[bartlomieju]

Split out of #8065, rebuilt on top of #8051.

workerd run <file> serves one JavaScript or TypeScript file as a Worker, with no config file:

workerd run worker.ts
workerd run worker.js --port 9000 --watch
workerd run worker.ts --compat-date 2026-01-01 --compat-flag nodejs_compat

• Bundling: the file is served as a wrangler config whose only field is main. So it is bundled with esbuild exactly as serve wrangler.jsonc bundles it, using WORKERD_ESBUILD, else the nearest node_modules/.bin/esbuild, else esbuild on PATH. TypeScript, relative imports and npm packages work, and --watch reloads when any input to the bundle changes.
• Defaults: the Worker gets the newest compatibility date this build supports, the same date as workerd bench --compat-date=latest, and listens on localhost:8787, as a wrangler config does.
• Options: --port, --compat-date and --compat-flag change those defaults. The rest of serve's options apply, including --socket-addr.
• Errors: a file that isn't JS/TS, or that doesn't exist, is a usage error. A failed bundle is a config error, so --watch waits for a fix.

Implementation: wrangler::load() is split into parsing and build(), which load_file() reuses with a synthesized config. The socket address is now a parameter. The esbuild-not-found message no longer assumes a wrangler config.

Out of scope here: Python entry modules, which esbuild can't bundle and so need their own path, and --print-config.

Tests: unit tests for the argument parsing, the accepted extensions, and the config run builds, using a stub esbuild in node_modules/.bin. Checked by hand with esbuild 0.28: a .ts entry with a relative import serves, and an edit to the import reloads under --watch.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 87 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.15%. Comparing base (c564d8a) to head (1565d76).

Files with missing lines	Patch %	Lines
src/workerd/server/cli/lib.rs	0.00%	46 Missing ⚠️
src/workerd/server/cli/wrangler.rs	0.00%	41 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8084      +/-   ##
==========================================
- Coverage   68.18%   68.15%   -0.03%     
==========================================
  Files         712      712              
  Lines      185743   185821      +78     
  Branches    29255    29255              
==========================================
+ Hits       126645   126647       +2     
- Misses      42019    42096      +77     
+ Partials    17079    17078       -1     

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