Auto merge of rust-lang#149044 - clubby789:implicit-return-span, r=cjgillot

Reduce confusing `unreachable_code` lints

Closes rust-lang#149042

Attempts to be smarter about identifying 'real'/user-written unreachable code to raise the lint

Author: bors

compiler/rustc_mir_build/src/builder/mod.rs

@@ -861,20 +861,46 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
         }
 
+        /// Starting at a target unreachable block, find some user code to lint as unreachable
+        fn find_unreachable_code_from(
+            bb: BasicBlock,
+            bbs: &IndexVec<BasicBlock, BasicBlockData<'_>>,
+        ) -> Option<(SourceInfo, &'static str)> {
+            let bb = &bbs[bb];
+            for stmt in &bb.statements {
+                match &stmt.kind {
+                    // Ignore the implicit `()` return place assignment for unit functions/blocks
+                    StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(const_))))
+                        if const_.ty().is_unit() =>
+                    {
+                        continue;
+                    }
+                    StatementKind::StorageLive(_) | StatementKind::StorageDead(_) => {
+                        continue;
+                    }
+                    StatementKind::FakeRead(..) => return Some((stmt.source_info, "definition")),
+                    _ => return Some((stmt.source_info, "expression")),
+                }
+            }
+
+            let term = bb.terminator();
+            match term.kind {
+                // No user code in this bb, and our goto target may be reachable via other paths
+                TerminatorKind::Goto { .. } | TerminatorKind::Return => None,
+                _ => Some((term.source_info, "expression")),
+            }
+        }
+
         for (target_bb, orig_ty, orig_span) in lints {
             if orig_span.in_external_macro(self.tcx.sess.source_map()) {
                 continue;
             }
-            let target_bb = &self.cfg.basic_blocks[target_bb];
-            let (target_loc, descr) = target_bb
-                .statements
-                .iter()
-                .find_map(|stmt| match stmt.kind {
-                    StatementKind::StorageLive(_) | StatementKind::StorageDead(_) => None,
-                    StatementKind::FakeRead(..) => Some((stmt.source_info, "definition")),
-                    _ => Some((stmt.source_info, "expression")),
-                })
-                .unwrap_or_else(|| (target_bb.terminator().source_info, "expression"));
+
+            let Some((target_loc, descr)) =
+                find_unreachable_code_from(target_bb, &self.cfg.basic_blocks)
+            else {
+                continue;
+            };
             let lint_root = self.source_scopes[target_loc.scope]
                 .local_data
                 .as_ref()

tests/ui/uninhabited/unreachable.rs

@@ -0,0 +1,34 @@
+//! Test that a diverging function as the final expression in a block does not
+//! raise an 'unreachable code' lint.
+
+//@ check-pass
+#![deny(unreachable_code)]
+
+enum Never {}
+
+fn make_never() -> Never {
+    loop {}
+}
+
+fn func() {
+    make_never();
+}
+
+fn block() {
+    {
+        make_never();
+    }
+}
+
+fn branchy() {
+    if false {
+        make_never();
+    } else {
+        make_never();
+    }
+}
+
+fn main() {
+    func();
+    block();
+}

tests/ui/uninhabited/void-branch.rs

@@ -6,8 +6,9 @@ enum Void {}
 fn with_void() {
     if false {
         unsafe {
-            //~^ ERROR unreachable expression
             std::mem::uninitialized::<Void>();
+            println!();
+            //~^ ERROR unreachable expression
         }
     }
 
@@ -20,8 +21,9 @@ fn infallible() -> std::convert::Infallible {
 
 fn with_infallible() {
     if false {
-        //~^ ERROR unreachable expression
         infallible();
+        println!()
+        //~^ ERROR unreachable expression
     }
 
     println!()

tests/ui/uninhabited/void-branch.stderr

@@ -1,15 +1,13 @@
 error: unreachable expression
-  --> $DIR/void-branch.rs:8:9
+  --> $DIR/void-branch.rs:10:13
    |
-LL | /         unsafe {
-LL | |
-LL | |             std::mem::uninitialized::<Void>();
-   | |             --------------------------------- any code following this expression is unreachable
-LL | |         }
-   | |_________^ unreachable expression
+LL |             std::mem::uninitialized::<Void>();
+   |             --------------------------------- any code following this expression is unreachable
+LL |             println!();
+   |             ^^^^^^^^^^ unreachable expression
    |
 note: this expression has type `Void`, which is uninhabited
-  --> $DIR/void-branch.rs:10:13
+  --> $DIR/void-branch.rs:9:13
    |
 LL |             std::mem::uninitialized::<Void>();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -18,23 +16,22 @@ note: the lint level is defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `$crate::format_args` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: unreachable expression
-  --> $DIR/void-branch.rs:22:14
+  --> $DIR/void-branch.rs:25:9
    |
-LL |       if false {
-   |  ______________^
-LL | |
-LL | |         infallible();
-   | |         ------------ any code following this expression is unreachable
-LL | |     }
-   | |_____^ unreachable expression
+LL |         infallible();
+   |         ------------ any code following this expression is unreachable
+LL |         println!()
+   |         ^^^^^^^^^^ unreachable expression
    |
 note: this expression has type `Infallible`, which is uninhabited
   --> $DIR/void-branch.rs:24:9
    |
 LL |         infallible();
    |         ^^^^^^^^^^^^
+   = note: this error originates in the macro `$crate::format_args` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 2 previous errors