Add check for correct closing method of html element

CHANGELOG.md

@@ -10,6 +10,8 @@
   [#492](https://github.com/lambda-fairy/maud/pull/492)
 - Remove Tide support and documentation references.
   [#501](https://github.com/lambda-fairy/maud/pull/501)
+- Add check for correct closing method of html element. `html_unchecked!` skips this check.
+  [#505](https://github.com/lambda-fairy/maud/pull/505)
 
 ## [0.27.0] - 2025-02-02
 

maud/src/lib.rs

@@ -14,7 +14,7 @@ extern crate alloc;
 use alloc::{borrow::Cow, boxed::Box, string::String};
 use core::fmt::{self, Arguments, Display, Write};
 
-pub use maud_macros::html;
+pub use maud_macros::{html, html_unchecked};
 
 mod escape;
 

maud/tests/unchecked.rs

@@ -0,0 +1,21 @@
+use maud::html_unchecked;
+
+// These tests purposefully produce invalid html
+
+#[test]
+fn unchecked_accepts_non_void_element_with_semicolon() {
+    let result = html_unchecked! {
+        div;
+        p;
+    };
+    assert_eq!(result.into_string(), "<div><p>");
+}
+
+#[test]
+fn unchecked_accepts_void_element_with_braces() {
+    let result = html_unchecked! {
+        br {}
+        img {}
+    };
+    assert_eq!(result.into_string(), "<br></br><img></img>");
+}

maud/tests/warnings/block-element-with-semicolon.rs

@@ -0,0 +1,13 @@
+use maud::html;
+
+fn main() {
+    html! {
+        div;
+        p;
+        my-custom-element;
+        .thing;
+        @if true {
+            span;
+        }
+    };
+}

maud/tests/warnings/block-element-with-semicolon.stderr

@@ -0,0 +1,39 @@
+error: `<div>` is not a void element, so it cannot be closed with `;`
+ --> tests/warnings/block-element-with-semicolon.rs:5:12
+  |
+5 |         div;
+  |            ^
+  |
+  = help: change this to `{}`
+
+error: `<p>` is not a void element, so it cannot be closed with `;`
+ --> tests/warnings/block-element-with-semicolon.rs:6:10
+  |
+6 |         p;
+  |          ^
+  |
+  = help: change this to `{}`
+
+error: `<my-custom-element>` is not a void element, so it cannot be closed with `;`
+ --> tests/warnings/block-element-with-semicolon.rs:7:26
+  |
+7 |         my-custom-element;
+  |                          ^
+  |
+  = help: change this to `{}`
+
+error: `<div>` is not a void element, so it cannot be closed with `;`
+ --> tests/warnings/block-element-with-semicolon.rs:8:15
+  |
+8 |         .thing;
+  |               ^
+  |
+  = help: change this to `{}`
+
+error: `<span>` is not a void element, so it cannot be closed with `;`
+  --> tests/warnings/block-element-with-semicolon.rs:10:17
+   |
+10 |             span;
+   |                 ^
+   |
+   = help: change this to `{}`

maud/tests/warnings/void-element-with-block.rs

@@ -0,0 +1,12 @@
+use maud::html;
+
+fn main() {
+    html! {
+        br {}
+        img alt="some text" {}
+        meta charset="utf-8" {}
+        div {
+            a { hr {} }
+        }
+    };
+}

maud/tests/warnings/void-element-with-block.stderr

@@ -0,0 +1,31 @@
+error: `<br>` is a void element and cannot have a closing tag
+ --> $DIR/void-element-with-block.rs:5:12
+  |
+5 |         br {}
+  |            ^
+  |
+  = help: change this to `;`
+
+error: `<img>` is a void element and cannot have a closing tag
+ --> $DIR/void-element-with-block.rs:6:29
+  |
+6 |         img alt="some text" {}
+  |                             ^
+  |
+  = help: change this to `;`
+
+error: `<meta>` is a void element and cannot have a closing tag
+ --> $DIR/void-element-with-block.rs:7:30
+  |
+7 |         meta charset="utf-8" {}
+  |                              ^
+  |
+  = help: change this to `;`
+
+error: `<hr>` is a void element and cannot have a closing tag
+ --> $DIR/void-element-with-block.rs:9:20
+  |
+9 |             a { hr {} }
+  |                    ^
+  |
+  = help: change this to `;`

