Improved verbosity of data.table:::test() function for mismatched expected warnings

NEWS.md

@@ -48,6 +48,8 @@
 
 7. `fread()` would not give a warning when every second line of input was empty, [#3339](https://github.com/Rdatatable/data.table/issues/3339). Now, a warning message 'The rows in this file appear to be separated by blank lines.' is given and suggests to set `blank.lines.skip` to `TRUE`. Thanks to @Henrik-P for the report and @Asa-Henry for the fix.
 
+8. `test()` now reports multiple expected warnings more clearly when `warning=` has length greater than 1L, instead of printing a collapsed or repeated mismatch summary after messages like `Test 1 produced 1 warnings but expected 2`, [#7092](https://github.com/Rdatatable/data.table/issues/7092). Expected and observed warnings are now printed on separate aligned lines, making small differences easier to spot. Thanks @MichaelChirico for the report, @ben-schwen for assistance, and @lucaslarson25, @tjdavis51, @D3VTHSTVR, and @car723 for the fix.
+
 ### Notes
 
 1. {data.table} now depends on R 3.5.0 (2018).

R/test.data.table.R

@@ -576,7 +576,16 @@ test = function(num, x, y=TRUE,
     }
     if (length(expected) != length(observed) && (!foreign || is.null(ignore.warning))) {
       # nocov start
-      catf("Test %s produced %d %ss but expected %d\n%s\n%s\n", numStr, length(observed), type, length(expected), paste("Expected:", expected), paste("Observed:", observed, collapse = "\n"))
+      align_messages = function(label, x) paste(
+        c(
+          paste0(label, x[1L]),
+          if (length(x) > 1L) paste0(strrep(" ", nchar(label)), x[-1L])
+        ),
+        collapse = "\n"
+      )
+      expected_text = align_messages("Expected: ", expected)
+      observed_text = align_messages("Observed: ", observed)
+      catf("Test %s produced %d %ss but expected %d\n%s\n%s\n", numStr, length(observed), type, length(expected), expected_text, observed_text)
       fail = TRUE
       # nocov end
     } else if (!foreign) {

inst/tests/tests.Rraw

@@ -21607,3 +21607,9 @@ test(2370.2, yearmon(x, format="numeric"), yearmon(x))  # numeric is the default
 test(2370.3, yearmon(x, format="character"), c("1111M11", "2019M01", "2019M02", "2019M03", "2019M12", "2020M02", "2020M03", "2020M12", "2040M01", "2040M12", "2100M03", NA_character_))
 test(2370.4, yearmon("2016-08-03 01:02:03.45", format="character"), "2016M08")
 test(2370.5, yearmon(NA, format="character"), NA_character_)
+
+# multiple expected/observed warnings in test() are printed on aligned lines, #7092
+test(2371.1, test(0, {warning("a"); 2L}, 2L, warning=c("a", "b")), FALSE,
+             output="Test 0 produced 1 warnings but expected 2\nExpected: a\n          b\nObserved: a")
+test(2372.2, test(0, {warning("a"); warning("b"); 2L}, 2L, warning="a"), FALSE,
+             output="Test 0 produced 2 warnings but expected 1\nExpected: a\nObserved: a\n          b")