Rollup merge of #121135 - Zalathar:no-whole-body-span, r=wesleywiser

coverage: Discard spans that fill the entire function body

While debugging some other coverage changes, I discovered a frustrating inconsistency that occurs in functions containing closures, if they end with an implicit `()` return instead of an explicit trailing-expression.

This turns out to have been caused by the corresponding node in MIR having a span that covers the entire function body. When preparing coverage spans, any span that fills the whole body tends to cause more harm than good, so this PR detects and discards those spans.

(This isn't the first time whole-body spans have caused problems; we also eliminated some of them in #118525.)

11 files changed, 117 insertions, 28 deletions

diff --git a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
index 9517ede288f..2db358379fe 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
@@ -132,18 +132,23 @@ fn bcb_to_initial_coverage_spans<'a, 'tcx>(
     bcb_data.basic_blocks.iter().flat_map(move |&bb| {
         let data = &mir_body[bb];
 
+        let unexpand = move |expn_span| {
+            unexpand_into_body_span_with_visible_macro(expn_span, body_span)
+                // Discard any spans that fill the entire body, because they tend
+                // to represent compiler-inserted code, e.g. implicitly returning `()`.
+                .filter(|(span, _)| !span.source_equal(body_span))
+        };
+
         let statement_spans = data.statements.iter().filter_map(move |statement| {
             let expn_span = filtered_statement_span(statement)?;
-            let (span, visible_macro) =
-                unexpand_into_body_span_with_visible_macro(expn_span, body_span)?;
+            let (span, visible_macro) = unexpand(expn_span)?;
 
             Some(SpanFromMir::new(span, visible_macro, bcb, is_closure_like(statement)))
         });
 
         let terminator_span = Some(data.terminator()).into_iter().filter_map(move |terminator| {
             let expn_span = filtered_terminator_span(terminator)?;
-            let (span, visible_macro) =
-                unexpand_into_body_span_with_visible_macro(expn_span, body_span)?;
+            let (span, visible_macro) = unexpand(expn_span)?;
 
             Some(SpanFromMir::new(span, visible_macro, bcb, false))
         });
diff --git a/tests/coverage/closure_unit_return.cov-map b/tests/coverage/closure_unit_return.cov-map
new file mode 100644
index 00000000000..c97b4a44dd6
--- /dev/null
+++ b/tests/coverage/closure_unit_return.cov-map
@@ -0,0 +1,34 @@
+Function name: closure_unit_return::explicit_unit
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 07, 01, 01, 10, 01, 05, 05, 02, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 2
+- Code(Counter(0)) at (prev + 7, 1) to (start + 1, 16)
+- Code(Counter(0)) at (prev + 5, 5) to (start + 2, 2)
+
+Function name: closure_unit_return::explicit_unit::{closure#0} (unused)
+Raw bytes (9): 0x[01, 01, 00, 01, 00, 08, 16, 02, 06]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Zero) at (prev + 8, 22) to (start + 2, 6)
+
+Function name: closure_unit_return::implicit_unit
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 10, 01, 01, 10, 01, 05, 05, 02, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 2
+- Code(Counter(0)) at (prev + 16, 1) to (start + 1, 16)
+- Code(Counter(0)) at (prev + 5, 5) to (start + 2, 2)
+
+Function name: closure_unit_return::implicit_unit::{closure#0} (unused)
+Raw bytes (9): 0x[01, 01, 00, 01, 00, 11, 16, 02, 06]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 1
+- Code(Zero) at (prev + 17, 22) to (start + 2, 6)
+
diff --git a/tests/coverage/closure_unit_return.coverage b/tests/coverage/closure_unit_return.coverage
new file mode 100644
index 00000000000..1056baa385c
--- /dev/null
+++ b/tests/coverage/closure_unit_return.coverage
@@ -0,0 +1,30 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |// edition: 2021
+   LL|       |
+   LL|       |// Regression test for an inconsistency between functions that return the value
+   LL|       |// of their trailing expression, and functions that implicitly return `()`.
+   LL|       |
+   LL|      1|fn explicit_unit() {
+   LL|      1|    let closure = || {
+   LL|      0|        ();
+   LL|      0|    };
+   LL|       |
+   LL|      1|    drop(closure);
+   LL|      1|    () // explicit return of trailing value
+   LL|      1|}
+   LL|       |
+   LL|      1|fn implicit_unit() {
+   LL|      1|    let closure = || {
+   LL|      0|        ();
+   LL|      0|    };
+   LL|       |
+   LL|      1|    drop(closure);
+   LL|      1|    // implicit return of `()`
+   LL|      1|}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |fn main() {
+   LL|       |    explicit_unit();
+   LL|       |    implicit_unit();
+   LL|       |}
+
diff --git a/tests/coverage/closure_unit_return.rs b/tests/coverage/closure_unit_return.rs
new file mode 100644
index 00000000000..d2b4ab8bd9f
--- /dev/null
+++ b/tests/coverage/closure_unit_return.rs
@@ -0,0 +1,29 @@
+#![feature(coverage_attribute)]
+// edition: 2021
+
+// Regression test for an inconsistency between functions that return the value
+// of their trailing expression, and functions that implicitly return `()`.
+
+fn explicit_unit() {
+    let closure = || {
+        ();
+    };
+
+    drop(closure);
+    () // explicit return of trailing value
+}
+
+fn implicit_unit() {
+    let closure = || {
+        ();
+    };
+
+    drop(closure);
+    // implicit return of `()`
+}
+
+#[coverage(off)]
+fn main() {
+    explicit_unit();
+    implicit_unit();
+}
diff --git a/tests/coverage/coverage_attr_closure.cov-map b/tests/coverage/coverage_attr_closure.cov-map
index 2208b28fd41..5d2c6b00b40 100644
--- a/tests/coverage/coverage_attr_closure.cov-map
+++ b/tests/coverage/coverage_attr_closure.cov-map
@@ -15,14 +15,14 @@ Number of file 0 mappings: 1
 - Code(Zero) at (prev + 29, 19) to (start + 2, 6)
 
