No syntax highlight in embedded language scope

Issue Type: Bug

I'm contributing to an extension that supports an embedded DSL written in Haskell. Currently the extension doesn't define a language ID and suggests adding a file association to Haskell to utilize existing syntax highlighters.

I'd like to define a proper language ID but we don't want to lose syntax highlighting, so I came up with a workaround: defining a grammar file with a single global scope and embedding Haskell into the global scope.

Prerequisites: Install a Haskell syntax highlighter (e.g. this one).

Expected behavior: highlight the source code as Haskell.

Actual behavior: I don't get syntax highlight although the languge scope utility correctly identifies the scope as haskell.

scope-sample

A .hs file in the same directory is displayed with correct syntax highlight.

scope-haskell

I made a minimal extension to reproduce the issue. I copy relevant files here.

package.json

{
    "name": "vscode-syntax-ext",
    "displayName": "Sample extension for VSCode",
    "description": "...",
    "version": "1.0.0",
    "engines": {
        "vscode": "^1.30.0"
    },
    "categories": [
        "Other"
    ],
    "contributes": {
        "languages": [
            {
                "id": "sample",
                "aliases": [
                    "sample"
                ],
                "extensions": [
                    ".sample"
                ]
            }
        ],
        "grammars": [
            {
                "language": "sample",
                "scopeName": "source.sample",
                "path": "./syntaxes/sample.tmLanguage.json",
                "embeddedLanguages": {
                    "source.sample": "haskell"
                }
            }
        ]
    }
}

syntaxes/sample.tmLanguage.json

{
    "scopeName": "source.sample",
    "patterns": [],
    "repository": {}
}

VS Code version: Code 1.53.2 (622cb03f7e070a9670c94bae1a45d78d7181fbd4, 2021-02-11T11:48:44.518Z) OS version: Linux x64 5.4.0-65-generic

System Info

|Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz (4 x 800)| |GPU Status|2d_canvas: enabled
gpu_compositing: enabled
multiple_raster_threads: enabled_on
oop_rasterization: disabled_off
opengl: enabled_on
protected_video_decode: unavailable_off
rasterization: disabled_software
skia_renderer: enabled_on
video_decode: disabled_software
vulkan: disabled_off
webgl: enabled
webgl2: enabled| |Load (avg)|1, 1, 1| |Memory (System)|7.65GB (0.17GB free)| |Process Argv|--no-sandbox --unity-launch --crash-reporter-id 222d795a-5647-4ace-82aa-10aed6154287| |Screen Reader|no| |VM|0%| |DESKTOP_SESSION|unity| |XDG_CURRENT_DESKTOP|Unity| |XDG_SESSION_DESKTOP|unity| |XDG_SESSION_TYPE|x11|

Extensions (27)

Extension|Author (truncated)|Version ---|---|--- ohm|att|1.0.0 vscode-postgres|cko|1.1.17 vscode-eslint|dba|2.1.14 tslint|eg2|1.0.44 vsc-material-theme|Equ|33.1.2 vsc-material-theme-icons|equ|1.2.2 reasonml|fre|1.0.38 vscode-graphql|Gra|0.3.15 haskell|has|1.2.0 hoogle-vscode|jca|0.0.7 svn-scm|joh|2.13.5 language-haskell|jus|3.3.0 theme-karyfoundation-themes|kar|20.0.3 AWK|lug|0.0.2 gnuplot|Mar|1.0.11 template-string-converter|meg|0.4.7 vscode-docker|ms-|1.10.0 cpptools|ms-|1.2.1 java|red|0.75.0 code-spell-checker|str|1.10.2 mib|Ver|0.0.4 vscodeintellicode|Vis|1.2.11 vscode-java-debug|vsc|0.31.0 vscode-java-dependency|vsc|0.18.0 vscode-java-pack|vsc|0.12.1 vscode-java-test|vsc|0.28.0 vscode-maven|vsc|0.27.1 (4 theme extensions excluded)

A/B Experiments

``` vsliv368cf:30146710 vsreu685:30147344 python383cf:30185419 vspor879:30202332 vspor708:30202333 vspor363:30204092 pythonvsdeb440:30248342 pythonvsded773:30248341 pythonvspyt875:30259475 ```

microsoft / vscode

No syntax highlight in embedded language scope #117190