Emit a warning if the doctest `main` function will not be run

10 files changed, 103 insertions, 17 deletions

diff --git a/src/librustdoc/doctest.rs b/src/librustdoc/doctest.rs
index 829a9ca6e7d..5b85eb54a5c 100644
--- a/src/librustdoc/doctest.rs
+++ b/src/librustdoc/doctest.rs
@@ -23,9 +23,9 @@ use rustc_hir::def_id::LOCAL_CRATE;
 use rustc_interface::interface;
 use rustc_session::config::{self, CrateType, ErrorOutputType, Input};
 use rustc_session::lint;
-use rustc_span::FileName;
 use rustc_span::edition::Edition;
 use rustc_span::symbol::sym;
+use rustc_span::{FileName, Span};
 use rustc_target::spec::{Target, TargetTuple};
 use tempfile::{Builder as TempFileBuilder, TempDir};
 use tracing::debug;
@@ -239,7 +239,7 @@ pub(crate) fn run(dcx: DiagCtxtHandle<'_>, input: Input, options: RustdocOptions
                 }
             } else {
                 let mut collector = CreateRunnableDocTests::new(options, opts);
-                tests.into_iter().for_each(|t| collector.add_test(t));
+                tests.into_iter().for_each(|t| collector.add_test(t, Some(compiler.sess.dcx())));
 
                 Ok(Some(collector))
             }
@@ -872,6 +872,7 @@ pub(crate) struct ScrapedDocTest {
     langstr: LangString,
     text: String,
     name: String,
+    span: Span,
 }
 
 impl ScrapedDocTest {
@@ -881,6 +882,7 @@ impl ScrapedDocTest {
         logical_path: Vec<String>,
         langstr: LangString,
         text: String,
+        span: Span,
     ) -> Self {
         let mut item_path = logical_path.join("::");
         item_path.retain(|c| c != ' ');
@@ -890,7 +892,7 @@ impl ScrapedDocTest {
         let name =
             format!("{} - {item_path}(line {line})", filename.prefer_remapped_unconditionaly());
 
-        Self { filename, line, langstr, text, name }
+        Self { filename, line, langstr, text, name, span }
     }
     fn edition(&self, opts: &RustdocOptions) -> Edition {
         self.langstr.edition.unwrap_or(opts.edition)
@@ -946,7 +948,7 @@ impl CreateRunnableDocTests {
         }
     }
 
-    fn add_test(&mut self, scraped_test: ScrapedDocTest) {
+    fn add_test(&mut self, scraped_test: ScrapedDocTest, dcx: Option<DiagCtxtHandle<'_>>) {
         // For example `module/file.rs` would become `module_file_rs`
         let file = scraped_test
             .filename
@@ -977,6 +979,8 @@ impl CreateRunnableDocTests {
             self.can_merge_doctests,
             Some(test_id),
             Some(&scraped_test.langstr),
+            dcx,
+            scraped_test.span,
         );
         let is_standalone = !doctest.can_be_merged
             || scraped_test.langstr.compile_fail
diff --git a/src/librustdoc/doctest/extracted.rs b/src/librustdoc/doctest/extracted.rs
index ce362eabfc4..d82bca3279d 100644
--- a/src/librustdoc/doctest/extracted.rs
+++ b/src/librustdoc/doctest/extracted.rs
@@ -3,6 +3,7 @@
 //! This module contains the logic to extract doctests and output a JSON containing this
 //! information.
 
+use rustc_span::DUMMY_SP;
 use serde::Serialize;
 
 use super::{DocTestBuilder, ScrapedDocTest};
@@ -35,7 +36,7 @@ impl ExtractedDocTests {
     ) {
         let edition = scraped_test.edition(options);
 
-        let ScrapedDocTest { filename, line, langstr, text, name } = scraped_test;
+        let ScrapedDocTest { filename, line, langstr, text, name, .. } = scraped_test;
 
         let doctest = DocTestBuilder::new(
             &text,
@@ -44,6 +45,8 @@ impl ExtractedDocTests {
             false,
             None,
             Some(&langstr),
+            None,
+            DUMMY_SP,
         );
         let (full_test_code, size) = doctest.generate_unique_doctest(
             &text,
diff --git a/src/librustdoc/doctest/make.rs b/src/librustdoc/doctest/make.rs
index d4fbfb12582..759b139e288 100644
--- a/src/librustdoc/doctest/make.rs
+++ b/src/librustdoc/doctest/make.rs
@@ -8,14 +8,14 @@ use std::sync::Arc;
 use rustc_ast::token::{Delimiter, TokenKind};
 use rustc_ast::tokenstream::TokenTree;
 use rustc_ast::{self as ast, AttrStyle, HasAttrs, StmtKind};
-use rustc_errors::ColorConfig;
 use rustc_errors::emitter::stderr_destination;
+use rustc_errors::{ColorConfig, DiagCtxtHandle};
 use rustc_parse::new_parser_from_source_str;
 use rustc_session::parse::ParseSess;
 use rustc_span::edition::Edition;
 use rustc_span::source_map::SourceMap;
 use rustc_span::symbol::sym;
-use rustc_span::{FileName, kw};
+use rustc_span::{FileName, Span, kw};
 use tracing::debug;
 