maud_macros/src/check.rs

@@ -0,0 +1,93 @@
+use proc_macro2_diagnostics::{Diagnostic, SpanDiagnosticExt};
+use syn::spanned::Spanned;
+
+use crate::ast::{
+    Block, ControlFlow, ControlFlowKind, Element, ElementBody, IfExpr, IfOrBlock, Markup, Markups,
+};
+
+/// The void elements as defined by the HTML Living Standard.
+const VOID_ELEMENTS: &[&str] = &[
+    "area", "base", "br", "col", "embed", "hr", "img", "input", "link", "meta", "source", "track",
+    "wbr",
+];
+
+/// Checks whether each element is closed in a valid way. Non-void elements
+/// must be closed with `{ ... }`, and void elements with `;`.
+///
+/// Emits a diagnostic for each mismatch found.
+pub fn check_elements(markups: &Markups<Element>, diagnostics: &mut Vec<Diagnostic>) {
+    for markup in &markups.markups {
+        check_markup(markup, diagnostics);
+    }
+}
+
+fn check_markup(markup: &Markup<Element>, diagnostics: &mut Vec<Diagnostic>) {
+    match markup {
+        Markup::Block(block) => check_block(block, diagnostics),
+        Markup::Element(element) => {
+            check_element(element, diagnostics);
+            if let ElementBody::Block(block) = &element.body {
+                check_block(block, diagnostics);
+            }
+        }
+        Markup::ControlFlow(control_flow) => check_control_flow(control_flow, diagnostics),
+        Markup::Lit(_) | Markup::Splice { .. } | Markup::Semi(_) => {}
+    }
+}
+
+fn check_block(block: &Block<Element>, diagnostics: &mut Vec<Diagnostic>) {
+    check_elements(&block.markups, diagnostics);
+}
+
+fn check_element(element: &Element, diagnostics: &mut Vec<Diagnostic>) {
+    let name = element
+        .name
+        .as_ref()
+        .map(|name| name.to_string().to_lowercase())
+        .unwrap_or_else(|| "div".to_owned());
+
+    match &element.body {
+        ElementBody::Void(semi) if !VOID_ELEMENTS.contains(&name.as_str()) => diagnostics.push(
+            semi.span()
+                .error(format!(
+                    "`<{name}>` is not a void element, so it cannot be closed with `;`"
+                ))
+                .help("change this to `{}`"),
+        ),
+        ElementBody::Block(block) if VOID_ELEMENTS.contains(&name.as_str()) => diagnostics.push(
+            block
+                .brace_token
+                .span
+                .open()
+                .error(format!(
+                    "`<{name}>` is a void element and cannot have a closing tag"
+                ))
+                .help("change this to `;`"),
+        ),
+        _ => {}
+    }
+}
+
+fn check_control_flow(control_flow: &ControlFlow<Element>, diagnostics: &mut Vec<Diagnostic>) {
+    match &control_flow.kind {
+        ControlFlowKind::Let(_) => {}
+        ControlFlowKind::If(if_) => check_if(if_, diagnostics),
+        ControlFlowKind::For(for_) => check_block(&for_.body, diagnostics),
+        ControlFlowKind::While(while_) => check_block(&while_.body, diagnostics),
+        ControlFlowKind::Match(match_) => {
+            for arm in &match_.arms {
+                check_markup(&arm.body, diagnostics);
+            }
+        }
+    }
+}
+
+fn check_if(if_: &IfExpr<Element>, diagnostics: &mut Vec<Diagnostic>) {
+    check_block(&if_.then_branch, diagnostics);
+    if let Some((_, _, else_branch)) = &if_.else_branch {
+        match &**else_branch {
+            IfOrBlock::If(if_) => check_if(if_, diagnostics),
+            IfOrBlock::Block(block) => check_block(block, diagnostics),
+        }
+    }
+}

maud_macros/src/lib.rs

@@ -5,6 +5,7 @@
 extern crate proc_macro;
 
 mod ast;
+mod check;
 mod escape;
 mod generate;
 
@@ -16,10 +17,17 @@ use syn::parse::{ParseStream, Parser};
 
 #[proc_macro]
 pub fn html(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
-    expand(input.into()).into()
+    expand(input.into(), true).into()
 }
 
-fn expand(input: TokenStream) -> TokenStream {
+/// Like [`html`], but does not check that elements are closed in a way that
+/// is valid for void elements.
+#[proc_macro]
+pub fn html_unchecked(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    expand(input.into(), false).into()
+}
+
+fn expand(input: TokenStream, check: bool) -> TokenStream {
     // Heuristic: the size of the resulting markup tends to correlate with the
     // code size of the template itself
     let size_hint = input.to_string().len();
@@ -41,6 +49,10 @@ fn expand(input: TokenStream) -> TokenStream {
         }
     };
 
+    if check {
+        check::check_elements(&markups, &mut diagnostics);
+    }
+
     let diag_tokens = diagnostics.into_iter().map(Diagnostic::emit_as_expr_tokens);
 
     let output_ident = Ident::new("__maud_output", Span::mixed_site());