1 files changed, 142 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/explicit_write.rs b/src/tools/clippy/clippy_lints/src/explicit_write.rs
new file mode 100644
index 000000000..5bf4313b4
--- /dev/null
+++ b/src/tools/clippy/clippy_lints/src/explicit_write.rs
@@ -0,0 +1,142 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::macros::FormatArgsExpn;
+use clippy_utils::source::snippet_with_applicability;
+use clippy_utils::{is_expn_of, match_function_call, paths};
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::def::Res;
+use rustc_hir::{BindingAnnotation, Block, BlockCheckMode, Expr, ExprKind, Node, PatKind, QPath, Stmt, StmtKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `write!()` / `writeln()!` which can be
+    /// replaced with `(e)print!()` / `(e)println!()`
+    ///
+    /// ### Why is this bad?
+    /// Using `(e)println! is clearer and more concise
+    ///
+    /// ### Example
+    /// ```rust
+    /// # use std::io::Write;
+    /// # let bar = "furchtbar";
+    /// writeln!(&mut std::io::stderr(), "foo: {:?}", bar).unwrap();
+    /// writeln!(&mut std::io::stdout(), "foo: {:?}", bar).unwrap();
+    /// ```
+    ///
+    /// Use instead:
+    /// ```rust
+    /// # use std::io::Write;
+    /// # let bar = "furchtbar";
+    /// eprintln!("foo: {:?}", bar);
+    /// println!("foo: {:?}", bar);
+    /// ```
+    #[clippy::version = "pre 1.29.0"]
+    pub EXPLICIT_WRITE,
+    complexity,
+    "using the `write!()` family of functions instead of the `print!()` family of functions, when using the latter would work"
+}
+
+declare_lint_pass!(ExplicitWrite => [EXPLICIT_WRITE]);
+
+impl<'tcx> LateLintPass<'tcx> for ExplicitWrite {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if_chain! {
+            // match call to unwrap
+            if let ExprKind::MethodCall(unwrap_fun, [write_call], _) = expr.kind;
+            if unwrap_fun.ident.name == sym::unwrap;
+            // match call to write_fmt
+            if let ExprKind::MethodCall(write_fun, [write_recv, write_arg], _) = look_in_block(cx, &write_call.kind);
+            if write_fun.ident.name == sym!(write_fmt);
+            // match calls to std::io::stdout() / std::io::stderr ()
+            if let Some(dest_name) = if match_function_call(cx, write_recv, &paths::STDOUT).is_some() {
+                Some("stdout")
+            } else if match_function_call(cx, write_recv, &paths::STDERR).is_some() {
+                Some("stderr")
+            } else {
+                None
+            };
+            if let Some(format_args) = FormatArgsExpn::parse(cx, write_arg);
+            then {
+                let calling_macro =
+                    // ordering is important here, since `writeln!` uses `write!` internally
+                    if is_expn_of(write_call.span, "writeln").is_some() {
+                        Some("writeln")
+                    } else if is_expn_of(write_call.span, "write").is_some() {
+                        Some("write")
+                    } else {
+                        None
+                    };
+                let prefix = if dest_name == "stderr" {
+                    "e"
+                } else {
+                    ""
+                };
+
+                // We need to remove the last trailing newline from the string because the
+                // underlying `fmt::write` function doesn't know whether `println!` or `print!` was
+                // used.
+                let (used, sugg_mac) = if let Some(macro_name) = calling_macro {
+                    (
+                        format!("{}!({}(), ...)", macro_name, dest_name),
+                        macro_name.replace("write", "print"),
+                    )
+                } else {
+                    (
+                        format!("{}().write_fmt(...)", dest_name),
+                        "print".into(),
+                    )
+                };
+                let mut applicability = Applicability::MachineApplicable;
+                let inputs_snippet = snippet_with_applicability(
+                    cx,
+                    format_args.inputs_span(),
+                    "..",
+                    &mut applicability,
+                );
+                span_lint_and_sugg(
+                    cx,
+                    EXPLICIT_WRITE,
+                    expr.span,
+                    &format!("use of `{}.unwrap()`", used),
+                    "try this",
+                    format!("{}{}!({})", prefix, sugg_mac, inputs_snippet),
+                    applicability,
+                )
+            }
+        }
+    }
+}
+
+/// If `kind` is a block that looks like `{ let result = $expr; result }` then
+/// returns $expr. Otherwise returns `kind`.
+fn look_in_block<'tcx, 'hir>(cx: &LateContext<'tcx>, kind: &'tcx ExprKind<'hir>) -> &'tcx ExprKind<'hir> {
+    if_chain! {
+        if let ExprKind::Block(block, _label @ None) = kind;
+        if let Block {
+            stmts: [Stmt { kind: StmtKind::Local(local), .. }],
+            expr: Some(expr_end_of_block),
+            rules: BlockCheckMode::DefaultBlock,
+            ..
+        } = block;
+
+        // Find id of the local that expr_end_of_block resolves to
+        if let ExprKind::Path(QPath::Resolved(None, expr_path)) = expr_end_of_block.kind;
+        if let Res::Local(expr_res) = expr_path.res;
+        if let Some(Node::Pat(res_pat)) = cx.tcx.hir().find(expr_res);
+
+        // Find id of the local we found in the block
+        if let PatKind::Binding(BindingAnnotation::Unannotated, local_hir_id, _ident, None) = local.pat.kind;
+
+        // If those two are the same hir id
+        if res_pat.hir_id == local_hir_id;
+
+        if let Some(init) = local.init;
+        then {
+            return &init.kind;
+        }
+    }
+    kind
+}