feat(presets): add error parsers for built-in presets

Problem: none of the four presets defined an error_parser, so the
diagnostic infrastructure went unused out of the box.

Solution: add parsers for typst (file:line:col short format), latexmk
(pdflatex file-line-error + summary), and pandoc (parse errors, YAML
exceptions, generic errors). Enable machine-parseable output flags in
typst and latex args. Pandoc parser is shared between markdown and
github presets.

lua/preview/presets.lua

@@ -1,15 +1,108 @@
 local M = {}
 
+---@param stderr string
+---@return preview.Diagnostic[]
+local function parse_typst(stderr)
+  local diagnostics = {}
+  for line in stderr:gmatch('[^\r\n]+') do
+    local file, lnum, col, severity, msg = line:match('^(.+):(%d+):(%d+): (%w+): (.+)$')
+    if lnum then
+      local sev = vim.diagnostic.severity.ERROR
+      if severity == 'warning' then
+        sev = vim.diagnostic.severity.WARN
+      end
+      table.insert(diagnostics, {
+        lnum = tonumber(lnum) - 1,
+        col = tonumber(col) - 1,
+        message = msg,
+        severity = sev,
+        source = file,
+      })
+    end
+  end
+  return diagnostics
+end
+
+---@param stderr string
+---@return preview.Diagnostic[]
+local function parse_latexmk(stderr)
+  local diagnostics = {}
+  for line in stderr:gmatch('[^\r\n]+') do
+    local _, lnum, msg = line:match('^%.?/?(.+%.tex):(%d+): (.+)$')
+    if lnum then
+      table.insert(diagnostics, {
+        lnum = tonumber(lnum) - 1,
+        col = 0,
+        message = msg,
+        severity = vim.diagnostic.severity.ERROR,
+      })
+    else
+      local rule_msg = line:match('^%s+(%S.+gave return code %d+)$')
+      if rule_msg then
+        table.insert(diagnostics, {
+          lnum = 0,
+          col = 0,
+          message = rule_msg,
+          severity = vim.diagnostic.severity.ERROR,
+        })
+      end
+    end
+  end
+  return diagnostics
+end
+
+---@param stderr string
+---@return preview.Diagnostic[]
+local function parse_pandoc(stderr)
+  local diagnostics = {}
+  for line in stderr:gmatch('[^\r\n]+') do
+    local lnum, col, msg = line:match('Error at .+ %(line (%d+), column (%d+)%): (.+)$')
+    if lnum then
+      table.insert(diagnostics, {
+        lnum = tonumber(lnum) - 1,
+        col = tonumber(col) - 1,
+        message = msg,
+        severity = vim.diagnostic.severity.ERROR,
+      })
+    else
+      local ylnum, ycol, ymsg =
+        line:match('YAML parse exception at line (%d+), column (%d+)[,:]%s*(.+)$')
+      if ylnum then
+        table.insert(diagnostics, {
+          lnum = tonumber(ylnum) - 1,
+          col = tonumber(ycol) - 1,
+          message = ymsg,
+          severity = vim.diagnostic.severity.ERROR,
+        })
+      else
+        local errmsg = line:match('^pandoc: (.+)$')
+        if errmsg and not errmsg:match('^Error at') then
+          table.insert(diagnostics, {
+            lnum = 0,
+            col = 0,
+            message = errmsg,
+            severity = vim.diagnostic.severity.ERROR,
+          })
+        end
+      end
+    end
+  end
+  return diagnostics
+end
+
 ---@type preview.ProviderConfig
 M.typst = {
   ft = 'typst',
   cmd = { 'typst', 'compile' },
   args = function(ctx)
-    return { ctx.file }
+    return { '--diagnostic-format', 'short', ctx.file }
   end,
   output = function(ctx)
     return (ctx.file:gsub('%.typ$', '.pdf'))
   end,
+  error_parser = function(stderr)
+    return parse_typst(stderr)
+  end,
   open = { 'xdg-open' },
 }
 
