Rollup merge of rust-lang#79816 - poliorcetics:rustdoc-fail-on-deny, …

…r=jyn514

Declare a new lint to properly deny warnings in rustdoc

This declares a new lint: `INVALID_RUST_CODEBLOCK` that is used by `rustdoc` to properly follow `-D warnings` instead of unconditionally emitting a warning.

## Todo List

- [x] Declare lint.
- [x] Document lint (file: `src/doc/rustdoc/src/lints.md`).
- [x] Use lint in `rustdoc` (file: `src/librustdoc/passes/check_code_block_syntax.rs`, maybe others).
- [x] Write tests.
  - [x] Note: one for the behaviour of the new lint when the error is in a dependency, not the crate being tested (rust-lang#79816 (comment))
- [x] Refactor things.

## Questions

- How/where are lints tested ?
- Where are lints for `rustdoc` tested ?

Fix rust-lang#79792.

```@rustbot``` label T-rustdoc A-lint

compiler/rustc_lint/src/lib.rs

@@ -70,7 +70,7 @@ use rustc_middle::ty::TyCtxt;
 use rustc_session::lint::builtin::{
     BARE_TRAIT_OBJECTS, BROKEN_INTRA_DOC_LINKS, ELIDED_LIFETIMES_IN_PATHS,
     EXPLICIT_OUTLIVES_REQUIREMENTS, INVALID_CODEBLOCK_ATTRIBUTES, INVALID_HTML_TAGS,
-    MISSING_DOC_CODE_EXAMPLES, NON_AUTOLINKS, PRIVATE_DOC_TESTS,
+    INVALID_RUST_CODEBLOCK, MISSING_DOC_CODE_EXAMPLES, NON_AUTOLINKS, PRIVATE_DOC_TESTS,
 };
 use rustc_span::symbol::{Ident, Symbol};
 use rustc_span::Span;
@@ -319,6 +319,7 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         BROKEN_INTRA_DOC_LINKS,
         PRIVATE_INTRA_DOC_LINKS,
         INVALID_CODEBLOCK_ATTRIBUTES,
+        INVALID_RUST_CODEBLOCK,
         MISSING_DOC_CODE_EXAMPLES,
         PRIVATE_DOC_TESTS,
         INVALID_HTML_TAGS

compiler/rustc_lint_defs/src/builtin.rs

@@ -1846,6 +1846,18 @@ declare_lint! {
     "codeblock attribute looks a lot like a known one"
 }
 
+declare_lint! {
+    /// The `invalid_rust_codeblock` lint detects Rust code blocks in
+    /// documentation examples that are invalid (e.g. empty, not parsable as
+    /// Rust code). This is a `rustdoc` only lint, see the documentation in the
+    /// [rustdoc book].
+    ///
+    /// [rustdoc book]: ../../../rustdoc/lints.html#invalid_rust_codeblock
+    pub INVALID_RUST_CODEBLOCK,
+    Warn,
+    "codeblock could not be parsed as valid Rust or is empty"
+}
+
 declare_lint! {
     /// The `missing_crate_level_docs` lint detects if documentation is
     /// missing at the crate root. This is a `rustdoc` only lint, see the
@@ -2846,6 +2858,7 @@ declare_lint_pass! {
         BROKEN_INTRA_DOC_LINKS,
         PRIVATE_INTRA_DOC_LINKS,
         INVALID_CODEBLOCK_ATTRIBUTES,
+        INVALID_RUST_CODEBLOCK,
         MISSING_CRATE_LEVEL_DOCS,
         MISSING_DOC_CODE_EXAMPLES,
         INVALID_HTML_TAGS,

compiler/rustc_trait_selection/src/opaque_types.rs

@@ -46,6 +46,7 @@ pub struct OpaqueTypeDecl<'tcx> {
     /// type Foo = impl Baz;
     /// fn bar() -> Foo {
     /// //          ^^^ This is the span we are looking for!
+    /// # }
     /// ```
     ///
     /// In cases where the fn returns `(impl Trait, impl Trait)` or

src/doc/rustdoc/src/lints.md

@@ -285,6 +285,41 @@ warning: unclosed HTML tag `h1`
 
 warning: 2 warnings emitted
 ```
+                
+## invalid_rust_codeblock
+
+This lint **warns by default**. It detects Rust code blocks in documentation
+examples that are invalid (e.g. empty, not parsable as Rust). For example:
+
+```rust
+/// Empty code block, with and without Rust:
+///
+/// ```rust
+/// ```
+///
+/// Unclosed code block:
+///
+/// ```rust
+fn main() {}
+```
+
+Which will give:
+
+```text
+warning: Rust code block is empty
+ --> src/lib.rs:3:5
+  |
+3 |   /// ```rust
+  |  _____^
+4 | | /// ```
+  | |_______^
+
+warning: Rust code block is empty
+ --> src/lib.rs:8:5
+  |
+8 | /// ```rust
+  |     ^^^^^^^
+```
 
