fix: compile-check says how to get its changed-files check back

Packages/src/Editor/CliOnlyTools~/CompileCheck/Skill/SKILL.md

@@ -54,8 +54,10 @@ again: the run reports the diagnostics recorded then, errors included. `--all` c
 everything from scratch, which is the way out if a reused result ever looks wrong.
 
 In a large assembly whose last compile-check compile succeeded, an edit that keeps every
-declaration is checked on the changed files only, in a few seconds. Such a check skips a few
-assembly-wide rules, so run once with `--full` before finishing. The first body edit after a Unity
+declaration is checked on the changed files only, in a few seconds. Adding or removing a using
+directive counts as a declaration change. Such a check skips a few assembly-wide rules, so run once
+with `--full` before finishing. When the project treats warnings as errors, a rule listed in
+`UncheckedCodes`, such as CS0414, can fail that `--full` run only. The first body edit after a Unity
 build, after a `--full` that had errors, after a run that skipped the assembly, or after
 `--all` in an assembly that references another of the project is compiled in full and records what
 the next check compares against; `FastPathFallbacks` says why.

cli/dispatcher/internal/compilecheck/fast_path_test.go

@@ -150,7 +150,7 @@ func TestFailedFullCompileLeavesNoBaseline(t *testing.T) {
 	recorder := newCachedSchedulerRecorder("Alpha", "Beta")
 	outcome := project.run(t, recorder, false)
 
-	assertFallback(t, outcome, recorder, "no baseline: the last full compile had errors")
+	assertFallback(t, outcome, recorder, "no baseline: the last full compile had errors; the first full compile without errors records one")
 }
 
 // Verifies that a change to an input other than source contents, such as a new source, compiles in full.

cli/dispatcher/internal/compilecheck/resultstore/baseline.go

@@ -130,7 +130,7 @@ func (s Store[D]) missingBaselineReason(assembly string) string {
 		return notOnRecord
 	}
 	if !manifest.Succeeded {
-		return "no baseline: the last full compile had errors"
+		return "no baseline: the last full compile had errors; the first full compile without errors records one"
 	}
 
 	return "no baseline: the last full compile did not record one"

cli/dispatcher/internal/compilecheck/resultstore/baseline_contract_test.go

@@ -181,7 +181,7 @@ func TestLoadBaselineExplainsAMissingBaseline(t *testing.T) {
 		},
 		"failed compile": {
 			record: func(t *testing.T, project projectStore) { project.recordCompile(t, "input", false) },
-			want:   "no baseline: the last full compile had errors",
+			want:   "no baseline: the last full compile had errors; the first full compile without errors records one",
 		},
 		"successful compile": {
 			record: func(t *testing.T, project projectStore) { project.recordCompile(t, "input", true) },

cli/dispatcher/internal/compilecheck/resultstore/store.go

@@ -19,7 +19,7 @@
 //     same key, a successful compile, and outputs that still match. Otherwise LoadBaseline says why
 //     there is none, in one of these reasons:
 //     "no baseline: no full compile of this assembly is on record",
-//     "no baseline: the last full compile had errors",
+//     "no baseline: the last full compile had errors; the first full compile without errors records one",
 //     "no baseline: the last full compile did not record one",
 //     "no baseline: it was recorded by another version of compile-check",
 //     "baseline compile is no longer on disk".

docs/compile-check.md

@@ -138,7 +138,9 @@ The check is used when all of these hold:
   list of source paths.
 - 1 to 30 of its sources changed.
 - Every changed file keeps its declarations: only method bodies, accessor bodies, expression
-  bodies, non-constant initializers outside partial types, comments, or whitespace changed.
+  bodies, non-constant initializers outside partial types, comments, or whitespace changed. Adding
+  or removing a `using` directive counts as a declaration change, because it can change what every
+  name in the file resolves to.
 
 The changed files are checked by a worker built once per Editor toolchain with the Editor's own
 dotnet and Roslyn. It reports their compiler and analyzer diagnostics, with `DiagnosticSuppressor`s
@@ -155,8 +157,9 @@ Otherwise the assembly is compiled in full as before, and `FastPathFallbacks` sa
 - `no baseline: no full compile of this assembly is on record` (none ran yet, a later run
   skipped the assembly, which removes its result and baseline, or an input changed while
   the last full compile ran, so it left no record; `FastPathBaselineNotes` of that run says which)
-- `no baseline: the last full compile had errors` (a full compile that fails drops the baseline an
-  earlier one left)
+- `no baseline: the last full compile had errors; the first full compile without errors records one`
+  (a full compile that fails drops the baseline an earlier one left, and the assembly is
+  compiled in full until a compile succeeds)
 - `no baseline: the last full compile did not record one` (`FastPathBaselineNotes` of that run
   says why, or the assembly was compiled only because a reference changed)
 - `no baseline: it was recorded by another version of compile-check`
@@ -205,7 +208,8 @@ What the check does not guarantee, so run `--full` before finishing:
   assembly, and the rules of analyzers that use Compilation, CompilationEnd, SemanticModel, or
   SymbolStart actions. `FastChecks[].UncheckedCodes` lists the IDs it could not run, leaving out
   the ones the response file already turns off (with `-nowarn` or a ruleset) and analyzer rules
-  that are disabled, so an assembly may list fewer than these four.
+  that are disabled, so an assembly may list fewer than these four. In a project that treats
+  warnings as errors (`-warnaserror`), these are errors that only `--full` reports.
 - Errors the compiler reports only while emitting, such as CS8178 and CS0656.
 - Source generators. The check reads the generator output saved by the last full compile and does
   not run them again; `FastChecks[].GeneratorsNotRerun` says so.