 Function name: coverage_attr_closure::contains_closures_on
-Raw bytes (19): 0x[01, 01, 00, 03, 01, 0f, 01, 02, 05, 01, 04, 06, 02, 05, 01, 04, 06, 01, 02]
+Raw bytes (19): 0x[01, 01, 00, 03, 01, 0f, 01, 01, 1a, 01, 05, 09, 00, 1b, 01, 04, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 3
-- Code(Counter(0)) at (prev + 15, 1) to (start + 2, 5)
-- Code(Counter(0)) at (prev + 4, 6) to (start + 2, 5)
-- Code(Counter(0)) at (prev + 4, 6) to (start + 1, 2)
+- Code(Counter(0)) at (prev + 15, 1) to (start + 1, 26)
+- Code(Counter(0)) at (prev + 5, 9) to (start + 0, 27)
+- Code(Counter(0)) at (prev + 4, 1) to (start + 0, 2)
 
 Function name: coverage_attr_closure::contains_closures_on::{closure#0} (unused)
 Raw bytes (9): 0x[01, 01, 00, 01, 00, 11, 13, 02, 06]
diff --git a/tests/coverage/coverage_attr_closure.coverage b/tests/coverage/coverage_attr_closure.coverage
index 32c75b40d83..3474ad65063 100644
--- a/tests/coverage/coverage_attr_closure.coverage
+++ b/tests/coverage/coverage_attr_closure.coverage
@@ -14,13 +14,13 @@
    LL|       |#[coverage(on)]
    LL|      1|fn contains_closures_on() {
    LL|      1|    let _local_closure_on = #[coverage(on)]
-   LL|      1|    |input: &str| {
+   LL|      0|    |input: &str| {
    LL|      0|        println!("{input}");
-   LL|      1|    };
+   LL|      0|    };
    LL|      1|    let _local_closure_off = #[coverage(off)]
-   LL|      1|    |input: &str| {
+   LL|       |    |input: &str| {
    LL|       |        println!("{input}");
-   LL|      1|    };
+   LL|       |    };
    LL|      1|}
    LL|       |
    LL|       |#[coverage(off)]
diff --git a/tests/coverage/inline-dead.cov-map b/tests/coverage/inline-dead.cov-map
index c669b7245ea..f77781ca028 100644
--- a/tests/coverage/inline-dead.cov-map
+++ b/tests/coverage/inline-dead.cov-map
@@ -22,13 +22,13 @@ Number of file 0 mappings: 4
     = (Zero + (c0 - Zero))
 
