-use crate::utils::SpanlessEq;
 use crate::utils::{
     is_expn_of, match_def_path, match_qpath, match_type, method_calls, paths, run_lints, snippet, span_lint,
-    span_lint_and_help, span_lint_and_sugg, walk_ptrs_ty,
+    span_lint_and_help, span_lint_and_sugg, walk_ptrs_ty, SpanlessEq,
 };
 use if_chain::if_chain;
 use rustc_ast::ast::{Crate as AstCrate, ItemKind, LitKind, NodeId};

clippy_lints/src/utils/mod.rs

@@ -21,7 +21,7 @@ pub mod sugg;
 pub mod usage;
 pub use self::attrs::*;
 pub use self::diagnostics::*;
-pub use self::hir_utils::{both, over, SpanlessEq, SpanlessHash};
+pub use self::hir_utils::{both, eq_expr_value, over, SpanlessEq, SpanlessHash};
 
 use std::borrow::Cow;
 use std::mem;