 use super::GlobalTestOptions;
@@ -61,6 +61,8 @@ impl DocTestBuilder {
         // If `test_id` is `None`, it means we're generating code for a code example "run" link.
         test_id: Option<String>,
         lang_str: Option<&LangString>,
+        dcx: Option<DiagCtxtHandle<'_>>,
+        span: Span,
     ) -> Self {
         let can_merge_doctests = can_merge_doctests
             && lang_str.is_some_and(|lang_str| {
@@ -69,7 +71,7 @@ impl DocTestBuilder {
 
         let result = rustc_driver::catch_fatal_errors(|| {
             rustc_span::create_session_if_not_set_then(edition, |_| {
-                parse_source(source, &crate_name)
+                parse_source(source, &crate_name, dcx, span)
             })
         });
 
@@ -289,7 +291,12 @@ fn reset_error_count(psess: &ParseSess) {
 
 const DOCTEST_CODE_WRAPPER: &str = "fn f(){";
 
-fn parse_source(source: &str, crate_name: &Option<&str>) -> Result<ParseSourceInfo, ()> {
+fn parse_source(
+    source: &str,
+    crate_name: &Option<&str>,
+    parent_dcx: Option<DiagCtxtHandle<'_>>,
+    span: Span,
+) -> Result<ParseSourceInfo, ()> {
     use rustc_errors::DiagCtxt;
     use rustc_errors::emitter::{Emitter, HumanEmitter};
     use rustc_span::source_map::FilePathMapping;
@@ -466,8 +473,17 @@ fn parse_source(source: &str, crate_name: &Option<&str>) -> Result<ParseSourceIn
                 }
             }
             if has_non_items {
-                // FIXME: if `info.has_main_fn` is `true`, emit a warning here to mention that
-                // this code will not be called.
+                if info.has_main_fn
+                    && let Some(dcx) = parent_dcx
+                    && !span.is_dummy()
+                {
+                    dcx.span_warn(
+                        span,
+                        "the `main` function of this doctest won't be run as it contains \
+                         expressions at the top level, meaning that the whole doctest code will be \
+                         wrapped in a function",
+                    );
+                }
                 info.has_main_fn = false;
             }
             Ok(info)
diff --git a/src/librustdoc/doctest/markdown.rs b/src/librustdoc/doctest/markdown.rs
index 497a8d7c4a7..b877e34583f 100644
--- a/src/librustdoc/doctest/markdown.rs
+++ b/src/librustdoc/doctest/markdown.rs
@@ -4,7 +4,7 @@ use std::fs::read_to_string;
 use std::sync::{Arc, Mutex};
 
 use rustc_session::config::Input;
-use rustc_span::FileName;
+use rustc_span::{DUMMY_SP, FileName};
 use tempfile::tempdir;
 
 use super::{
@@ -24,7 +24,14 @@ impl DocTestVisitor for MdCollector {
         let filename = self.filename.clone();
         // First line of Markdown is line 1.
         let line = 1 + rel_line.offset();
-        self.tests.push(ScrapedDocTest::new(filename, line, self.cur_path.clone(), config, test));
+        self.tests.push(ScrapedDocTest::new(
+            filename,
+            line,
+            self.cur_path.clone(),
+            config,
+            test,
+            DUMMY_SP,
+        ));
     }
 
     fn visit_header(&mut self, name: &str, level: u32) {
@@ -107,7 +114,7 @@ pub(crate) fn test(input: &Input, options: Options) -> Result<(), String> {
     find_testable_code(&input_str, &mut md_collector, codes, None);
 
     let mut collector = CreateRunnableDocTests::new(options.clone(), opts);
-    md_collector.tests.into_iter().for_each(|t| collector.add_test(t));
+    md_collector.tests.into_iter().for_each(|t| collector.add_test(t, None));
     let CreateRunnableDocTests { opts, rustdoc_options, standalone_tests, mergeable_tests, .. } =
         collector;
     crate::doctest::run_tests(
diff --git a/src/librustdoc/doctest/rust.rs b/src/librustdoc/doctest/rust.rs
index 43dcfab880b..f9d2aa3d3b4 100644
--- a/src/librustdoc/doctest/rust.rs
+++ b/src/librustdoc/doctest/rust.rs
@@ -1,5 +1,6 @@
 //! Doctest functionality used only for doctests in `.rs` source files.
 