 Function name: inline_dead::main
-Raw bytes (14): 0x[01, 01, 00, 02, 01, 04, 01, 03, 0d, 01, 05, 06, 02, 02]
+Raw bytes (14): 0x[01, 01, 00, 02, 01, 04, 01, 03, 0a, 01, 06, 05, 01, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 2
-- Code(Counter(0)) at (prev + 4, 1) to (start + 3, 13)
-- Code(Counter(0)) at (prev + 5, 6) to (start + 2, 2)
+- Code(Counter(0)) at (prev + 4, 1) to (start + 3, 10)
+- Code(Counter(0)) at (prev + 6, 5) to (start + 1, 2)
 
 Function name: inline_dead::main::{closure#0}
 Raw bytes (23): 0x[01, 01, 02, 00, 06, 01, 00, 03, 01, 07, 17, 01, 16, 00, 01, 17, 00, 18, 03, 01, 05, 00, 06]
diff --git a/tests/coverage/macro_name_span.cov-map b/tests/coverage/macro_name_span.cov-map
index a18e5f14861..f3ee44d2a5a 100644
--- a/tests/coverage/macro_name_span.cov-map
+++ b/tests/coverage/macro_name_span.cov-map
@@ -1,10 +1,10 @@
 Function name: macro_name_span::affected_function
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 16, 1c, 02, 06]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 16, 1c, 01, 40]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 22, 28) to (start + 2, 6)
+- Code(Counter(0)) at (prev + 22, 28) to (start + 1, 64)
 
 Function name: macro_name_span::main
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 0b, 01, 02, 02]
diff --git a/tests/coverage/macro_name_span.coverage b/tests/coverage/macro_name_span.coverage
index 28c88b1defa..0e12ce4a27c 100644
--- a/tests/coverage/macro_name_span.coverage
+++ b/tests/coverage/macro_name_span.coverage
@@ -21,6 +21,6 @@
    LL|       |macro_name_span_helper::macro_that_defines_a_function! {
    LL|      1|    fn affected_function() {
    LL|      1|        macro_with_an_unreasonably_and_egregiously_long_name!();
-   LL|      1|    }
+   LL|       |    }
    LL|       |}
 
diff --git a/tests/coverage/unicode.cov-map b/tests/coverage/unicode.cov-map
index cd40194a083..aedfb2071c1 100644
--- a/tests/coverage/unicode.cov-map
+++ b/tests/coverage/unicode.cov-map
@@ -27,14 +27,6 @@ Number of file 0 mappings: 9
 - Code(Expression(5, Add)) at (prev + 2, 5) to (start + 1, 2)
     = (c4 + ((((c0 + c1) - c1) - c2) + c3))
 
-Function name: unicode::サビ
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 1e, 14, 00, 18]
-Number of files: 1
-- file 0 => global file 1
-Number of expressions: 0
-Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 30, 20) to (start + 0, 24)
-
 Function name: unicode::他 (unused)
 Raw bytes (9): 0x[01, 01, 00, 01, 00, 1e, 19, 00, 25]
 Number of files: 1
diff --git a/tests/coverage/unicode.coverage b/tests/coverage/unicode.coverage
index b284a557d57..af586111ba3 100644
--- a/tests/coverage/unicode.coverage
+++ b/tests/coverage/unicode.coverage
@@ -29,8 +29,7 @@
    LL|       |
    LL|       |macro_rules! macro_that_defines_a_function {
    LL|       |    (fn $名:ident () $体:tt) => {
-   LL|      1|        fn $名 () $体 fn 他 () {}
-                                      ^0
+   LL|      0|        fn $名 () $体 fn 他 () {}
    LL|       |    }
    LL|       |}
    LL|       |