compiler: fix few needless_pass_by_ref_mut clippy lints

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_session\src\config.rs:2013:16
     |
2013 |     early_dcx: &mut EarlyDiagCtxt,
     |                ^^^^^^^^^^^^^^^^^^ help: consider changing to: `&EarlyDiagCtxt`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_ast_passes\src\ast_validation.rs:1555:11
     |
1555 |     this: &mut AstValidator<'_>,
     |           ^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&AstValidator<'_>`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_infer\src\infer\snapshot\fudge.rs:16:12
   |
16 |     table: &mut UnificationTable<'_, 'tcx, T>,
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&UnificationTable<'_, 'tcx, T>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_expand\src\expand.rs:961:13
    |
961 |     parser: &mut Parser<'a>,
    |             ^^^^^^^^^^^^^^^ help: consider changing to: `&Parser<'a>`
    |
    = warning: changing this function will impact semver compatibility
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1552,7 +1552,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 /// When encountering an equality constraint in a `where` clause, emit an error. If the code seems
 /// like it's setting an associated type, provide an appropriate suggestion.
 fn deny_equality_constraints(
-    this: &mut AstValidator<'_>,
+    this: &AstValidator<'_>,
     predicate: &WhereEqPredicate,
     generics: &Generics,
 ) {

compiler/rustc_expand/src/expand.rs

@@ -871,7 +871,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         let mut parser = self.cx.new_parser_from_tts(toks);
         match parse_ast_fragment(&mut parser, kind) {
             Ok(fragment) => {
-                ensure_complete_parse(&mut parser, path, kind.name(), span);
+                ensure_complete_parse(&parser, path, kind.name(), span);
                 fragment
             }
             Err(mut err) => {
@@ -958,7 +958,7 @@ pub fn parse_ast_fragment<'a>(
 }
 
 pub fn ensure_complete_parse<'a>(
-    parser: &mut Parser<'a>,
+    parser: &Parser<'a>,
     macro_path: &ast::Path,
     kind_name: &str,
     span: Span,

compiler/rustc_infer/src/infer/snapshot/fudge.rs

@@ -13,7 +13,7 @@ use ut::UnifyKey;
 use std::ops::Range;
 
 fn vars_since_snapshot<'tcx, T>(
-    table: &mut UnificationTable<'_, 'tcx, T>,
+    table: &UnificationTable<'_, 'tcx, T>,
     snapshot_var_len: usize,
 ) -> Range<T>
 where
@@ -124,11 +124,11 @@ impl<'tcx> InferCtxt<'tcx> {
                     let type_vars =
                         inner.type_variables().vars_since_snapshot(variable_lengths.type_var_len);
                     let int_vars = vars_since_snapshot(
-                        &mut inner.int_unification_table(),
+                        &inner.int_unification_table(),
                         variable_lengths.int_var_len,
                     );
                     let float_vars = vars_since_snapshot(
-                        &mut inner.float_unification_table(),
+                        &inner.float_unification_table(),
                         variable_lengths.float_var_len,
                     );
                     let region_vars = inner

compiler/rustc_session/src/config.rs

@@ -2010,7 +2010,7 @@ pub fn parse_crate_edition(early_dcx: &EarlyDiagCtxt, matches: &getopts::Matches
 }
 
 fn check_error_format_stability(
-    early_dcx: &mut EarlyDiagCtxt,
+    early_dcx: &EarlyDiagCtxt,
     unstable_opts: &UnstableOptions,
     error_format: ErrorOutputType,
 ) {
@@ -2574,7 +2574,7 @@ fn parse_remap_path_prefix(
 }
 
 fn parse_logical_env(
-    early_dcx: &mut EarlyDiagCtxt,
+    early_dcx: &EarlyDiagCtxt,
     matches: &getopts::Matches,
 ) -> FxIndexMap<String, String> {
     let mut vars = FxIndexMap::default();