Rollup merge of rust-lang#64581 - ztlpn:fix-ok-wrapping-unreachable-code, r=Centril

Fix unreachable_code warnings for try{} block ok-wrapped expressions

Fixes rust-lang#54165 and fixes rust-lang#63324.

Author: Centril

src/librustc/hir/lowering/expr.rs

@@ -392,32 +392,49 @@ impl LoweringContext<'_> {
         )
     }
 
+    /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_ok(<expr>) }`,
+    /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_ok(()) }`
+    /// and save the block id to use it as a break target for desugaring of the `?` operator.
     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind {
         self.with_catch_scope(body.id, |this| {
-            let unstable_span = this.mark_span_with_reason(
+            let mut block = this.lower_block(body, true).into_inner();
+
+            let try_span = this.mark_span_with_reason(
                 DesugaringKind::TryBlock,
                 body.span,
                 this.allow_try_trait.clone(),
             );
-            let mut block = this.lower_block(body, true).into_inner();
-            let tail = block.expr.take().map_or_else(
-                || this.expr_unit(this.sess.source_map().end_point(unstable_span)),
+
+            // Final expression of the block (if present) or `()` with span at the end of block
+            let tail_expr = block.expr.take().map_or_else(
+                || this.expr_unit(this.sess.source_map().end_point(try_span)),
                 |x: P<hir::Expr>| x.into_inner(),
             );
-            block.expr = Some(this.wrap_in_try_constructor(sym::from_ok, tail, unstable_span));
+
+            let ok_wrapped_span = this.mark_span_with_reason(
+                DesugaringKind::TryBlock,
+                tail_expr.span,
+                None
+            );
+
+            // `::std::ops::Try::from_ok($tail_expr)`
+            block.expr = Some(this.wrap_in_try_constructor(
+                sym::from_ok, try_span, tail_expr, ok_wrapped_span));
+
             hir::ExprKind::Block(P(block), None)
         })
     }
 
     fn wrap_in_try_constructor(
         &mut self,
         method: Symbol,
-        e: hir::Expr,
-        unstable_span: Span,
+        method_span: Span,
+        expr: hir::Expr,
+        overall_span: Span,
     ) -> P<hir::Expr> {
         let path = &[sym::ops, sym::Try, method];
-        let from_err = P(self.expr_std_path(unstable_span, path, None, ThinVec::new()));
-        P(self.expr_call(e.span, from_err, hir_vec![e]))
+        let constructor = P(self.expr_std_path(method_span, path, None, ThinVec::new()));
+        P(self.expr_call(overall_span, constructor, hir_vec![expr]))
     }
 
     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm {
@@ -1244,7 +1261,7 @@ impl LoweringContext<'_> {
                 self.expr_call_std_path(try_span, from_path, hir_vec![err_expr])
             };
             let from_err_expr =
-                self.wrap_in_try_constructor(sym::from_error, from_expr, unstable_span);
+                self.wrap_in_try_constructor(sym::from_error, unstable_span, from_expr, try_span);
             let thin_attrs = ThinVec::from(attrs);
             let catch_scope = self.catch_scopes.last().map(|x| *x);
             let ret_expr = if let Some(catch_node) = catch_scope {

src/librustc/hir/mod.rs

@@ -861,7 +861,7 @@ pub struct Block {
     pub span: Span,
     /// If true, then there may exist `break 'a` values that aim to
     /// break out of this block early.
-    /// Used by `'label: {}` blocks and by `catch` statements.
+    /// Used by `'label: {}` blocks and by `try {}` blocks.
     pub targeted_by_break: bool,
 }
 

src/librustc_typeck/check/expr.rs

@@ -18,6 +18,7 @@ use crate::util::nodemap::FxHashMap;
 use crate::astconv::AstConv as _;
 
 use errors::{Applicability, DiagnosticBuilder, pluralise};
+use syntax_pos::hygiene::DesugaringKind;
 use syntax::ast;
 use syntax::symbol::{Symbol, kw, sym};
 use syntax::source_map::Span;
@@ -150,8 +151,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         debug!(">> type-checking: expr={:?} expected={:?}",
                expr, expected);
 
+        // True if `expr` is a `Try::from_ok(())` that is a result of desugaring a try block
+        // without the final expr (e.g. `try { return; }`). We don't want to generate an
+        // unreachable_code lint for it since warnings for autogenerated code are confusing.
+        let is_try_block_generated_unit_expr = match expr.node {
+            ExprKind::Call(_, ref args) if expr.span.is_desugaring(DesugaringKind::TryBlock) =>
+                args.len() == 1 && args[0].span.is_desugaring(DesugaringKind::TryBlock),
+
+            _ => false,
+        };
+
         // Warn for expressions after diverging siblings.
-        self.warn_if_unreachable(expr.hir_id, expr.span, "expression");
+        if !is_try_block_generated_unit_expr {
+            self.warn_if_unreachable(expr.hir_id, expr.span, "expression");
+        }
 