@@ -18,11 +111,19 @@ M.latex = {
   ft = 'tex',
   cmd = { 'latexmk' },
   args = function(ctx)
-    return { '-pdf', '-interaction=nonstopmode', ctx.file }
+    return {
+      '-pdf',
+      '-interaction=nonstopmode',
+      '-pdflatex=pdflatex -file-line-error -interaction=nonstopmode %O %S',
+      ctx.file,
+    }
   end,
   output = function(ctx)
     return (ctx.file:gsub('%.tex$', '.pdf'))
   end,
+  error_parser = function(stderr)
+    return parse_latexmk(stderr)
+  end,
   clean = function(ctx)
     return { 'latexmk', '-c', ctx.file }
   end,
@@ -40,6 +141,9 @@ M.markdown = {
   output = function(ctx)
     return (ctx.file:gsub('%.md$', '.html'))
   end,
+  error_parser = function(stderr)
+    return parse_pandoc(stderr)
+  end,
   clean = function(ctx)
     return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }
   end,
@@ -67,6 +171,9 @@ M.github = {
   output = function(ctx)
     return (ctx.file:gsub('%.md$', '.html'))
   end,
+  error_parser = function(stderr)
+    return parse_pandoc(stderr)
+  end,
   clean = function(ctx)
     return { 'rm', '-f', (ctx.file:gsub('%.md$', '.html')) }
   end,

spec/presets_spec.lua

@@ -21,10 +21,10 @@ describe('presets', function()
       assert.are.same({ 'typst', 'compile' }, presets.typst.cmd)
     end)
 
-    it('returns args with file path', function()
+    it('returns args with diagnostic format and file path', function()
       local args = presets.typst.args(ctx)
       assert.is_table(args)
-      assert.are.same({ '/tmp/document.typ' }, args)
+      assert.are.same({ '--diagnostic-format', 'short', '/tmp/document.typ' }, args)
     end)
 
     it('returns pdf output path', function()
@@ -36,6 +36,30 @@ describe('presets', function()
     it('has open enabled', function()
       assert.are.same({ 'xdg-open' }, presets.typst.open)
     end)
+
+    it('parses errors from stderr', function()
+      local stderr = table.concat({
+        'main.typ:5:23: error: unexpected token',
+        'main.typ:12:1: warning: unused variable',
+      }, '\n')
+      local diagnostics = presets.typst.error_parser(stderr, ctx)
+      assert.is_table(diagnostics)
+      assert.are.equal(2, #diagnostics)
+      assert.are.equal(4, diagnostics[1].lnum)
+      assert.are.equal(22, diagnostics[1].col)
+      assert.are.equal('unexpected token', diagnostics[1].message)
+      assert.are.equal(vim.diagnostic.severity.ERROR, diagnostics[1].severity)
+      assert.are.equal('main.typ', diagnostics[1].source)
+      assert.are.equal(11, diagnostics[2].lnum)
+      assert.are.equal(0, diagnostics[2].col)
+      assert.are.equal('unused variable', diagnostics[2].message)
+      assert.are.equal(vim.diagnostic.severity.WARN, diagnostics[2].severity)
+    end)
+
+    it('returns empty table for clean stderr', function()
+      local diagnostics = presets.typst.error_parser('', ctx)
+      assert.are.same({}, diagnostics)
+    end)
   end)
 
   describe('latex', function()
@@ -57,7 +81,12 @@ describe('presets', function()
     it('returns args with pdf flag and file path', function()
       local args = presets.latex.args(tex_ctx)
       assert.is_table(args)
-      assert.are.same({ '-pdf', '-interaction=nonstopmode', '/tmp/document.tex' }, args)
+      assert.are.same({
+        '-pdf',
+        '-interaction=nonstopmode',
+        '-pdflatex=pdflatex -file-line-error -interaction=nonstopmode %O %S',
+        '/tmp/document.tex',
+      }, args)
     end)
 