+use std::cell::Cell;
 use std::env;
 use std::sync::Arc;
 
@@ -47,13 +48,33 @@ impl RustCollector {
 
 impl DocTestVisitor for RustCollector {
     fn visit_test(&mut self, test: String, config: LangString, rel_line: MdRelLine) {
-        let line = self.get_base_line() + rel_line.offset();
+        let base_line = self.get_base_line();
+        let line = base_line + rel_line.offset();
+        let count = Cell::new(base_line);
+        let span = if line > base_line {
+            match self.source_map.span_extend_while(self.position, |c| {
+                if c == '\n' {
+                    let count_v = count.get();
+                    count.set(count_v + 1);
+                    if count_v >= line {
+                        return false;
+                    }
+                }
+                true
+            }) {
+                Ok(sp) => self.source_map.span_extend_to_line(sp.shrink_to_hi()),
+                _ => self.position,
+            }
+        } else {
+            self.position
+        };
         self.tests.push(ScrapedDocTest::new(
             self.get_filename(),
             line,
             self.cur_path.clone(),
             config,
             test,
+            span,
         ));
     }
 
diff --git a/src/librustdoc/doctest/tests.rs b/src/librustdoc/doctest/tests.rs
index 49add73e9d6..ce27a20540e 100644
--- a/src/librustdoc/doctest/tests.rs
+++ b/src/librustdoc/doctest/tests.rs
@@ -1,5 +1,6 @@
 use std::path::PathBuf;
 
+use rustc_span::DUMMY_SP;
 use rustc_span::edition::DEFAULT_EDITION;
 
 use super::{DocTestBuilder, GlobalTestOptions};
@@ -18,6 +19,8 @@ fn make_test(
         false,
         test_id.map(|s| s.to_string()),
         None,
+        None,
+        DUMMY_SP,
     );
     let (code, line_offset) =
         doctest.generate_unique_doctest(test_code, dont_insert_main, opts, crate_name);
diff --git a/src/librustdoc/html/markdown.rs b/src/librustdoc/html/markdown.rs
index fc46293e7ea..5014a5198c8 100644
--- a/src/librustdoc/html/markdown.rs
+++ b/src/librustdoc/html/markdown.rs
@@ -45,7 +45,7 @@ use rustc_middle::ty::TyCtxt;
 pub(crate) use rustc_resolve::rustdoc::main_body_opts;
 use rustc_resolve::rustdoc::may_be_doc_link;
 use rustc_span::edition::Edition;
-use rustc_span::{Span, Symbol};
+use rustc_span::{DUMMY_SP, Span, Symbol};
 use tracing::{debug, trace};
 
 use crate::clean::RenderedLink;
@@ -303,7 +303,9 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
                 attrs: vec![],
                 args_file: PathBuf::new(),
             };
-            let doctest = doctest::DocTestBuilder::new(&test, krate, edition, false, None, None);
+            let doctest = doctest::DocTestBuilder::new(
+                &test, krate, edition, false, None, None, None, DUMMY_SP,
+            );
             let (test, _) = doctest.generate_unique_doctest(&test, false, &opts, krate);
             let channel = if test.contains("#![feature(") { "&amp;version=nightly" } else { "" };
 
diff --git a/tests/rustdoc-ui/doctest/failed-doctest-extra-semicolon-on-item.stderr b/tests/rustdoc-ui/doctest/failed-doctest-extra-semicolon-on-item.stderr
new file mode 100644
index 00000000000..113fb7ccb60
--- /dev/null
+++ b/tests/rustdoc-ui/doctest/failed-doctest-extra-semicolon-on-item.stderr
@@ -0,0 +1,8 @@
+warning: the `main` function of this doctest won't be run as it contains expressions at the top level, meaning that the whole doctest code will be wrapped in a function
+  --> $DIR/failed-doctest-extra-semicolon-on-item.rs:11:1
+   |
+11 | /// ```rust
+   | ^^^^^^^^^^^
+
+warning: 1 warning emitted
+
diff --git a/tests/rustdoc-ui/doctest/main-alongside-stmts.stderr b/tests/rustdoc-ui/doctest/main-alongside-stmts.stderr
new file mode 100644
index 00000000000..d90a289ca69
--- /dev/null
+++ b/tests/rustdoc-ui/doctest/main-alongside-stmts.stderr
@@ -0,0 +1,14 @@
+warning: the `main` function of this doctest won't be run as it contains expressions at the top level, meaning that the whole doctest code will be wrapped in a function
+  --> $DIR/main-alongside-stmts.rs:17:1
+   |
+17 | //! ```
+   | ^^^^^^^
+
+warning: the `main` function of this doctest won't be run as it contains expressions at the top level, meaning that the whole doctest code will be wrapped in a function
+  --> $DIR/main-alongside-stmts.rs:26:1
+   |
+26 | //! ```
+   | ^^^^^^^
+
+warning: 2 warnings emitted
+
diff --git a/tests/rustdoc-ui/doctest/test-main-alongside-exprs.stderr b/tests/rustdoc-ui/doctest/test-main-alongside-exprs.stderr
new file mode 100644
index 00000000000..0dc7c2a2eea
--- /dev/null
+++ b/tests/rustdoc-ui/doctest/test-main-alongside-exprs.stderr
@@ -0,0 +1,8 @@
+warning: the `main` function of this doctest won't be run as it contains expressions at the top level, meaning that the whole doctest code will be wrapped in a function
+  --> $DIR/test-main-alongside-exprs.rs:15:1
+   |
+15 | //! ```
+   | ^^^^^^^
+
+warning: 1 warning emitted
+