         // Hide the outer diverging and has_errors flags.
         let old_diverges = self.diverges.get();
@@ -164,6 +177,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Warn for non-block expressions with diverging children.
         match expr.kind {
             ExprKind::Block(..) | ExprKind::Loop(..) | ExprKind::Match(..) => {},
+            // If `expr` is a result of desugaring the try block and is an ok-wrapped
+            // diverging expression (e.g. it arose from desugaring of `try { return }`),
+            // we skip issuing a warning because it is autogenerated code.
+            ExprKind::Call(..) if expr.span.is_desugaring(DesugaringKind::TryBlock) => {},
             ExprKind::Call(ref callee, _) =>
                 self.warn_if_unreachable(expr.hir_id, callee.span, "call"),
             ExprKind::MethodCall(_, ref span, _) =>

src/librustc_typeck/check/mod.rs

@@ -472,7 +472,7 @@ pub enum Diverges {
     WarnedAlways
 }
 
-// Convenience impls for combinig `Diverges`.
+// Convenience impls for combining `Diverges`.
 
 impl ops::BitAnd for Diverges {
     type Output = Self;

src/test/ui/try-block/try-block-unreachable-code-lint.rs

@@ -0,0 +1,76 @@
+// Test unreachable_code lint for `try {}` block ok-wrapping. See issues #54165, #63324.
+
+// compile-flags: --edition 2018
+// check-pass
+#![feature(try_blocks)]
+#![warn(unreachable_code)]
+
+fn err() -> Result<u32, ()> {
+    Err(())
+}
+
+// In the following cases unreachable code is autogenerated and should not be reported.
+
+fn test_ok_wrapped_divergent_expr_1() {
+    let res: Result<u32, ()> = try {
+        loop {
+            err()?;
+        }
+    };
+    println!("res: {:?}", res);
+}
+
+fn test_ok_wrapped_divergent_expr_2() {
+    let _: Result<u32, ()> = try {
+        return
+    };
+}
+
+fn test_autogenerated_unit_after_divergent_expr() {
+    let _: Result<(), ()> = try {
+        return;
+    };
+}
+
+// In the following cases unreachable code should be reported.
+
+fn test_try_block_after_divergent_stmt() {
+    let _: Result<u32, ()> = {
+        return;
+
+        try {
+            loop {
+                err()?;
+            }
+        }
+        // ~^^^^^ WARNING unreachable expression
+    };
+}
+
+fn test_wrapped_divergent_expr() {
+    let _: Result<u32, ()> = {
+        Err(return)
+        // ~^ WARNING unreachable call
+    };
+}
+
+fn test_expr_after_divergent_stmt_in_try_block() {
+    let res: Result<u32, ()> = try {
+        loop {
+            err()?;
+        }
+
+        42
+        // ~^ WARNING unreachable expression
+    };
+    println!("res: {:?}", res);
+}
+
+fn main() {
+    test_ok_wrapped_divergent_expr_1();
+    test_ok_wrapped_divergent_expr_2();
+    test_autogenerated_unit_after_divergent_expr();
+    test_try_block_after_divergent_stmt();
+    test_wrapped_divergent_expr();
+    test_expr_after_divergent_stmt_in_try_block();
+}

src/test/ui/try-block/try-block-unreachable-code-lint.stderr

@@ -0,0 +1,38 @@
+warning: unreachable expression
+  --> $DIR/try-block-unreachable-code-lint.rs:41:9
+   |
+LL |           return;
+   |           ------ any code following this expression is unreachable
+LL | 
+LL | /         try {
+LL | |             loop {
+LL | |                 err()?;
+LL | |             }
+LL | |         }
+   | |_________^ unreachable expression
+   |
+note: lint level defined here
+  --> $DIR/try-block-unreachable-code-lint.rs:6:9
+   |
+LL | #![warn(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+
+warning: unreachable call
+  --> $DIR/try-block-unreachable-code-lint.rs:52:9
+   |
+LL |         Err(return)
+   |         ^^^ ------ any code following this expression is unreachable
+   |         |
+   |         unreachable call
+
+warning: unreachable expression
+  --> $DIR/try-block-unreachable-code-lint.rs:63:9
+   |
+LL | /         loop {
+LL | |             err()?;
+LL | |         }
+   | |_________- any code following this expression is unreachable
+LL | 
+LL |           42
+   |           ^^ unreachable expression
+