Rollup merge of #64267 - ehuss:rustdoc-fix-mixed-code-block, r=GuillaumeGomez

rustdoc: fix diagnostic with mixed code block styles

This fixes a relatively obscure issue where the diagnostic (emitted [here](https://github.com/rust-lang/rust/blame/ef54f57c5b9d894a38179d09b00610c1b337b086/src/librustdoc/passes/check_code_block_syntax.rs#L69)) would get confused since the "is_fenced" flag wasn't reset properly.

Author: Centril

src/librustdoc/html/markdown.rs

@@ -931,7 +931,10 @@ crate fn rust_code_blocks(md: &str) -> Vec<RustCodeBlock> {
                             is_fenced = true;
                             previous_offset + fence_idx
                         }
-                        None => offset,
+                        None => {
+                            is_fenced = false;
+                            offset
+                        }
                     };
                 }
             }

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

@@ -74,3 +74,11 @@ pub fn empty_rust() {}
 ///
 /// ```
 pub fn empty_rust_with_whitespace() {}
+
+/// ```
+/// let x = 1;
+/// ```
+///
+///     \____/
+///
+pub fn indent_after_fenced() {}

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

@@ -201,6 +201,24 @@ help: mark blocks that do not contain Rust code as text
 LL | /// ```text
    |     ^^^^^^^
 
+error: unknown start of token: \
+ --> <doctest>:1:1
+  |
+1 | \____/
+  | ^
+
+warning: could not parse code block as Rust code
+  --> $DIR/invalid-syntax.rs:82:9
+   |
+LL | ///     \____/
+   |         ^^^^^^
+
+error: unknown start of token: \
+ --> <rustdoc-highlighting>:1:1
+  |
+1 | \____/
+  | ^
+
 error: unknown start of token: \
  --> <rustdoc-highlighting>:1:1
   |