Auto merge of #4729 - rust-lang:needless-doc-main-syn, r=<try>

Use syn in needless_doctest_main lint

changelog: none

This fixes #4698 by only linting non-empty `fn main()`s. This is not a perfect solution, but I don't want to omit linting if any macro call or attribute is detected, as that would result in many false negatives.

clippy_lints/Cargo.toml

@@ -25,13 +25,14 @@ matches = "0.1.7"
 pulldown-cmark = "0.6.0"
 quine-mc_cluskey = "0.2.2"
 regex-syntax = "0.6"
+semver = "0.9.0"
 serde = { version = "1.0", features = ["derive"] }
 smallvec = { version = "1", features = ["union"] }
+syn = { version = "1.0", features = ["full"] }
 toml = "0.5.3"
 unicode-normalization = "0.1"
-semver = "0.9.0"
-# NOTE: cargo requires serde feat in its url dep
-# see <https://github.com/rust-lang/rust/pull/63587#issuecomment-522343864>
+# cargo requires serde feat in its url dep
+# see https://github.com/rust-lang/rust/pull/63587#issuecomment-522343864
 url = { version =  "2.1.0", features = ["serde"] }
 
 [features]

clippy_lints/src/doc.rs

@@ -342,9 +342,29 @@ fn check_doc<'a, Events: Iterator<Item = (pulldown_cmark::Event<'a>, Range<usize
     safety_header
 }
 
-fn check_code(cx: &LateContext<'_, '_>, text: &str, span: Span) {
-    if text.contains("fn main() {") {
-        span_lint(cx, NEEDLESS_DOCTEST_MAIN, span, "needless `fn main` in doctest");
+// check a syn Item for non-empty `fn main() { .. }`
+fn is_default_main_fn(item: &syn::Item) -> bool {
+    match item {
+        syn::Item::Fn(syn::ItemFn { ref sig, ref block, .. }) => {
+            !block.stmts.is_empty()
+                && sig.ident == "main"
+                && match sig.output {
+                    syn::ReturnType::Default => true,
+                    syn::ReturnType::Type(_, ref ty) => match **ty {
+                        syn::Type::Tuple(syn::TypeTuple { ref elems, .. }) => elems.is_empty(),
+                        _ => false,
+                    },
+                }
+        },
+        _ => false,
+    }
+}
+
+fn check_code(cx: &LateContext<'_, '_>, code: &str, span: Span) {
+    if let Ok(file) = syn::parse_file(code) {
+        if file.items.iter().any(is_default_main_fn) {
+            span_lint(cx, NEEDLESS_DOCTEST_MAIN, span, "needless `fn main() {}` in doctest");
+        }
     }
 }
 

tests/ui/needless_doc_main.rs

@@ -0,0 +1,27 @@
+/// This method does nothing. It's just there for testing.
+///
+/// # Examples:
+///
+/// ```text,ignore
+/// This is not Rust code
+/// Just check that parsing won't crash
+/// R€fridger@tør.
+/// ```
+///
+/// ```
+/// fn main() {
+///     let this_is_linted = 42u8;
+///     assert_eq!(this_is_linted / 2, 21);
+/// }
+/// ```
+///
+/// ```
+/// #[allow(unused)]
+/// const I = 1;
+/// fn main() {
+///     // this main method contains no code and is thus not linted.
+/// }
+/// ```
+fn main() {
+    // nothing to see here, move along.
+}

tests/ui/needless_doc_main.stderr

@@ -0,0 +1,10 @@
+error: needless `fn main() {}` in doctest
+  --> $DIR/needless_doc_main.rs:12:4
+   |
+LL | /// fn main() {
+   |    ^^^^^^^^^^^^
+   |
+   = note: `-D clippy::needless-doctest-main` implied by `-D warnings`
+
+error: aborting due to previous error
+