Allow explicit_write in tests (rust-lang#15862)

Resolves rust-lang#15780.

I didn't get any feedback on that issue. So I hope it is okay that I
charged ahead and addressed it.

changelog: Allow `explicit_write` in tests

Author: Jarcho

clippy_lints/src/explicit_write.rs

@@ -2,7 +2,7 @@ use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::macros::{FormatArgsStorage, format_args_inputs_span};
 use clippy_utils::res::MaybeResPath;
 use clippy_utils::source::snippet_with_applicability;
-use clippy_utils::{is_expn_of, sym};
+use clippy_utils::{is_expn_of, is_in_test, sym};
 use rustc_errors::Applicability;
 use rustc_hir::def::Res;
 use rustc_hir::{BindingMode, Block, BlockCheckMode, Expr, ExprKind, Node, PatKind, QPath, Stmt, StmtKind};
@@ -72,6 +72,11 @@ impl<'tcx> LateLintPass<'tcx> for ExplicitWrite {
                 return;
             };
 
+            // Performing an explicit write in a test circumvent's libtest's capture of stdio and stdout.
+            if is_in_test(cx.tcx, expr.hir_id) {
+                return;
+            }
+
             // ordering is important here, since `writeln!` uses `write!` internally
             let calling_macro = if is_expn_of(write_call.span, sym::writeln).is_some() {
                 Some("writeln")

tests/ui/explicit_write_in_test.rs

@@ -0,0 +1,9 @@
+//@ check-pass
+#![warn(clippy::explicit_write)]
+
+#[test]
+fn test() {
+    use std::io::Write;
+    writeln!(std::io::stderr(), "I am an explicit write.").unwrap();
+    eprintln!("I am not an explicit write.");
+}