 ## non_autolinks
 

src/librustdoc/core.rs

@@ -308,6 +308,7 @@ crate fn create_config(
     let private_doc_tests = rustc_lint::builtin::PRIVATE_DOC_TESTS.name;
     let no_crate_level_docs = rustc_lint::builtin::MISSING_CRATE_LEVEL_DOCS.name;
     let invalid_codeblock_attributes_name = rustc_lint::builtin::INVALID_CODEBLOCK_ATTRIBUTES.name;
+    let invalid_rust_codeblock = rustc_lint::builtin::INVALID_RUST_CODEBLOCK.name;
     let invalid_html_tags = rustc_lint::builtin::INVALID_HTML_TAGS.name;
     let renamed_and_removed_lints = rustc_lint::builtin::RENAMED_AND_REMOVED_LINTS.name;
     let non_autolinks = rustc_lint::builtin::NON_AUTOLINKS.name;
@@ -323,6 +324,7 @@ crate fn create_config(
         private_doc_tests.to_owned(),
         no_crate_level_docs.to_owned(),
         invalid_codeblock_attributes_name.to_owned(),
+        invalid_rust_codeblock.to_owned(),
         invalid_html_tags.to_owned(),
         renamed_and_removed_lints.to_owned(),
         unknown_lints.to_owned(),

src/librustdoc/passes/check_code_block_syntax.rs

@@ -1,6 +1,8 @@
 use rustc_data_structures::sync::{Lock, Lrc};
 use rustc_errors::{emitter::Emitter, Applicability, Diagnostic, Handler};
+use rustc_middle::lint::LintDiagnosticBuilder;
 use rustc_parse::parse_stream_from_source_str;
+use rustc_session::lint;
 use rustc_session::parse::ParseSess;
 use rustc_span::source_map::{FilePathMapping, SourceMap};
 use rustc_span::{FileName, InnerSpan};
@@ -47,51 +49,76 @@ impl<'a, 'tcx> SyntaxChecker<'a, 'tcx> {
         .unwrap_or(false);
         let buffer = buffer.borrow();
 
-        if buffer.has_errors || is_empty {
-            let mut diag = if let Some(sp) =
-                super::source_span_for_markdown_range(self.cx, &dox, &code_block.range, &item.attrs)
-            {
-                let warning_message = if buffer.has_errors {
+        if !(buffer.has_errors || is_empty) {
+            // No errors in a non-empty program.
+            return;
+        }
+
+        let local_id = match item.def_id.as_local() {
+            Some(id) => id,
+            // We don't need to check the syntax for other crates so returning
+            // without doing anything should not be a problem.
+            None => return,
+        };
+
+        let hir_id = self.cx.tcx.hir().local_def_id_to_hir_id(local_id);
+        let mark_with_text = code_block.syntax.is_none() && code_block.is_fenced;
+
+        // The span and whether it is precise or not.
+        let (sp, precise_span) = match super::source_span_for_markdown_range(
+            self.cx,
+            &dox,
+            &code_block.range,
+            &item.attrs,
+        ) {
+            Some(sp) => (sp, true),
+            None => (super::span_of_attrs(&item.attrs).unwrap_or(item.source.span()), false),
+        };
+
+        // lambda that will use the lint to start a new diagnostic and add
+        // a suggestion to it when needed.
+        let diag_builder = |lint: LintDiagnosticBuilder<'_>| {
+            let mut diag = if precise_span {
+                let msg = if buffer.has_errors {
                     "could not parse code block as Rust code"
                 } else {
                     "Rust code block is empty"
                 };
 
-                let mut diag = self.cx.sess().struct_span_warn(sp, warning_message);
-
-                if code_block.syntax.is_none() && code_block.is_fenced {
-                    let sp = sp.from_inner(InnerSpan::new(0, 3));
+                let mut diag = lint.build(msg);
+                if mark_with_text {
                     diag.span_suggestion(
-                        sp,
+                        sp.from_inner(InnerSpan::new(0, 3)),
                         "mark blocks that do not contain Rust code as text",
                         String::from("```text"),
                         Applicability::MachineApplicable,
                     );
                 }
-
                 diag
             } else {
-                // We couldn't calculate the span of the markdown block that had the error, so our
-                // diagnostics are going to be a bit lacking.
-                let mut diag = self.cx.sess().struct_span_warn(
-                    super::span_of_attrs(&item.attrs).unwrap_or(item.source.span()),
-                    "doc comment contains an invalid Rust code block",
-                );
-
-                if code_block.syntax.is_none() && code_block.is_fenced {
+                let mut diag = lint.build("doc comment contains an invalid Rust code block");
+                if mark_with_text {
                     diag.help("mark blocks that do not contain Rust code as text: ```text");
                 }
 
                 diag
             };
-
             // FIXME(#67563): Provide more context for these errors by displaying the spans inline.
             for message in buffer.messages.iter() {
                 diag.note(&message);
             }
-
             diag.emit();
-        }
+        };
+
+        // Finally build and emit the completed diagnostic.
+        // All points of divergence have been handled earlier so this can be
+        // done the same way whether the span is precise or not.
+        self.cx.tcx.struct_span_lint_hir(
+            lint::builtin::INVALID_RUST_CODEBLOCK,
+            hir_id,
+            sp,
+            diag_builder,
+        );
     }
 }
 

src/test/rustdoc-ui/invalid-syntax.stderr

@@ -7,6 +7,7 @@ LL | | /// \__________pkt->size___________/          \_result->size_/ \__pkt->si
 LL | | /// ```
    | |_______^
    |
+   = note: `#[warn(invalid_rust_codeblock)]` on by default
    = note: error from rustc: unknown start of token: \
    = note: error from rustc: unknown start of token: \
    = note: error from rustc: unknown start of token: \