     it('returns pdf output path', function()
@@ -75,6 +104,44 @@ describe('presets', function()
     it('has open enabled', function()
       assert.are.same({ 'xdg-open' }, presets.latex.open)
     end)
+
+    it('parses file-line-error format from stderr', function()
+      local stderr = table.concat({
+        './document.tex:10: Undefined control sequence.',
+        'l.10 \\badcommand',
+        'Collected error summary (may duplicate other messages):',
+        "  pdflatex: Command for 'pdflatex' gave return code 256",
+      }, '\n')
+      local diagnostics = presets.latex.error_parser(stderr, tex_ctx)
+      assert.is_table(diagnostics)
+      assert.is_true(#diagnostics > 0)
+      assert.are.equal(9, diagnostics[1].lnum)
+      assert.are.equal(0, diagnostics[1].col)
+      assert.are.equal('Undefined control sequence.', diagnostics[1].message)
+      assert.are.equal(vim.diagnostic.severity.ERROR, diagnostics[1].severity)
+    end)
+
+    it('parses collected error summary', function()
+      local stderr = table.concat({
+        'Latexmk: Errors, so I did not complete making targets',
+        'Collected error summary (may duplicate other messages):',
+        "  pdflatex: Command for 'pdflatex' gave return code 256",
+      }, '\n')
+      local diagnostics = presets.latex.error_parser(stderr, tex_ctx)
+      assert.is_table(diagnostics)
+      assert.are.equal(1, #diagnostics)
+      assert.are.equal(0, diagnostics[1].lnum)
+      assert.are.equal(0, diagnostics[1].col)
+      assert.are.equal(
+        "pdflatex: Command for 'pdflatex' gave return code 256",
+        diagnostics[1].message
+      )
+    end)
+
+    it('returns empty table for clean stderr', function()
+      local diagnostics = presets.latex.error_parser('', tex_ctx)
+      assert.are.same({}, diagnostics)
+    end)
   end)
 
   describe('markdown', function()
@@ -117,6 +184,43 @@ describe('presets', function()
     it('has open enabled', function()
       assert.are.same({ 'xdg-open' }, presets.markdown.open)
     end)
+
+    it('parses pandoc parse errors from stderr', function()
+      local stderr = 'Error at "source" (line 75, column 1): unexpected end of input'
+      local diagnostics = presets.markdown.error_parser(stderr, md_ctx)
+      assert.is_table(diagnostics)
+      assert.are.equal(1, #diagnostics)
+      assert.are.equal(74, diagnostics[1].lnum)
+      assert.are.equal(0, diagnostics[1].col)
+      assert.are.equal('unexpected end of input', diagnostics[1].message)
+      assert.are.equal(vim.diagnostic.severity.ERROR, diagnostics[1].severity)
+    end)
+
+    it('parses YAML parse exceptions from stderr', function()
+      local stderr =
+        'YAML parse exception at line 3, column 2, while scanning a block scalar: did not find expected comment or line break'
+      local diagnostics = presets.markdown.error_parser(stderr, md_ctx)
+      assert.is_table(diagnostics)
+      assert.are.equal(1, #diagnostics)
+      assert.are.equal(2, diagnostics[1].lnum)
+      assert.are.equal(1, diagnostics[1].col)
+      assert.is_string(diagnostics[1].message)
+    end)
+
+    it('parses generic pandoc errors from stderr', function()
+      local stderr = 'pandoc: Could not find data file templates/default.html5'
+      local diagnostics = presets.markdown.error_parser(stderr, md_ctx)
+      assert.is_table(diagnostics)
+      assert.are.equal(1, #diagnostics)
+      assert.are.equal(0, diagnostics[1].lnum)
+      assert.are.equal(0, diagnostics[1].col)
+      assert.are.equal('Could not find data file templates/default.html5', diagnostics[1].message)
+    end)
+
+    it('returns empty table for clean stderr', function()
+      local diagnostics = presets.markdown.error_parser('', md_ctx)
+      assert.are.same({}, diagnostics)
+    end)
   end)
 
   describe('github', function()
@@ -179,5 +283,21 @@ describe('presets', function()
     it('has open enabled', function()
       assert.are.same({ 'xdg-open' }, presets.github.open)
     end)
+
+    it('parses pandoc parse errors from stderr', function()
+      local stderr = 'Error at "document.md" (line 12, column 5): unexpected "}" expecting letter'
+      local diagnostics = presets.github.error_parser(stderr, md_ctx)
+      assert.is_table(diagnostics)
+      assert.are.equal(1, #diagnostics)
+      assert.are.equal(11, diagnostics[1].lnum)
+      assert.are.equal(4, diagnostics[1].col)
+      assert.are.equal('unexpected "}" expecting letter', diagnostics[1].message)
+      assert.are.equal(vim.diagnostic.severity.ERROR, diagnostics[1].severity)
+    end)
+
+    it('returns empty table for clean stderr', function()
+      local diagnostics = presets.github.error_parser('', md_ctx)
+      assert.are.same({}, diagnostics)
+    end)
   end)
 end)